Category: Structural Engineering & Framing

A deck ledger — the board that carries the deck your stairs lead down from — attaches to the house band joist with 1/2 in / 12.7 mm lag screws or through-bolts, spaced according to joist span per IRC Table R507.9.1.3(1). Nails and standard deck screws are not acceptable fasteners for this connection; they don’t develop enough withdrawal and shear strength, and a ledger that pulls away from the house is the single most common cause of catastrophic deck and stair collapse. Confirm your spacing with the Ledger Board Attachment Calculator before you predrill.

How to size ledger fastener spacing

Fastener spacing under IRC Table R507.9.1.3(1) is set by joist span (measured from the center of the ledger-side joist hanger to the center of the beam the joists rest on, not including cantilevers), fastener type, and sheathing thickness between the ledger and the band joist. As a rough planning rule before checking the exact table value: on-center spacing in inches for 1/2 in lag screws works out to about 190 divided by the joist span in feet, and for 1/2 in through-bolts, about 330 divided by the joist span in feet.

Worked example: a deck with a 12 ft / 3.66 m joist span and 1/2 in / 12.7 mm maximum sheathing between ledger and band joist needs through-bolts at roughly 29 in / 737 mm on center, or lag screws at roughly 15 in / 381 mm on center for the same span — lag screws need tighter spacing than through-bolts because they don’t clamp the connection the same way a nut and washer do on the inside face. Fasteners are staggered in two rows, one near the top edge and one near the bottom edge of the ledger, alternating along its length rather than running in a single straight line.

Minimum edge and end distances matter as much as the spacing number: 2 in / 51 mm minimum from the top edge and ledger ends, 3/4 in / 19 mm minimum from the bottom edge, and at least 1 5/8 in / 41 mm between the two rows, per Table R507.9.1.3(2). Get any of those wrong and the connection doesn’t meet the prescriptive table even if the on-center spacing is correct.

What the code actually requires

IRC Section R507.9.1.3 (2021 edition) governs the ledger-to-band-joist connection. Fasteners must be hot-dip galvanized or stainless steel, installed per Table R507.9.1.3(1) for spacing and Table R507.9.1.3(2) for placement. Holes for 1/2 in lag screws are predrilled with two different bit sizes — a full 1/2 in hole through the ledger and any sheathing, then a 5/16 in to 3/8 in / 8–9.5 mm pilot hole through the band joist itself — and the tip of the lag screw has to fully extend beyond the inside face of the band joist.

The table’s prescriptive values assume a 40 psf / 1.9 kPa deck live load, 10 psf / 0.5 kPa dead load, and snow load not exceeding 40 psf, applied concurrently. Heavier snow loads, hot tubs, or other unusual loading push the ledger connection outside the prescriptive table and into engineered design territory. Where the house side is framed with engineered rim joists or manufactured trusses instead of a solid sawn band joist, the manufacturer’s own attachment recommendations govern instead of the standard IRC table, and in some truss conditions a full engineered design is required because there’s no solid band joist to bolt into at all.

Lateral load — the deck’s tendency to rack sideways or pull away from the house under people moving on it, wind, or seismic forces — is a separate requirement from the vertical ledger bolts. IRC R507.2 and related provisions require tension ties connecting the deck framing back to the house structure independent of the ledger bolts, since the ledger fasteners alone aren’t credited with resisting that lateral force in the prescriptive path.

Common mistakes

Using nails or standard deck screws instead of lag screws or through-bolts. This remains the most common ledger failure mode. Nails and general-purpose deck screws don’t have the shear or withdrawal capacity the connection needs, and the failure is often silent until the ledger suddenly separates from the house under load. Fix: use only 1/2 in lag screws, through-bolts, or a structural screw specifically load-rated for ledger attachment (such as Simpson SDWS or FastenMaster LedgerLOK) installed at the manufacturer’s specified spacing.

Skipping flashing behind the ledger. Bolting the ledger tight to the house without flashing lets water track behind the board and into the band joist year after year. The wood rots from the inside, and by the time it’s visible from outside, the fasteners may already be holding into punky, decayed wood with a fraction of their rated capacity. Fix: install flashing per IRC R703.4 before the ledger goes up, not as an afterthought after fasteners are already driven.

Bolting through brick veneer or foam sheathing without the right hardware. Standard lag screws through more than about 1 in of non-structural sheathing or veneer lose bearing and can crush the sheathing over time rather than clamping the ledger tight. Fix: use through-bolts with load-rated standoff or spacer hardware designed for veneer conditions, sized per the manufacturer’s engineering letter for that wall assembly.

Guessing spacing instead of measuring actual joist span. Builders sometimes default to a generic 16 in on-center pattern regardless of span, which is fine for a short-span deck but under-fastened for anything with a longer joist run. Fix: measure the actual joist span from ledger-side hanger center to beam center, then pull the correct spacing from Table R507.9.1.3(1) for that span, not a memorized default.

Related calculators you might need

Once the ledger connection is sized, the joists it carries need hangers rated for the same load path — the Joist Hanger Count Calculator works out how many you need for the framed landing. If the stair stringers land on that same structure, check the Stringer Spacing Calculator to confirm stringer count against the stair width. For the stairs themselves, the Deck Stair Calculator lays out rise, run, and stringer geometry, and the Stair Footing Size Calculator sizes the pads at the base. Run the full assembly past the IRC Stair Code Checker before you finalize the plan.

Frequently asked questions

Can I attach a deck ledger with screws instead of lag bolts?

Only with structural screws specifically load-rated for ledger connections, such as Simpson SDWS or FastenMaster LedgerLOK, installed at the fastener manufacturer’s engineered spacing — not with standard deck screws or wood screws. Those screws are evaluated separately from the IRC’s lag-bolt table and typically have their own published spacing chart tied to the same joist-span logic.

How far apart should deck ledger bolts be?

It depends on your joist span and fastener type, per IRC Table R507.9.1.3(1) — typical residential spans land somewhere between about 15 in and 30 in on center. Run your actual joist span through the Ledger Board Attachment Calculator rather than using a fixed number, since spacing tightens as span increases.

Do I need flashing behind a deck ledger board?

Yes. IRC R703.4 requires flashing at the ledger-to-house connection to keep water out of the band joist, and skipping it is one of the fastest ways to rot out a structural connection that looks fine from the outside for years before it fails.

What happens if my house has a manufactured truss instead of a solid band joist?

The standard IRC ledger table doesn’t apply directly. In many truss conditions the manufacturer provides specific attachment details, and in some cases there’s no solid band joist at all, which means the ledger connection needs an engineered design rather than the prescriptive path.

Is a ledger connection enough to resist lateral loads on a deck?

No. The ledger bolts are sized for vertical (gravity) load only. Lateral load — from people moving on the deck, wind, or seismic forces — requires separate tension ties connecting the deck framing back to the house structure, independent of the ledger fasteners themselves.

A residential stair frame has to survive two separate load cases at once: a 40 psf uniform live load spread across every tread, and a 300 lb concentrated load applied over a 4 sq in patch anywhere on a tread. IRC Table R301.5 (2021 edition) requires the frame to be designed for whichever produces the greater stress — usually the concentrated load near the tread’s unsupported edge. Everything else in stair framing — stringer size, spacing, beam sizing, footings — exists to carry that number safely to the ground.

Three subsystems do the work: the stringers that carry tread loads down the slope, the top and bottom connections that anchor the stringers to the structure and the ground, and the landings that break up long runs and provide a level transition. Get any one of the three wrong and the stair either fails a code inspection or fails structurally, and the two aren’t always the same failure.

How Load Moves Through a Stair Frame

Stringers are the spine of the stair. Cut stringers (the sawtooth-profile boards most DIYers picture) and solid stringers (uncut boards with cleats or brackets carrying the treads) both trace back to the same source document contractors actually build from: the American Wood Council’s DCA6 prescriptive deck guide, which most jurisdictions adopt by reference for stair framing since the IRC itself doesn’t publish a stringer span table.

The governing numbers: cut 2×12 stringers must be spaced no more than 18 in / 457 mm on center, with a minimum throat depth of 5 in / 127 mm after notching. A solid stringer pair (no notching) can span up to 16 ft 6 in in southern pine or 13 ft 3 in in other commonly used species, but only two solid stringers are permitted on a 36 in / 914 mm wide stair — width beyond that needs a third stringer regardless of species.

Worked example: a 40 in / 1016 mm wide exterior stair using cut 2×12 stringers. Divide the width by the 18 in maximum spacing: 40 ÷ 18 = 2.2, which rounds up to 3 spaces, meaning 4 stringers minimum, not the 2 or 3 that most DIY framing crews default to. Run the exact count for your own width, tread material, and species through the Stringer Spacing Calculator before you buy lumber — composite and thinner tread stock tighten the spacing further.

Why Every Stair Needs a Complete Load Path

A stringer that’s perfectly sized still fails if either end has nowhere to transfer its load. At the top, stringers need a positive connection — through-bolted to a ledger, seated in an engineered stringer hanger, or bearing directly on a beam — because a load path that relies on toe-nails alone loses capacity under the lateral, racking forces a stair actually sees when people run down it. IRC R301.7 sets the deflection ceiling for the framing at L/360 under live load and L/240 under total load, which is the practical limit on how far a stringer or beam can span before it feels springy underfoot even if it isn’t overstressed.

At the bottom, the stringer needs to bear on something that won’t move: a poured footing, a concrete pad at least 3.5 in / 89 mm thick and 8 in / 203 mm across, or a structural landing. Skip the footing and an exterior stringer resting on soil or a paver will heave with the frost cycle and walk out of alignment within a season or two. Where the total vertical rise of a run gets long — many jurisdictions draw the line around 12 ft / 3.7 m of rise, though this is a local amendment rather than a universal IRC baseline — a mid-run landing is required, which re-establishes bearing and shortens the effective stringer span at the same time.

Common Mistakes

Guessing stringer spacing on wide stairs. A crew builds a 48 in wide exterior stair with two stringers because “that’s what the last one had,” without checking that the last one was 36 in wide. The result is a bouncy tread and, over years, visible sag between stringers. The fix is arithmetic, not judgment: width ÷ 18 in, round up, add one — confirmed against the Stringer Spacing Calculator.

Notching past the minimum throat. Cutting a stringer’s tooth profile too deep to fit a particular riser height leaves less than the 5 in / 127 mm minimum throat at the exact point where bending stress peaks. The stringer doesn’t fail immediately — it fails quietly, as a hairline crack that widens over a few seasons of load cycling. The fix is to hold the throat minimum and adjust riser height or stringer depth instead, never the throat.

Toe-nailing the top connection. Stringers nailed at an angle into a ledger or rim joist instead of hung or through-bolted rely entirely on nail withdrawal resistance to resist racking. It’s the single most common failure point building inspectors flag on stair reframes. Use a rated stringer hanger or through-bolt per the connector manufacturer’s table, sized with the Ledger Board Attachment Calculator.

Skipping the footing at grade. Exterior stringers set directly on a paver or compacted gravel without a frost-depth footing look fine at handoff and start rocking within a year in any climate with real freeze-thaw cycles. Size the footing to local frost depth and the tributary load actually landing on it with the Stair Footing Size Calculator, not a fixed “12 inches down” rule of thumb.

Related Calculators You Might Need

Once stringer count and spacing are locked in, the next constraint is usually whatever the stringers land on. If that’s a beam rather than a ledger, the Stair Beam Span Calculator sizes the horizontal support member for the tributary width and species you’re using. If the run breaks partway up, the Landing Framing Calculator works out joist size and beam span for the platform itself, treating it structurally as a small deck. Because every one of these decisions ultimately answers to a code minimum, it’s worth running the finished layout through the IRC Stair Code Checker before cutting lumber. If the stair will see unusually heavy or commercial-grade foot traffic, confirm the frame’s actual capacity with the Stair Load/Weight Capacity Calculator rather than assuming residential defaults apply. For a full picture of every tool in this category, the structural calculators section covers stringers, beams, footings, and framing together.

Frequently Asked Questions

How many stringers does a 36 inch wide stair need? A 36 in / 914 mm wide stair using cut 2×12 stringers at the maximum 18 in on-center spacing needs 3 stringers — two outer stringers plus one centered between them. This is also the minimum most inspectors expect on any stair at or near the 36 in code-minimum width, even before running the numbers, because 36 ÷ 18 = 2 spaces, which always resolves to 3 stringers. Verify with the Stringer Spacing Calculator if your tread material is thinner than standard lumber.

What size lumber is used for stair stringers? Cut stringers are almost always framed from 2×12 stock, since that’s the smallest dimension lumber that leaves a code-minimum 5 in throat after the tread-and-riser notch is cut. Smaller stock like 2×10 doesn’t leave enough material once notched for a standard 7.5 in riser and 10 in tread. Solid, uncut stringers can sometimes use engineered lumber (LVL) for longer clear spans.

Do stair stringers need their own footing? Yes, wherever a stringer lands at or near grade. Interior stringers landing on a structural floor don’t need a separate footing, but any stringer terminating outdoors — deck stairs, porch steps, basement entries — needs to bear on a footing or slab rated for the local frost depth, not just a paver set on gravel. Undersized footings are one of the most common causes of exterior stair movement.

Can you use two stringers on a wide staircase? Only up to 36 in / 914 mm of width when using solid, uncut stringers spanning within the DCA6 limits — 16 ft 6 in for southern pine, 13 ft 3 in for most other species. Beyond 36 in wide, or with any cut/notched stringer configuration, a third stringer is required regardless of span length, because the 18 in maximum spacing rule applies independently of the two-stringer allowance.

How is stair framing different from deck framing? Stair framing carries the same live-load table as decks (40 psf, IRC R301.5) but adds the 300 lb concentrated load specific to individual treads, plus the diagonal geometry that puts stringers in combined bending and shear rather than simple bending. A joist span table sized for a flat deck will not transfer directly to a stringer span — the two use different prescriptive tables for that reason.

Every stringer that hangs off a rim joist or header — rather than bearing directly on a post or footing — needs its own hanger, and every landing joist framed between two headers needs a hanger at each end. For a typical deck stair with 2 stringers hung off the rim joist and a landing framed with 4 joists between double headers, that’s 2 stringer hangers plus 8 joist hangers — 10 total, not counting any hangers needed where the landing itself connects back to the main deck frame. Run your own configuration through the Joist Hanger Count Calculator to get an exact count before you order hardware.

How to count joist hangers for a stair

Start by identifying every connection where a framing member’s end is supported by hanging off another member instead of resting on top of a post, beam, or footing. Stair stringers hung from a header need a hanger specifically rated for the sloped connection — IRC R507.13.2 requires a minimum capacity of 625 lb / 2,835 N for these — one per stringer. A landing framed independently, with joists spanning between two header beams, needs a hanger at both ends of every joist, since neither end simply rests on top of anything.

Worked example: a stair 42 in / 1,067 mm wide is built with 3 stringers (outer two plus a center stringer for the extra width), all hung from a header with stair-rated hangers — that’s 3 hangers. The landing at the base is framed with 4 joists spanning between a pair of double 2×10 headers, hung at both ends — that’s 4 joists x 2 ends = 8 hangers. Total for this configuration: 11 joist hangers, before accounting for any hanger needed where that landing header ties back into the main deck or house framing.

Hanger counts by stair configuration

These figures assume a straight-run stair with stringers hung from a header at the top, and an independently framed landing at the bottom with joists hung at both ends. Actual counts shift with landing size and whether the landing bears on posts instead of hanging from headers.

Stair width	Stringers (hung)	Landing joists	Total hangers
36 in / 914 mm	2	3	8
42 in / 1,067 mm	3	4	11
48 in / 1,219 mm	3	5	13
60 in / 1,524 mm	4	6	16

Common mistakes

Using standard face-mount joist hangers instead of stair-rated hangers. A generic 2x hanger built for a level, square joist connection doesn’t seat correctly on an angled stringer and isn’t rated for the sloped connection load IRC R507.13.2 requires. Fix: buy hangers specifically designed and load-rated for stringer-to-header connections, not repurposed floor-joist hardware.

Leaving hanger nail holes partially empty. A hanger’s published load rating assumes every specified hole is filled with the correct fastener type and length. Skipping half the holes to save a few minutes doesn’t cut the capacity in half evenly — it can drop it well below what the connection actually needs to carry, since the remaining fasteners take a disproportionate share of the load. Fix: fill every hole with the manufacturer-specified nail or structural screw, no exceptions.

Mixing incompatible hanger and fastener coatings. Installing a hot-dip galvanized (HDG) or ZMAX hanger with plain, uncoated nails — or the reverse — sets up galvanic corrosion, especially in ACQ-treated lumber, which is common in deck and stair construction. The connection can look fine for a couple of years while corroding from the inside. Fix: match hanger coating to fastener coating — HDG or ZMAX hangers with hot-dip galvanized nails (ASTM A153), stainless hangers with stainless fasteners.

Skipping hangers on interior landing joists because the ends ‘look supported.’ On a landing with more than two joists, it’s easy to hang the outer two carefully and let the interior joists rest on a ledger strip or get a couple of toe-nails instead of a proper hanger. Those joists sag under load over time, which shows up as a soft spot or slight slope across the landing. Fix: hang every joist-to-header connection shown on the framing plan, interior joists included.

Related calculators you might need

Before counting hangers, confirm the ledger or header they attach to is fastened correctly — the Ledger Board Attachment Calculator checks that connection. If you haven’t settled on stringer count yet, the Stringer Spacing Calculator determines how many stringers a given stair width needs. For the landing framing itself, run dimensions through the Landing Framing Calculator, and check the header spanning to the stringers with the Stair Beam Span Calculator. If the whole assembly is part of a deck, the Deck Stair Calculator ties the geometry together.

Frequently asked questions

How many joist hangers do I need for deck stairs?

It depends on stringer count and landing size, but a typical 36 in wide stair with 2 hung stringers and a 3-joist landing needs around 8 hangers total. Wider stairs with a center stringer and larger landings need more — run your specific dimensions through the Joist Hanger Count Calculator for an exact number.

Do stair stringers need special joist hangers?

Yes. IRC R507.13.2 requires hangers specifically designed for sloped stringer connections, rated for a minimum capacity of 625 lb. A standard flat joist hanger isn’t shaped for the stringer angle and isn’t rated for this connection.

What size nails go in a stair stringer hanger?

Use the exact nail or structural screw type and length specified by the hanger manufacturer for that hanger model — hanger load ratings are tested with a specific fastener, and substituting a different nail size or type invalidates the published capacity.

Can I toe-nail a landing joist instead of using a hanger?

Toe-nailing alone doesn’t meet code for a joist framed between two headers where the joist end isn’t bearing on top of anything — it needs a rated hanger at each end. Toe-nailing is only appropriate for specific blocking conditions the code calls out separately, not as a substitute for a hanger.

Why do my stringer hangers need a 625 lb rating?

IRC R507.13.2 sets that minimum specifically for hangers supporting sloped stringer connections, since the angled load path puts different stress on the hanger than a standard level joist connection. A lower-rated or standard hanger doesn’t meet this requirement even if it physically fits.

A stair beam carries the combined reaction load of every stringer landing on it, plus whatever the landing or deck structure above contributes — and that load is almost always higher than a same-size floor or deck beam sees, because IRC Table R301.5 requires the design to account for a 300 lb concentrated load on top of the standard 40 psf live load. Getting stair beam span wrong doesn’t usually show up as collapse — it shows up as a beam that bounces, a landing that feels loose, or a beam that’s been oversized so far past what’s needed that the footing and post sizing balloon with it.

How Stair Beam Span Is Actually Calculated

There’s no separate “stair beam” table in the IRC — stair support beams are sized using the same tributary-load method as any deck or floor beam, adjusted for the fact that a stair’s tributary width is measured differently than a flat deck’s. Tributary width is half the distance to the next support on each side: half the stringer span on one side, plus half the landing joist span on the other, if the beam also carries a landing.

The load per linear foot on the beam is the tributary width multiplied by the governing load — 40 psf live plus roughly 10 psf dead for a typical wood-framed landing, per the same load basis DCA6 uses for deck beam design. A beam carrying a 6 ft tributary width at 50 psf combined load sees 300 plf (pounds per linear foot) of uniform load, before the concentrated tread load is checked separately at the point of maximum stress.

Worked example: a landing beam with a 6 ft / 1.8 m tributary width carrying an 8 ft / 2.4 m clear span. That’s 300 plf across an 8 ft span — a load and span combination that typically lands in the range of a built-up 2-ply 2×10 to 2×12 in southern pine, depending on grade and whether the lumber is wet-service rated for exterior exposure. The exact allowable span for a given size, species, and grade comes from the same species-specific span tables used for deck beams (DCA6 Table 4 or an engineered lumber manufacturer’s table) — plug your tributary width and species into the Stair Beam Span Calculator to get the minimum size for your actual numbers rather than guessing from a similar-looking project.

What Actually Changes the Required Beam Size

Four variables move the required beam size more than anything else. Tributary width has the most direct effect — doubling the tributary width roughly doubles the load per foot, which usually means going up a full lumber size or adding a ply, not just tightening the span slightly. Species and grade matter because a southern pine No. 2 beam and a lower-grade spruce-pine-fir beam of the same dimension can carry meaningfully different spans for the same load.

Wet-service conditions — any beam exposed to weather, which covers essentially every exterior stair landing — reduce the allowable span compared to the same lumber used indoors, because treated and weather-exposed lumber tables apply a wet-service adjustment factor. And concentrated versus uniform loading matters because a beam that passes the uniform 40 psf check can still fail the 300 lb concentrated-load check if the beam is undersized or the span is long — both cases need to be checked, and the smaller resulting span governs.

Common Mistakes

Sizing the beam for joists instead of stair reactions. A beam gets pulled straight from a deck joist span table without adjusting for the fact that a stair beam often carries a concentrated stringer reaction rather than a smoothly distributed joist load. The fix is to calculate the actual tributary width and load at the beam, not to copy a table entry sized for a different load pattern.

Not doubling tributary width where two stair runs meet at one landing beam. On an L-shaped or U-shaped layout, a single landing beam sometimes picks up stringers from two separate runs, which roughly doubles its tributary width compared to a single-run stair. Treating it as a single-run beam under-sizes it. Recalculate tributary width for every load actually landing on that specific beam.

Single-ply beams where a built-up beam is required. A built-up 2- or 3-ply beam is specified in the design but built as a single wide member (or vice versa) because the framer had different stock on hand. Ply count changes the effective section and the allowable span — a single 2×12 is not structurally equivalent to a built-up 2-ply 2×10 of similar overall depth. Match what’s actually built to what was calculated.

Ignoring wet-service reduction on exterior beams. A span pulled from an interior-use table gets applied to an uncovered exterior landing beam without adjusting for wet-service conditions, which quietly erodes the safety margin the original calculation assumed. Any beam exposed to weather needs the wet-service-adjusted span, not the dry interior figure.

Related Calculators You Might Need

Once the beam is sized, the next step is usually the landing it supports — the Landing Framing Calculator works out joist size and layout for the platform the beam carries. Whatever the beam sits on needs its own footing, sized with the Stair Footing Size Calculator, and the connection between joists and beam typically runs through hangers — use the Joist Hanger Count Calculator to get an accurate hardware count. If the stringers landing on this beam haven’t been sized yet, start with the Stringer Spacing Calculator, and for commercial applications, check the beam and connections against the IBC Commercial Stair Code Calculator rather than residential defaults.

Frequently Asked Questions

How do I size a beam for a stair landing? Calculate the tributary width the beam actually carries — half the stringer span plus half the landing joist span on the other side — multiply by the combined live and dead load (typically 50 psf total), and match the resulting load-per-foot and clear span against a species-specific beam span table. The Stair Beam Span Calculator does this calculation directly from your dimensions and species selection.

Does a stair beam need to be doubled? Usually, yes, for anything beyond a very short span and light tributary width. Built-up beams of 2 or 3 plies are the standard approach for stair landing beams because a single-ply member of the same overall depth typically can’t match the section properties of a built-up beam at realistic residential spans. The exact ply count depends on the calculated load, not a fixed rule of thumb.

What size beam do I need for a 6 foot stair landing? It depends entirely on tributary width, species, and grade — there’s no single universal answer. A landing beam with a modest 4–6 ft tributary width and an 6–8 ft clear span commonly ends up in the 2-ply 2×10 to 2×12 range in a common structural species, but the only reliable way to confirm a size is to run your specific tributary width, span, and species through a beam span calculation rather than matching a number from an unrelated project.

Can a stair beam be smaller than a deck beam of the same span? No — if anything, expect it to need to be the same size or larger, because stair beams carry the same uniform load basis as deck beams plus a concentrated tread load check that flat deck beams don’t need to satisfy. Assuming a stair beam can be undersized relative to an equivalent deck beam is a common and risky shortcut.

IRC R311.7.6 sets two non-negotiable minimums for any stair landing: the width perpendicular to travel must be at least as wide as the stair it serves, and where the run is a straight flight, the depth in the direction of travel must be at least 36 in / 914 mm. A 36 in wide stair therefore needs a landing that’s a minimum of 36 in square — and landing framing is what actually holds that platform up, structurally, it’s built exactly like a small deck: joists, a beam, posts, and a footing.

How a Landing Is Actually Framed

A landing carries the same live-load basis as the rest of the stair: 40 psf uniform plus a 300 lb concentrated load check per IRC Table R301.5, applied because the landing functions as a walking surface exactly like a tread does. That load gets picked up by joists spanning between a ledger (or beam) on one side and a beam on the other, with the beam typically carried on posts down to footings, unless the landing is small enough to cantilever or is directly supported by a masonry wall.

Worked example: a 42 in / 1067 mm wide stair terminating at a landing that’s 42 in wide by 48 in / 1219 mm deep — deeper than the 36 in code minimum to give a comfortable turn radius. That landing needs joists spanning the 42 in width, typically doubled or tripled where the stringers attach, carried by a beam sized for the landing’s tributary load using the same beam-span method used for the rest of the stair frame. Enter your landing’s actual dimensions into the Landing Framing Calculator to get joist size, spacing, and beam requirements for that specific footprint.

Why Landing Dimensions and Framing Are Linked

The code-minimum dimensions in R311.7.6 exist for a reason that directly affects framing: a landing narrower than the stair it serves creates a pinch point exactly where someone is transitioning from a sloped surface to a flat one, and a landing shallower than 36 in doesn’t give a full stride of flat surface before the next flight starts. Framing has to accommodate whatever dimension the code requires, not the other way around — undersizing the platform to simplify the framing is not a legitimate shortcut.

Many jurisdictions also require an intermediate landing partway up any run where the total vertical rise exceeds roughly 12 ft / 3.7 m — this is a common state and local amendment rather than a universal IRC baseline, so it’s worth confirming the exact trigger height with the local building department before finalizing a tall exterior stair design. Where it applies, that mid-run landing gets framed identically to a top or bottom landing: joists, beam, posts, footings, sized for the same load table.

Common Mistakes

Undersizing the landing footprint. A landing gets built to whatever scrap decking or leftover joist length is on hand rather than to the code-minimum width-of-the-stair and 36 in depth rule. An inspector will flag this at final, and retrofitting a landing after the stringers and railings are already installed is far more disruptive than sizing it correctly the first time.

Leaving one edge of the landing unsupported. The ledger-side edge gets properly attached, but the outer edge is left resting on fill dirt or an unrated paver instead of a beam on footings. The landing then settles unevenly, which shows up as a landing that’s no longer level with the top of the stringers it’s supposed to meet flush.

Weak ledger connection at the house side. A landing ledger gets nailed to rim joist or siding the same way a deck ledger sometimes is, without through-bolting or lag-screwing into solid framing per a rated connection table. This is the same failure mode responsible for most deck-ledger collapses, and a landing ledger carries an added concentrated tread load on top of the standard uniform load a deck ledger sees. Size the connection with the Ledger Board Attachment Calculator, not a generic nailing schedule.

Mismatched landing height at the top riser. The landing gets framed slightly high or low relative to the last stringer cut, creating a top or bottom riser that’s noticeably taller or shorter than the rest — a trip hazard and, past a small tolerance, a code violation on riser uniformity. Confirm the finished landing surface height against the stringer layout before the stringers are permanently fastened, not after.

Related Calculators You Might Need

Landing framing depends directly on the beam that carries it — use the Stair Beam Span Calculator to size that member for the landing’s specific tributary load. Whatever the beam rests on needs a footing sized for local frost depth and the tributary load, using the Stair Footing Size Calculator. The joist-to-beam and joist-to-ledger connections typically need rated hangers — the Joist Hanger Count Calculator works out an accurate hardware count from your joist layout. Before finalizing dimensions, confirm the platform meets the minimum footprint with the Landing Size Calculator, and run the full assembly through the IRC Stair Code Checker to catch anything the framing alone won’t flag.

Frequently Asked Questions

How big does a stair landing need to be? At minimum, as wide as the stair it serves and at least 36 in / 914 mm deep in the direction of travel for a straight-run stair, per IRC R311.7.6. Landings of other shapes need a depth and area at least equal to a quarter circle with a radius equal to the required width. Check your specific footprint against the Landing Size Calculator before framing.

Do I need a landing on outdoor stairs? Yes — the landing requirement in R311.7.6 applies at both the top and bottom of any stairway, interior or exterior, with a narrow exception for the top of certain interior flights where no door swings over the stairs. Exterior stairs at a deck, porch, or entry door need a code-compliant landing at both ends just like an interior stair does.

Can a landing be smaller than the width of the stair? No. R311.7.6 explicitly ties landing width to the width of the flight it serves — the landing can’t be narrower than the stair. It can be wider, and often is, to accommodate a door swing or a turn in the stair layout, but never narrower.

Does a stair landing need its own footing? If the landing is elevated above grade — at a deck, porch, or second-story exterior stair — yes, it needs the same kind of footing support as the beam and posts holding up any other elevated structure. A landing directly poured as part of a concrete slab at grade doesn’t need a separate footing in the same sense, since the slab itself bears the load.

A stand-alone stair footing needs a minimum thickness of 6 in / 152 mm under IRC Section R403.1.1, with a footing projection of at least 2 in / 51 mm on each side that can’t exceed the footing’s own thickness. Width is the variable that actually changes project to project — it’s driven by the load landing on that footing and your soil’s bearing capacity, not a single fixed number. For most residential deck stairs on average soil (assumed 1,500 psf / 71.8 kPa bearing capacity unless tested), a 12 in x 12 in / 300 mm x 300 mm pad covers the load with margin to spare. Run your own numbers with the Stair Footing Size Calculator before you order concrete.

How to size a stair footing

Footing sizing comes down to one relationship: required footing area equals the load the footing carries divided by the allowable soil bearing pressure. The load is the tributary area at that bearing point multiplied by the design load (40 psf / 1.9 kPa live load plus roughly 10 psf / 0.5 kPa dead load for a residential stair, per IRC Table R301.5 and matching ASCE 7 values).

Take a real example: an exterior deck stair with a total rise of 108 in / 2,743 mm and a 7.5 in / 190 mm riser height works out to 14 risers. The stair is built with two stringers spaced 34 in / 864 mm apart, each landing on its own footing at the base. Each stringer’s tributary width is half the stair width plus a small margin — call it 1.4 ft / 0.43 m — over a run length of about 10.5 ft / 3.2 m. That’s a tributary area of roughly 14.7 sq ft / 1.37 sq m per stringer, split across the run but concentrated at the footing as a point reaction. Using 50 psf combined load, the reaction at the base of one stringer is in the range of 300–400 lb / 136–181 kg, depending on how the load is distributed along the stringer.

Divide that reaction by the assumed 1,500 psf soil bearing value and you get a required footing area under half a square foot — which is why a 12 in x 12 in pad (1 sq ft / 0.09 sq m) is the default prescriptive size accepted in most jurisdictions for a two-stringer residential stair. Wider stairs, three-plus stringers, masonry stair walls, or soft soil push the number up fast, which is where Table R403.1(1) and an actual soil bearing test start to matter instead of the default assumption.

What the code actually requires

IRC Section R403.1.1 (2021 edition) sets the 6 in minimum thickness for spread footings and caps the footing projection at the footing’s thickness. Table R403.1(1) then sets minimum footing width based on three inputs: the number of stories the footing supports, the construction type above it, and the load-bearing value of the soil per Table R401.4.1. Wider footings are required as soil bearing capacity drops — a footing sized for 3,000 psf soil can be significantly narrower than the same load on 1,500 psf soil.

Depth is a separate requirement from thickness. The footing base has to sit below the local frost line to avoid frost heave cracking the pad and lifting the stair over winter freeze-thaw cycles (IRC R403.1.4), and the IBC additionally sets a minimum depth of 12 in / 305 mm below undisturbed grade regardless of frost depth. Frost line depth is set locally, not by the base code — it can range from a few inches in the warmest coastal jurisdictions to 42 in / 1,067 mm or more in cold northern climates, and only your local building department’s published frost depth is authoritative for your address.

On sloped sites, footings can’t just be poured on an angle — IRC R403.1.5 requires stepped footings, with each horizontal step at least 2 ft / 610 mm long and each vertical rise no more than 0.75 times that horizontal run. Every step still has to hit the same frost depth and bear on undisturbed or properly compacted soil, which matters for exterior stairs built into a graded yard.

Typical footing widths by soil bearing capacity

These figures illustrate how footing width scales with assumed soil bearing capacity for a light, single-story load case per IRC Table R403.1(1) — always confirm the exact figure against your adopted code edition and local amendments before pouring.

Soil bearing capacity	Relative footing width	Typical use case	Note
1,500 psf / 71.8 kPa	Widest	Assumed default, untested soil	Conservative; used when no soil report exists
2,000 psf / 95.8 kPa	Moderate	Common inspected fill or native soil	Often needs a compaction or soil report
3,000 psf / 143.6 kPa	Narrower	Dense sand, gravel, hard clay	Verify with geotechnical data, not assumption
4,000 psf / 191.5 kPa	Narrowest	Rock or engineered fill	Rare for typical residential stair footings

Common mistakes

Pouring above the frost line. Builders working fast in summer often set stair footings at a shallow, convenient depth instead of the jurisdiction’s actual frost depth. The footing heaves during the first hard freeze-thaw cycle, cracking the pad and tilting the stair. Fix: pull the local frost depth from the building department before excavating, not from a general rule of thumb from a different climate.

Assuming 1,500 psf soil bearing on fill or soft clay. The default assumption only holds for reasonably competent, undisturbed native soil. On recently placed fill, wet clay, or organic topsoil, actual bearing capacity can be well below that number, and a prescriptive 12 in x 12 in footing will settle unevenly under one corner of the stair. Fix: order a soil test on questionable sites, or oversize the footing and add a compacted gravel base as a conservative buffer.

Skipping a footing under an intermediate stringer or landing post. On wide stairs with three or more stringers, or stairs with a mid-run landing, every bearing point needs its own footing — not just the two outside stringers. Leaving the center stringer resting on grade or a paver leads to differential settlement, a stringer that racks out of plane with the others, and treads that go out of level over a season or two. Fix: footing at every bearing point shown on the framing plan, sized individually for its own tributary load.

No reinforcement or connection dowels in freeze-thaw climates. A plain unreinforced pad in a region with real freeze-thaw cycling is more prone to surface spalling and cracking at the edges, and it gives the post base nothing to positively anchor into. Fix: add reinforcement and a post base or anchor bolt cast into the footing per your local amendment, even where the base code doesn’t explicitly mandate it for small footings.

Related calculators you might need

Once the footing size is set, the next question is usually whether the stringer spanning to that footing is adequately sized — the Stair Beam Span Calculator checks allowable span against lumber size and species. If the stair lands on a framed deck rather than a slab, the Landing Framing Calculator sizes the joists and header at that landing. For material takeoff on the pour itself, the Concrete Stairs Calculator estimates volume, and the Cubic Yard of Concrete Converter converts that into an order quantity. Before you finalize anything, run the layout past the IRC Stair Code Checker to confirm the rest of the stair meets rise, run, and landing requirements alongside the footing.

Frequently asked questions

How deep does a stair footing need to be?

Deep enough to sit below your local frost line, with an absolute minimum of 12 in / 305 mm below undisturbed grade regardless of frost depth. Frost depth is set by your local building department and ranges from a few inches in mild coastal climates to over 42 in / 1,067 mm in cold northern regions — check your jurisdiction’s published number rather than assuming a figure from a different area.

What size footing do I need for deck stairs?

For a typical two-stringer residential stair on average soil, a 12 in x 12 in x 6 in / 300 mm x 300 mm x 150 mm pad usually covers the load with margin. Wider stairs, extra stringers, masonry construction, or soft soil push that number up — run the specific load and soil bearing value through the Stair Footing Size Calculator rather than defaulting to the minimum on anything beyond a basic stair.

Can I use a precast paver instead of a poured concrete footing?

Some jurisdictions accept precast concrete deck blocks for light, low-rise stairs, but they generally aren’t accepted below the frost line and have a lower rated bearing capacity than a poured pad. Confirm with your local building department before substituting — many inspectors require a poured, frost-depth footing for any stair over a couple of steps.

Do stair footings need rebar?

The base code doesn’t universally mandate reinforcement in small residential footings, but many local amendments require it in freeze-thaw climates or seismic zones, and it’s cheap insurance against edge spalling either way. Stepped footings on sloped sites specifically require added horizontal reinforcement through the step transitions in higher seismic design categories.

Why does my inspector want a soil test before approving the footing?

Inspectors ask for soil data when the site shows signs of fill, recent grading, high organic content, or standing water — conditions where the standard 1,500 psf assumption isn’t reliable. A geotechnical report gives an actual bearing value, which can either reduce your footing size on strong soil or force a larger pad on weak soil.

Residential stairs are designed to carry a 40 psf / 1.9 kPa uniform live load or a 300 lb / 136 kg concentrated load applied to a single tread — whichever produces the greater stress on that tread — per IBC Table 1607.1 and matching ASCE 7 values. Public and commercial stairs step up to a 100 psf / 4.8 kPa uniform live load. On most residential stairs, the 300 lb concentrated load, not the uniform load, ends up governing the actual tread and stringer design. Check your own configuration against the Stair Load & Weight Capacity Calculator.

How stair load capacity is actually calculated

Two load cases are checked independently, and the tread has to pass both — they aren’t added together. The uniform case spreads 40 psf across the full tread area; the concentrated case applies 300 lb to a small area at the point on the tread that produces the worst-case bending stress, typically mid-span between supports.

Worked example: a tread 36 in / 914 mm wide and 10.5 in / 267 mm deep has an area of 2.625 sq ft / 0.24 sq m. At 40 psf uniform load, that’s only 105 lb / 47.6 kg spread across the tread — a light load. But the 300 lb concentrated load case applied to the same tread, centered between its supports, produces a much higher bending moment relative to that small footprint, which is why it typically governs the design of the tread material and the stringer supporting it, not the uniform load figure.

Deflection is checked alongside strength. IBC Table 1604.3 limits live-load deflection to L/360 and total-load deflection to L/240, where L is the span in inches between supports — a longer span between stringers or between a tread’s bearing points allows more absolute deflection, but the ratio limit stays constant.

What the code actually requires

IBC Table 1607.1 (2018/2021 editions) sets the 40 psf uniform live load for stairs in one- and two-family dwellings, and 100 psf for stairs in all other occupancies — offices, apartments, retail, and similar. ASCE 7 Table 4.3-1 lists the same values, since the IBC references ASCE 7 for its load provisions. The 300 lb concentrated load applies regardless of occupancy type and is meant to represent a single heavy footfall or a person carrying something bulky and dense at one point on a tread.

These two load cases — uniform and concentrated — are evaluated separately, not combined, and whichever produces the higher stress at a given point governs the design there. Live loads at or below 100 psf generally cannot be reduced for tributary area in the way some floor loads can, since stairs and corridors are treated as areas where full occupant loading is a realistic, non-transient condition rather than an averaged-out load. Local jurisdictions sometimes amend these base values upward for specific occupancy types, so always confirm against the adopted edition in your area rather than assuming the base IBC figures apply unmodified.

Common mistakes

Sizing stringers for width without checking span and deflection together. A 2×12 stringer that comfortably spans a narrow stair can be undersized once you widen the stair without adding a center stringer, because the same lumber is now covering more tributary tread area per stringer. Fix: add an intermediate stringer for stairs wider than about 36 in, or run the actual span and tributary width through a structural check rather than assuming the same lumber size scales indefinitely.

Notching stringers too deep at the throat. Cutting risers and treads into a solid stringer removes wood, and if the remaining uncut section — the throat — drops below roughly 3.5 in / 89 mm on a standard 2×12, the stringer’s bending strength drops sharply right at the point where load is concentrated. Fix: keep the minimum throat dimension on any notched stringer, or switch to a solid stringer with attached cleats or brackets instead of deep notching when the geometry doesn’t leave enough material.

Only checking the uniform load case. A stair can pass a 40 psf uniform-load calculation on paper while still failing the 300 lb concentrated load case at a single point, because the two cases stress the tread differently. Fix: always verify both load cases independently rather than assuming a passing uniform-load number means the stair is fully code-compliant.

Upgrading tread material without rechecking dead load. Swapping painted plywood treads for natural stone or thick tile adds significant dead load that the original stringer and framing may not have been designed to carry, even though the live load capacity hasn’t changed. Fix: total the new dead load plus the required live load and compare it against the original stringer’s rated capacity before installing a heavier finish material — a visual check is not a substitute for the calculation.

Related calculators you might need

Once you know the governing load, check whether your stringer size actually spans the distance safely with the Stair Beam Span Calculator. If you’re deciding between two or three stringers for a given width, the Stringer Spacing Calculator settles that. Confirm the full geometry and code requirements together with the IRC Stair Code Checker, and if you’re planning material or hardware, the Joist Hanger Count Calculator and Wood Stair Calculator round out the framing plan.

Frequently asked questions

How much weight can a residential stair hold?

Residential stairs are designed for a 40 psf uniform live load or a 300 lb concentrated load on a single tread, whichever governs at that point — not a single fixed weight limit for the whole stair. In practice, the 300 lb concentrated load case is usually the one that determines tread and stringer sizing.

Is 40 psf enough for a stair with a lot of foot traffic?

Yes, for residential use — the 40 psf figure already represents a conservative design load, not an average expected load. Commercial and public stairs with higher expected occupant density are designed to 100 psf instead, which is why occupancy type matters when selecting the correct load case.

Do stair treads need to pass both the uniform and concentrated load tests?

Yes. The two cases are checked independently rather than combined, and the tread and stringer have to satisfy whichever one produces the greater stress at each point. Skipping the concentrated load check is a common way stairs pass an informal calculation while still being under-designed.

How do I know if my stringers are undersized for a wide stair?

Compare your stringer spacing and span against a structural table for your lumber species and grade using the Stair Beam Span Calculator — as a rule, stairs wider than about 36 in need a third, center stringer rather than relying on two outer stringers alone.

Does adding tile or stone treads affect stair load capacity?

It affects dead load, which is added to live load when checking the stringer’s total capacity, even though the required live load rating itself doesn’t change. A stringer that was adequate for lightweight treads can be under capacity once heavier finish material is added, so recheck the total load before installing it.

A stair 36 in / 914 mm wide — the IRC code minimum — needs a minimum of 3 stringers when framed with cut 2x12s. That number comes directly from the American Wood Council’s DCA6 prescriptive limit: cut stringers can’t be spaced more than 18 in / 457 mm on center, and 36 in of width divided by 18 in leaves exactly two spaces, which always resolves to three stringers. Wider stairs, thinner tread material, or composite decking all push that number up.

The Stringer Spacing Formula

The math is simple once you know the governing limit. Take the stair’s total width, divide by the maximum on-center spacing for your stringer type and tread material, round the result up to the next whole number of spaces, then add one to convert spaces into a stringer count.

For standard cut 2×12 stringers under solid wood treads, that maximum spacing is 18 in / 457 mm. For thinner composite or PVC decking, manufacturers frequently tighten that to 12–16 in / 305–406 mm on center because the thinner board flexes more between supports — always check the deck board manufacturer’s span rating rather than assuming lumber spacing applies.

Worked example: a 42 in / 1067 mm wide interior stair using standard 2×12 cut stringers under 1x hardwood treads. 42 ÷ 18 = 2.33, rounded up to 3 spaces, plus one gives 4 stringers. Run your own width and tread material through the Stringer Spacing Calculator to get an exact count and center-to-center layout before you cut lumber.

Stringer Counts by Common Stair Widths

The table below covers the widths that come up most often in residential and light-commercial work, using the 18 in maximum spacing for standard cut lumber stringers under solid wood treads.

Stair Width	Spaces (÷18 in, rounded up)	Stringers Required	Notes
36 in / 914 mm	2	3	IRC code-minimum width
42 in / 1067 mm	3	4	Common interior main stair
48 in / 1219 mm	3	4	Wide interior or entry stair
60 in / 1524 mm	4	5	Grand or dual-handrail stair
36 in, solid stringers, ≤13 ft span	1	2	Solid uncut stringers only — DCA6 exception

That last row is the exception, not the rule: the two-stringer allowance only applies to solid, uncut stringers on a 36 in wide stair spanning within DCA6’s solid-stringer limit — 16 ft 6 in for southern pine, 13 ft 3 in for most other species. Any notched, cut-profile stringer at any width still follows the 18 in spacing rule, and any width above 36 in needs a third stringer regardless of stringer type.

Common Mistakes

Assuming two stringers is always enough. The two-stringer detail gets copied from one project to the next without checking whether the new stair is still 36 in wide and still using solid, uncut stringers. Widen the stair to 42 in or notch the stringers for treads, and two is no longer code-compliant. The fix is to recheck the width and stringer type every time, not to reuse a prior layout.

Ignoring tread material stiffness. Lumber spacing rules get applied to composite or PVC tread stock without checking the manufacturer’s own span rating, which is frequently tighter than 18 in. The result is a tread that deflects visibly underfoot even though the framing technically meets the lumber-based spacing rule. Always cross-check the decking manufacturer’s span table against the framing spacing, and use whichever is more restrictive.

Spacing stringers unevenly. Stringers get placed to match convenient joist locations on the structure below rather than evenly across the stair width, leaving one bay wider than the calculated maximum. An 18 in average spacing with one 24 in gap does not satisfy an 18 in maximum — the largest single gap is what code and the tread material actually see. Lay out stringers to an even spacing first, then adjust the supporting structure to match.

Landing intermediate stringers on unsupported blocking. An added center or intermediate stringer needs to bear on something continuous — a beam, ledger, or footing — not on a piece of blocking nailed between joists with no load path to the ground. A stringer that’s correctly spaced but poorly supported at either end still fails the same way an absent stringer would.

Related Calculators You Might Need

Once the stringer count is set, the next question is usually what those stringers land on. The Stair Beam Span Calculator sizes the beam or ledger carrying the full stringer load, and the Stair Footing Size Calculator sizes what that beam sits on at grade. If you’re estimating lumber for the stringers themselves, the 2×12 Stringer Board Calculator converts your stringer count and rise into a board count and cut list. For anything beyond standard residential foot traffic, confirm the finished frame against the Stair Load/Weight Capacity Calculator, and run the final layout through the IRC Stair Code Checker before you finalize the cut list.

Frequently Asked Questions

How many stringers do I need for a 4 foot wide stair? A 48 in / 1219 mm wide stair using standard cut 2×12 stringers at the 18 in maximum spacing needs 4 stringers: 48 ÷ 18 = 2.67, which rounds up to 3 spaces, plus one. If the stair uses composite or PVC treads, check the decking manufacturer’s span rating first — it may require an additional stringer even at this width.

What is the maximum spacing between stair stringers? For cut, notched 2×12 stringers under standard wood treads, the DCA6 prescriptive limit is 18 in / 457 mm on center, adopted by most jurisdictions since the IRC itself doesn’t publish a separate stringer span table. Thinner or composite tread material often requires tighter spacing — check the tread manufacturer’s rating against this maximum and use whichever number is smaller.

Can you build a wide staircase with only two stringers? Only if the stair is 36 in / 914 mm wide or narrower and the stringers are solid and uncut, spanning within the DCA6 limit of 16 ft 6 in for southern pine or 13 ft 3 in for most other species. Any stair wider than 36 in, or using notched cut stringers at any width, needs a minimum of three stringers regardless of span length.

Do composite deck stairs need more stringers than wood stairs? Often yes. Composite and PVC decking typically deflects more than solid lumber at the same span, so manufacturers frequently rate their boards for 12–16 in on-center support rather than the 18 in lumber maximum. Check the specific product’s span table — using lumber spacing rules with composite treads is one of the most common stair-building errors on deck stairs. Verify with the Stringer Spacing Calculator using your actual tread material.

Do stringers need to be evenly spaced? Yes — the maximum on-center spacing rule applies to the largest single gap between any two adjacent stringers, not an average across the stair width. Spacing stringers unevenly to match existing framing below can leave one bay exceeding the maximum even if the overall count looks correct on paper.