Getting the two numbers this calculator needs
Fastener spacing on a ledger connection is driven almost entirely by two measurements. Get these right and the rest follows.
Measure the ledger length
Measure the full length of ledger board that will be bolted flush against the house rim joist or band board — end to end, not including any gap left for corner trim.
Find the supported span
This is the horizontal distance the joists (or stair stringers) run away from this ledger before reaching their far support — a beam, a wall, or the bottom of the stair. Longer spans push more load onto the ledger and tighten the required fastener spacing.
Pick your fastener and check your framing
Confirm whether you're using 1/2" lag screws or 1/2" through-bolts, and identify the species of the house's existing rim joist — this is usually printed as a grade stamp on the lumber itself.
Read the compliance strip, not just the spacing number
Amber chips mean a figure is drawn from a generic prescriptive table and needs a quick check against your local adopted code edition before you buy hardware or drill a single hole.
The four calculations behind every result
A ledger connection problem is really two problems: how much load is actually arriving at the board, and how far apart the fasteners can be to safely carry it.
Tributary Load
Half of the supported span's load transfers back to this ledger; the other half goes to the far beam, wall, or stringer base.
Total Ledger Load
The distributed load per foot, multiplied out across the full length of the ledger board.
Fastener Spacing
Spacing isn't a formula you solve — it's read from a prescriptive table keyed to supported span, fastener type, and band joist species. See the Spacing Table section below.
Fastener Count
Spaces along the ledger, plus one for the starting fastener, split alternately between two staggered rows.
Walking through a 12 ft ledger, 10 ft span
Same four calculations, filled in for a 12-foot ledger supporting joists that run 10 feet to their far beam — the default this calculator loads with.
Tributary load
10 ft span ÷ 2 = 5 ft tributary width. At 40 psf: 5 × 40 = 200 lb per linear foot of ledger.
Total ledger load
200 lb/ft × 12 ft ledger = 2,400 lb total load this connection has to transfer into the house framing.
Spacing from the table
At a 10 ft span, 1/2" lag screws into a Douglas Fir-Larch band joist: 16" on center [VERIFY against your adopted code edition]. Through-bolts at the same span would allow noticeably wider spacing — see the reference table below.
Fastener count and load check
144" ÷ 16" = 9 spaces, plus one: 10 fasteners total, five per row, staggered. That's 2,400 ÷ 10 = 240 lb per fastener — this is the number to compare against your fastener manufacturer's published allowable shear value, since generic lag screw capacity varies by brand and thread design.
Notice the spacing didn't change proportionally with load — it comes from a stepped table, not a straight-line formula, so doubling the span doesn't simply halve the spacing. Always read the actual bucket your span falls into, not an interpolated value between two rows.
Prescriptive fastener spacing, by supported span
This is the shape of the table this calculator reads from — modeled on the prescriptive method used in IRC ledger attachment provisions and the AWC DCA6 guide, for a 40 psf live + 10 psf dead load ledger with a Douglas Fir-Larch or Southern Pine band joist.
| Supported Span | 1/2" Lag Screw | 1/2" Through-Bolt |
|---|---|---|
| Up to 6 ft | 30" o.c. | 36" o.c. |
| 6'1" – 8' | 23" o.c. | 36" o.c. |
| 8'1" – 10' | 18" o.c. | 34" o.c. |
| 10'1" – 12' | 15" o.c. | 29" o.c. |
| 12'1" – 14' | 13" o.c. | 24" o.c. |
| 14'1" – 16' | 11" o.c. | 21" o.c. |
| 16'1" – 18' | 10" o.c. | 19" o.c. |
[VERIFY: these spacing values are modeled on the general shape of published prescriptive ledger fastening tables (IRC ledger attachment provisions / AWC DCA6 Table 1) but are not guaranteed to match your specific adopted code edition, jurisdiction amendment, or exact load combination. Confirm every value here against your local building department's currently adopted table before fastening. Hem-Fir / Spruce-Pine-Fir band joists typically require tighter spacing than shown — this calculator applies an approximate reduction, also unconfirmed, when that species is selected.]
Rows, flashing & hardware selection
A ledger can hit every spacing number on the table and still fail if these four things aren't handled correctly.
Why two staggered rows
Fasteners are staggered into a top and bottom row rather than run in a single line, which spreads the load across more wood fiber and resists the board splitting along the grain. This calculator assumes a 2" edge distance from the top and bottom of the board [VERIFY: confirm minimum edge distance for your fastener diameter and species].
Flashing is not optional
A properly fastened ledger with no flashing still rots the band joist behind it. Z-flashing (or a self-adhering membrane plus metal drip cap) that sheds water over the ledger and away from the house wall is standard practice on every exterior ledger connection.
Lag screw vs. through-bolt
Through-bolts are the stronger connection and allow wider spacing, but the nut and washer need to be accessible from inside — usually through a basement or crawlspace ceiling. Where that access doesn't exist, lag screws are the practical choice at tighter spacing.
Proprietary structural screws
Some structural wood screw systems are engineered and code-listed with their own published spacing tables, which can allow wider spacing than the generic 1/2" lag/bolt table above. If you're using one, follow that manufacturer's table instead of the generic one here [VERIFY: confirm the product has current code-evaluation-report approval for this application].
These are planning-stage estimates, not a stamped structural design. A rim board that isn't solid-sawn lumber, a band joist behind brick veneer or stucco, or any load beyond what's shown in the table above should go to an engineer, not this calculator.
Common mistakes when attaching a ledger board
Ledger failures are one of the most common causes of deck collapse — nearly all of them trace back to one of these five errors, not to bad carpentry elsewhere on the structure.
Using deck screws or nails instead of lags/bolts
Standard deck screws and common nails aren't rated for this connection's shear and withdrawal loads — code requires 1/2" lag screws or through-bolts specifically, not general-purpose fasteners.
Fastening into brick veneer, stucco, or a rim board that isn't solid lumber
The fastener has to reach solid, structural framing behind the finish material. Veneer, stucco, and some engineered rim boards can't take a ledger connection without an engineer's detail.
Skipping flashing
A structurally perfect connection with no flashing still lets water track behind the ledger and rot the band joist from the inside — often invisible until the ledger is already failing.
Running fasteners in a single row instead of staggered
A single row concentrates the load along one line of grain and is far more prone to splitting the ledger under load than two properly staggered rows.
Ignoring cantilevered floor joists behind the ledger
If the house's floor joists are cantilevered out past the foundation at the ledger location, that framing generally cannot also carry a ledger load without additional engineering — a detail many DIY guides skip entirely.
Assuming lag and bolt spacing are interchangeable
Switching from through-bolts back to lag screws at the same spacing after a design was engineered for bolts silently under-builds the connection — always re-check the table for the fastener you're actually installing.
When to use a ledger attachment calculator
Ledger fastening comes up at the very start of nearly every deck or exterior stair project — before a single joist or stringer goes up.
New Deck Construction
Sizing the ledger connection before framing the rest of the deck.
Exterior Stair Ledger
Attaching the ledger that carries stair stringer hangers back to the house.
Deck Rebuilds & Re-Ledgering
Replacing an old, undersized, or improperly fastened ledger during a deck rebuild — one of the most common deck inspection failures.
Permit & Inspection Prep
Confirming spacing and fastener count before a plan check or framing inspection.
Porch & Addition Ledgers
Attaching a ledger for a covered porch, mudroom, or small addition floor system.
Engineer Consult Trigger
Spotting spans, loads, or framing conditions that fall outside the prescriptive table and need a stamped design instead.