IRC Table R507.9.1.3(1), all 84 cells, read from the code
Decks collapse at the ledger. Not the decking, not the joists, not the railing: the connection to the house. IRC Table R507.9.1.3(1) is the prescriptive answer and this reads all eighty-four cells of it. It also tells you when you have left the table, because past an 18 ft joist span the code says plainly that you may not extrapolate.
Ledger connection
ft
ft
Spacing from IRC Table R507.9.1.3(1). Interpolation between spans is permitted; extrapolation past the table is not.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
Ledger in section: the tip must pass the band joist
Worked example
A 16 ft ledger carrying joists that span 12 ft, 40 psf live load, 1/2 in lag screws through 1/2 in sheathing.
The cell. 40 psf live, 12 ft joist span, lag screws: 15 in on centre.
Count. 16 ft at 15 in is 13 lag screws, in two staggered rows 1 5/8 in apart.
Switch to bolts. The same connection with 1/2 in bolts goes to 29 in, less than half the fasteners. Bolts are far more efficient than lags here.
Push the span to 20 ft. The verdict fails: the table stops at 18 and footnote a forbids extrapolation. That connection needs an engineer.
Say the wall is brick veneer. The verdict fails again, and no spacing fixes it.
The two failure cases are the point of the page. Most ledger calculators will happily give you a number for a 24 ft joist span bolted to a brick veneer, and both of those are the code saying no rather than the code saying tighter.
The formula
There is no formula. It is a lookup, and the only arithmetic is the count and the interpolation the code allows:
n = ⌊ 12L / s ⌋ + 1 s(x) = s₁ + (s₂ − s₁)(x − x₁)/(x₂ − x₁)
s
spacing from Table R507.9.1.3(1), in, by load case, joist span and fastener
L
ledger length, ft
x, x₁, x₂
your joist span and the two table spans either side of it
rounding
interpolated spacing is rounded DOWN, because a tighter pattern is the safe direction
Rounding matters here in a way it does not on a materials takeoff. Every interpolated value on this page rounds down to a tighter spacing, because the consequence of being one fastener short on a ledger is not a second trip to the yard.
Why this page exists when nobody searches for it
This page was not built from keyword research. It has no meaningful search volume in the data behind the rest of this site, and it was built anyway, because three other calculators here already say some version of the same sentence: ledger failure is what collapses decks, and no number on this page speaks to it.
That was true and it was an unsatisfying place to leave a reader. The fastener page counts deck screws, the joist page sizes joists, the load page traces pounds down to the footings, and all three point at the ledger and stop. Now they point here.
The other reason is that the ledger is where the prescriptive path ends soonest. The joist table runs to long spans, the beam table too, but the ledger table stops at an 18 ft joist span and at a 70 psf ground snow load, and footnote a forbids reading past either edge. A calculator that quietly extrapolates is giving you a number the code specifically refused to give.
And the two disqualifying conditions in R507.9.1.1 are not spacing problems at all. A ledger carrying a beam and a ledger bolted to masonry veneer are both simply not permitted, whatever pattern you use, and both are common enough to be worth a verdict of their own.
The table stops at an 18 ft joist span, and you may not push past it
Deck collapses are ledger failures. Not decking, not joists, not railings: the connection between the deck and the house. IRC Table R507.9.1.3(1) is the prescriptive answer, and it carries a footnote that decides whether you may use it at all:
Interpolation permitted. Extrapolation is not permitted.
Table R507.9.1.3(1), footnote a
Interpolation is permitted, so a 9 ft joist span sits legitimately between the 8 ft and 10 ft rows. Extrapolation is not. A 20 ft joist span is not a slightly tighter version of the 18 ft row, it is outside the table, and the prescriptive path ends there. The same is true above a 70 psf ground snow load. Past either boundary the connection needs an engineer, not arithmetic.
Two more sentences in R507.9.1.1 fail more inspections than the spacing does:
Deck ledgers shall be a minimum 2-inch by 8-inch nominal, pressure-preservative-treated Southern pine, incised pressure-preservative-treated hem-fir, or approved, naturally durable, No. 2 grade or better lumber. Deck ledgers shall not support concentrated loads from beams or girders. Deck ledgers shall not be supported on stone or masonry veneer.
R507.9.1.1, ledger details
A ledger may not carry a beam, and may not be fastened to stone or masonry veneer. Veneer is not structure. It is hung on the building, and bolting a deck to it transfers the deck's load into a facing that was never designed to take it. If the wall is brick or stone veneer, the bolts have to reach the structure behind it, or the deck has to be freestanding.
Band joists supporting a ledger shall be a minimum 2-inch-nominal, solid-sawn, spruce-pine-fir or better lumber or a minimum 1-inch nominal engineered wood rim boards in accordance with Section R502.1.7.
R507.9.1.2, band joist details
And the rule that governs when nothing can be confirmed:
Where positive connection to the primary building structure cannot be verified during inspection, decks shall be self-supporting.
R507.9.1
Where the fasteners go
Spacing is only half of it. The table below is the minimum end, edge and row geometry, and the fasteners must be staggered top to bottom along the run:
Table R507.9.1.3(2), minimum end and edge distances and spacing between rows
Top edge
Bottom edge
Ends
Row spacing
Ledger
2 in
3/4 in
2 in
1 5/8 in
Band joist
3/4 in
2 in
2 in
1 5/8 in
Note that the ledger and the band joist have their edge distances the opposite way round: 2 in from the top of the ledger but 3/4 in from the top of the band joist. That is not a misprint, it is what staggering the rows produces.
The tip of the lag screw shall fully extend beyond the inside face of the band joist.
Table R507.9.1.3(1), footnote d
A lag screw that stops inside the band joist is not the fastener the table tested. This is the single most common way a correctly spaced ledger is still wrong.
Flashing, and the failure nobody sees coming
Ledgers shall be flashed in accordance with Section R703.4 to prevent water from contacting the house band joist.
Table R507.9.1.3(1), footnote b
Every ledger figure in the code assumes sound wood. A ledger that is correctly bolted into a band joist quietly rotting behind unflashed siding will hold its rated load right up until it does not. The flashing is not a detail, it is what keeps the numbers true.
Lateral load is a separate connection
Where the lateral load connection is provided in accordance with Figure R507.9.2(1), hold-down tension devices shall be installed in not less than two locations per deck, within 24 inches of each end of the deck. Each device shall have an allowable stress design capacity of not less than 1,500 pounds. Where the lateral load connections are provided in accordance with Figure R507.9.2(2), the hold-down tension devices shall be installed in not less than four locations per deck, and each device shall have an allowable stress design capacity of not less than 750 pounds.
R507.9.2, lateral connection
The vertical fasteners in the table above hold the deck up. They do not hold it against being pushed away from the house, and that is a different device: either two hold-downs of 1,500 lb within 24 in of each end, or four of 750 lb. A deck with a perfect bolt pattern and no lateral connection is not compliant.
Code text read from the Massachusetts Residential Code 10th edition, Chapter 5 Floors, reproducing the 2021 IRC.
Frequently asked questions
How far apart should deck ledger bolts be?
From IRC Table R507.9.1.3(1), by load case, joist span and fastener. At 40 psf live with a 12 ft joist span it is 15 in for 1/2 in lag screws and 29 in for 1/2 in bolts through 1/2 in sheathing. Bolts allow roughly twice the spacing of lags.
Can I use lag screws instead of bolts for a deck ledger?
Yes, and the table covers both, but lags need far tighter spacing for the same load: 15 in against 29 in for a 12 ft joist span at 40 psf. The tip of the lag must also fully extend beyond the inside face of the band joist, per footnote d.
What if my joist span is more than 18 feet?
The table stops at 18 ft and footnote a states that interpolation is permitted and extrapolation is not. There is no prescriptive answer past that point, so the connection needs an engineer. Tightening the 18 ft spacing further is exactly what the footnote forbids.
Can a deck ledger be attached to brick or stone veneer?
No. R507.9.1.1 states that deck ledgers shall not be supported on stone or masonry veneer. Veneer is hung on the building rather than part of its structure. The fasteners must reach the structure behind it, or the deck must be freestanding.
Can a beam land on the deck ledger?
No. R507.9.1.1 states that deck ledgers shall not support concentrated loads from beams or girders. The table distributes a uniform load along the ledger; a beam bearing on it is a point load the table never covered.
Do I need hold-downs as well as ledger bolts?
Yes. R507.9.2 requires either two hold-down tension devices of at least 1,500 lb within 24 in of each end of the deck, or four devices of at least 750 lb. The ledger fasteners carry vertical load; the hold-downs resist the deck being pushed away from the house.
What size does the ledger board have to be?
A minimum 2 by 8 nominal, in pressure-preservative-treated southern pine, incised treated hem-fir, or approved naturally durable No. 2 grade or better lumber, per R507.9.1.1.
Does the ledger need flashing?
Yes, per R703.4, and footnote b says so explicitly. Every figure in the table assumes sound wood. An unflashed ledger rots the band joist behind the siding, and the connection holds its rated load right up until it does not.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-07. Codes are amended locally; confirm against the edition your jurisdiction enforces.
This is the connection that kills people. Spacing is necessary and not sufficient: flashing, fastener tips, edge distances, band joist condition and lateral hold-downs all have to be right too. Past an 18 ft joist span or a 70 psf ground snow load there is no prescriptive answer and the connection needs an engineer. If you cannot verify what is behind the siding, the code says build freestanding.