Calculate deck ledger fastener spacing and the number of lag screws or bolts your ledger needs, using IRC and DCA6 on-center requirements by joist span.
Deck Ledger Fastener Spacing Formula
The calculator uses two steps. First it reads the maximum on-center spacing straight from the code table, based on your joist span and fastener type. Then it turns that spacing into a fastener count for your ledger length:
N = ceil((L - 2*e) / s) + 1
Variables:
- N is the number of fasteners needed along the ledger
- L is the ledger length in inches
- e is the end distance, the gap from each ledger end to the first fastener (code allows 2 in. to 5 in.)
- s is the maximum on-center spacing from the code table, in inches
- ceil means round up to the next whole number, because you cannot install a partial fastener
Pick what you want to find at the top of the calculator. In “Fasteners needed and their spacing” mode you enter the joist span, fastener type, and ledger length, and the tool returns the code maximum spacing, the fastener count, and an even spacing that fits the ledger. In “Check if my planned spacing passes” mode you enter a spacing you already have in mind and the tool tells you whether it meets the code maximum for your span. The fastener type matters because a 1/2 in. lag screw, a 1/2 in. through-bolt, and a structural timber screw each carry a different load, so each gets its own spacing.
The joist span is the value that drives everything. Measure it from the center of the ledger to the center of the beam the joists rest on, and leave out any cantilever past that beam. A longer span puts more load on each fastener, so the required spacing gets tighter as the span grows.
Code Maximum Fastener Spacing by Joist Span
This is the on-center spacing schedule from IRC Table R507.9.1.3(1) and AWC DCA6. Find your joist span band, then read across to your fastener type. Values are the maximum spacing, so closer is always allowed.
| Joist span | 1/2 in. lag screw | 1/2 in. through-bolt | Timber screw (SDWS) |
|---|---|---|---|
| 6 ft and less | 30 in. | 36 in. | 12 in. |
| 6 ft 1 in. to 8 ft | 23 in. | 36 in. | 9 in. |
| 8 ft 1 in. to 10 ft | 18 in. | 34 in. | 7 in. |
| 10 ft 1 in. to 12 ft | 15 in. | 29 in. | 6 in. |
| 12 ft 1 in. to 14 ft | 13 in. | 24 in. | 5 in. |
| 14 ft 1 in. to 16 ft | 11 in. | 21 in. | Not tabulated |
| 16 ft 1 in. to 18 ft | 10 in. | 19 in. | Not tabulated |
The next table skips the spacing math and gives the finished fastener count for common ledger lengths using 1/2 in. lag screws with a 2 in. end distance. Read down your joist span column to size an order before you start.
| Ledger length | 6 ft span (30 in.) | 10 ft span (18 in.) | 12 ft span (15 in.) | 16 ft span (11 in.) |
|---|---|---|---|---|
| 8 ft | 5 | 7 | 8 | 10 |
| 10 ft | 5 | 8 | 9 | 12 |
| 12 ft | 6 | 9 | 11 | 14 |
| 14 ft | 7 | 11 | 12 | 16 |
| 16 ft | 8 | 12 | 14 | 19 |
| 20 ft | 9 | 15 | 17 | 23 |
Example Problems
Example 1: Count the lag screws for a ledger.
Your joists span 12 ft, your ledger is 16 ft long, and you are using 1/2 in. lag screws with a 2 in. end distance. The table gives a 15 in. maximum spacing at a 12 ft span. Convert the ledger to inches, 16 * 12 = 192 in., and subtract the two end gaps, 192 – 4 = 188 in. Then N = ceil(188 / 15) + 1 = ceil(12.53) + 1 = 13 + 1 = 14 fasteners, which lands at about 14.5 in. on center.
Example 2: Check a planned spacing.
You plan to set 1/2 in. through-bolts at 24 in. on center for joists that span 15 ft. The 14 ft 1 in. to 16 ft band allows 21 in. for through-bolts. Because 24 in. is greater than 21 in., the spacing fails and you tighten it to 21 in. or less.
Frequently Asked Questions
Do I use the joist span or the ledger length to set the spacing?
The joist span sets the spacing, and the ledger length sets the count. Spacing comes from the code table based on how far the joists reach from the ledger to the beam, because that distance controls how much load each fastener carries. Once you know the spacing, you divide the ledger length by it and round up to get the number of fasteners.
Can I mix lag screws and through-bolts on one ledger?
Size the whole ledger to whichever fastener you are actually using, and do not average the two. If you switch fastener types partway along the ledger, apply the tighter of the two required spacings to the entire run so no section is under-fastened. Through-bolts carry more load than lag screws of the same diameter, which is why the bolt column allows wider spacing.
Where do the fasteners sit on the ledger?
Stagger them in two rows so no two fasteners share the same grain line, which keeps the board from splitting. Start about 2 in. in from each end, keep each fastener roughly 2 in. from the top and bottom edges, and for 1/2 in. lag screws pre-drill a 5/16 in. pilot hole through the ledger and into the band joist.
