Calculate deck ledger lag screw or through-bolt spacing and fastener count from joist span and ledger length, using the AWC DCA6 ledger table.
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
Choose fastener count or a spacing comparison. Built-in lag-screw and bolt values apply only to the solid-sawn band assembly and loading stated below. The structural-screw option requires its approved maximum spacing and exact model/report reference, plus the approved end distance for a count. A spacing comparison is not a complete code-compliance check.
Use the clear joist span between support faces, excluding cantilever. The selected table band rounds a span upward without interpolation.
Code Maximum Fastener Spacing by Joist Span
The lag and bolt columns reproduce the 1½-inch solid-sawn band rows of AWC DCA6-2015 Table 5: 40 psf live plus 10 psf dead load, qualifying minimum 2×8 No.2 treated ledger, permitted lumber species, and at most 15/32-inch sheathing. Engineered rims and other assemblies need their own approved schedule. Figure 19 placement, adequate supporting construction, flashing, corrosion resistance and lateral connections also apply; verify the locally adopted requirements.
| Joist span | 1/2 in. lag screw | 1/2 in. through-bolt | Structural screw |
|---|---|---|---|
| 6 ft and less | 30 in. | 36 in. | Approved schedule required |
| 6 ft 1 in. to 8 ft | 23 in. | 36 in. | Approved schedule required |
| 8 ft 1 in. to 10 ft | 18 in. | 34 in. | Approved schedule required |
| 10 ft 1 in. to 12 ft | 15 in. | 29 in. | Approved schedule required |
| 12 ft 1 in. to 14 ft | 13 in. | 24 in. | Approved schedule required |
| 14 ft 1 in. to 16 ft | 11 in. | 21 in. | Approved schedule required |
| 16 ft 1 in. to 18 ft | 10 in. | 19 in. | Approved schedule required |
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?
Do not average fastener schedules or assume mixed fasteners share load. Obtain an approved connection detail for mixed systems.
Where do the fasteners sit on the ledger?
Follow DCA6 Figure 19 and the approved connection detail for staggered rows and all edge/end clearances. A ½-inch lag uses a ½-inch clearance hole through the ledger and a 5/16-inch pilot in the band, with a washer and the tip extending past the band. The source advice to use a 5/16-inch hole through both members was incorrect. Structural screws follow their exact manufacturer instructions.
