How to use the deck calculator
- Enter the length along the house and the depth out from it. Boards usually run parallel to the house and joists run out to a beam.
- Pick board length to match your deck if you can. A 16 ft deck with 16 ft boards has no butt joints.
- Choose your joist lumber, size and spacing to see whether the span fits the DCA 6 table. The check treats the full depth as the joist span, with a beam at the outer edge and no overhang.
The formulas
Rows = depth (in) ÷ (board width + gap), rounded up. Boards = rows × (deck length ÷ board length, at least 1) × (1 + extra), rounded up.
Joists = deck length (in) ÷ spacing, rounded up, plus 1. Screws = rows × joists × 2, from DCA 6's two #8 screws per board at each joist.
Worked example: a 16 × 12 ft deck
144 in ÷ 5.6875 in = 25.3, so 26 rows of 16 ft boards. With 10% extra: 29 boards. Joists at 16 in on center: 192 ÷ 16 = 12, plus 1 = 13 joists. Screws: 26 × 13 × 2 = 676. Southern pine 2 × 8 joists span only 11 ft 10 in at 16 in on center, so a 12 ft span needs 2 × 10s (14 ft 0 in) or 12 in spacing (13 ft 1 in).
Maximum deck joist spans (DCA 6, Table 2)
No. 2 grade lumber, wet service, 40 psf live load plus 10 psf dead load.
| Species | Size | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|---|
| Southern pine | 2 × 6 | 9' 11" | 9' 0" | 7' 7" |
| Southern pine | 2 × 8 | 13' 1" | 11' 10" | 9' 8" |
| Southern pine | 2 × 10 | 16' 2" | 14' 0" | 11' 5" |
| Southern pine | 2 × 12 | 18' 0" | 16' 6" | 13' 6" |
| Douglas fir-larch, hem-fir, SPF | 2 × 6 | 9' 6" | 8' 4" | 6' 10" |
| Douglas fir-larch, hem-fir, SPF | 2 × 8 | 12' 6" | 11' 1" | 9' 1" |
| Douglas fir-larch, hem-fir, SPF | 2 × 10 | 15' 8" | 13' 7" | 11' 1" |
| Douglas fir-larch, hem-fir, SPF | 2 × 12 | 18' 0" | 15' 9" | 12' 10" |
| Redwood, western cedar, ponderosa or red pine | 2 × 6 | 8' 10" | 8' 0" | 6' 10" |
| Redwood, western cedar, ponderosa or red pine | 2 × 8 | 11' 8" | 10' 7" | 8' 8" |
| Redwood, western cedar, ponderosa or red pine | 2 × 10 | 14' 11" | 13' 0" | 10' 7" |
| Redwood, western cedar, ponderosa or red pine | 2 × 12 | 17' 5" | 15' 1" | 12' 4" |
These numbers are for planning. Beams, posts, footings, the ledger connection and guards all need their own design. Follow DCA 6 or your local code, and get a permit where one is required.