contractor · 2026-05-01
Calculate joist count, board feet of decking, and total lumber cost for a residential deck build.
| Deck length (ft) | 16 |
| Deck width (ft) | 12 |
| Joist spacing (inches OC) | 16 |
| Decking board width (inches) | 5.5 |
| Decking $ per linear foot | $4 |
| Joist $ per linear foot | $3 |
| Joist hangers + fasteners | $350 |
| Waste factor % | 12% |
| Joists needed | 13 |
| Decking boards needed (8 ft) | 61 |
| Decking linear feet (with waste) | 484 |
| Decking cost | $2,056 |
| Joist cost | $507 |
A deck takeoff is grade-school geometry that contractors charge $200 to do. Two pieces of lumber, one waste factor.
10-15% on straight runs. 20% on diagonals or herringbone patterns. The waste covers crown-removal cuts, end trim, and miscuts. If your contractor adds 25%+ on a basic rectangular deck, push back.
This is a decking + joist takeoff, which is the lumber that's hardest to estimate by eye. Posts/beams/stairs you can shop separately because the unit cost is well-known and the quantities are small.
Wood decking on 16 OC works for spans up to ~6 ft per joist. Composite manufacturers usually require 12 OC for proper support and warranty (Trex, TimberTech). Always check the decking manufacturer spec — 12 OC adds 25% to joist count but voiding the warranty costs more.
Beams support joists at the outer edge. For an 8-ft post spacing carrying 12-ft joist span, you typically need a triple 2×10 PT beam. Posts: 6×6 PT minimum, set on poured concrete footings 12 in below frost depth. Use a span calculator or your local code's prescriptive deck guide for sizing.
Pure lumber cost: composite is 2-3× pressure-treated. Lifetime cost: composite is cheaper at year 15 because PT decking needs sealing every 2-3 years and rebuilding around year 15-20. Composite holds up 25-30 years with no maintenance. If you'll own the house 10+ years, composite usually wins.
Lumber comes in 8/10/12/16 ft increments. A 14 ft deck means you cut from 16-footers and waste 2 ft per board. Crown-checking discards 3-5% of boards. Mistakes happen. 12% covers all of this — buying right at math means a $150 trip to the lumberyard mid-build.