How the numbers are worked out
No guesswork — the list is built from the same formulas carpenters use on site:
Boards = rows × ceil(board run ÷ stock length) × (1 + waste)
Joists = ceil(length (in) ÷ spacing) + 1, plus 2 rim joists
Beam lines = 1 per ~7 ft of projection from the ledger
Posts = (ceil(length ÷ 6 ft) + 1) per beam line · Footings = posts
Screws = 2 per board × every joist it crosses
Balusters ≈ 2.7 per linear ft of railing (4″ max spacing)
Framing lumber lengths are rounded up to the next standard stock length (8 / 10 / 12 / 14 / 16 ft), because a 13-ft span still needs a 14-ft board — and the offcut rarely goes to waste on a real build.
Three ways to cut the bill
- Size the deck to your boards. A 16-ft deck with 16-ft boards has zero decking offcuts. A 17-ft deck nearly doubles board joints.
- Don't over-space joists. 24″ spacing saves a few joists but makes composite feel spongy — most composite warranties require 16″ max.
- Buy the waste, skip the second trip. 10% extra lumber is far cheaper than a mid-weekend delivery run.
Deck building questions
How many boards do I need for a 12×16 deck?
With standard 5½″-wide boards and a 3/16″ gap, a 12×16 deck needs 26 rows. Using 16-ft boards that run the full length, that's 26 boards before waste — 29 with the standard 10% allowance. This calculator does that exact maths for any size.
Do I really need footings for every post?
Yes. Each post sits on its own footing sized to carry its share of the load down to undisturbed soil below the frost line. Deck collapses are overwhelmingly a footing and ledger problem, not a decking problem.
Composite vs. pressure-treated — which is cheaper?
Pressure-treated is roughly a third of the upfront board cost, but needs staining/sealing every 1–2 years. Composite costs more now and almost nothing to maintain. Over 15 years the two are surprisingly close — switch the material above and compare the totals.
What joist spacing should I use?
16″ on centre is the standard for wood and most composite. Go 12″ for diagonal boards, heavy loads (hot tubs), or a noticeably stiffer feel. 24″ is only suitable for 2×6 wood decking laid flat — never for composite.
Does this include the cost of labour or permits?
No — this is materials only. Professional installation typically adds $15–$35 per sq ft depending on region and complexity, and permits usually run $100–$500. The per-sq-ft material figure shown above is the number to compare against contractor material quotes.