How Much Lumber for a 16×20 Deck Frame?
A 16×20 ft deck frame needs about 30 joists at 16 in o.c., perimeter beams, 6 posts, and roughly 700 board feet of decking — before stairs and railings. This guide builds the lumber list member by member, then converts everything to board feet so yard quotes compare apples to apples.
Step-by-Step Takeoff
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1
Confirm span direction and joist size from a span table first (2x8 at 16 in o.c. spans ~12 ft; upsize for longer).
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2
Joists: 16 ft span direction — 20 ft ÷ 1.33 ft + 1 ≈ 16 joists of 16 ft 2x8.
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3
Beams: 2 perimeter beams of 20 ft doubled 2x10; posts: 6 × 4x4x8.
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4
Decking: 320 sq ft ÷ (5.5/12 ft width) ≈ 700 lin ft of 5/4x6 ≈ 436 bf.
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5
Hardware: joist hangers per joist end, post bases, and ledger flashing — the forgotten 10% of deck budgets.
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6
Convert every member to board feet and cost in the Lumber Calculator.
Guide FAQ
16 or 12 inch joist spacing?
16 in o.c. is standard; 12 in o.c. for diagonal decking or heavy loads.
How do board feet price a deck?
Convert each member (T×W×L÷12), sum, and multiply by the yard price — then compare against a materials package quote.
Pressure-treated or cedar?
Treated for framing always; cedar or composite for visible decking if the budget allows.
How many posts for a 16x20 deck?
Six 4x4 posts on footings for a freestanding deck; ledger-attached decks need fewer.
Do I need a permit for a deck?
Almost always for attached or elevated decks — bring the lumber list and a sketch to the permit desk.
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