How Much Lumber Do I Need for a Deck?

Enter your deck size to get a full framing take-off: joist size from the span table, joists, rim and ledger, beam, posts, footing concrete, decking boards and hardware.

What the Lumber List Includes

Most deck calculators stop at decking boards. The boards are the easy part; the framing underneath is where the money, the structural risk and most of the second trips to the lumber yard live. This calculator builds the whole take-off in the order you would frame it.

• Joist size is chosen from the span table below: the smallest size whose maximum span clears your projection at the spacing you picked. If nothing clears it, the calculator adds an intermediate beam line and tells you whether tighter spacing would avoid it.
• Joist count is the length along the house divided by the spacing, plus one for the end joist. The two end joists double as the side rim.
• Rim and ledger run the full length at the same depth as the joists. Ledgers are never smaller than 2x8.
• Beam and posts assume a 2-ply 2x10 Southern pine beam. The load on the beam (half the joist span either side) sets how far apart the posts can be, and that sets the post and footing count.
• Concrete is figured per footing at 12 inches in diameter and the depth you enter. Use your local frost depth, not a guess.
• Decking runs parallel to the house, so the rows stack across the projection, with a 3/16 in gap between boards and 5% for waste.
• Hardware: joist hangers on the ledger side, a hurricane tie wherever a joist crosses a beam, and two screws per board at every joist.

Joist Span Table (No. 2 Southern Pine)

Maximum clear span, measured from the face of the ledger to the face of the beam, for 40 psf live load plus 10 psf dead load with no cantilever. These are the IRC 2021 Table R507.6 values the calculator uses.

Douglas fir-larch, hem-fir and spruce-pine-fir all span less than Southern pine, SPF by roughly 15 to 18 percent. Check the grade stamp on your lumber: most pressure-treated framing sold east of the Rockies is Southern pine, and the difference can push a deck from 2x10 to 2x12.

A span that lands right at the table maximum is legal but will feel bouncy. Many builders design a foot short of the maximum. For every common size worked out in advance, see the deck lumber list by size.

Deck Lumber: Pressure-Treated vs. Composite

Pressure-treated lumber is the standard choice for deck framing (posts, beams, joists) and is often used for decking boards as well. It's the most affordable option — a basic 12×16 deck can be framed and decked in PT lumber for $2,000–$4,000 in materials. Requires annual sealing or staining to prevent weathering, and boards need to dry out for 3–6 months after installation before staining.

Composite decking (Trex, TimberTech, Fiberon) uses the same pressure-treated framing but replaces wood decking boards with a composite material. Composite costs $4–$12 per linear foot vs. $2–$4 for PT wood, but requires no staining, sealing, or splinter-proofing. Over 20 years, the maintenance savings often justify the higher upfront cost. Most composite products carry 25–30 year warranties.

Best practice: use pressure-treated lumber for all framing (posts, beams, joists) regardless of your decking choice. The framing is in contact with the ground or close to moisture and must be rated for ground contact (look for UC4B or UC4C rating).

Do You Need a Permit for a Deck?

Most jurisdictions require a building permit for attached decks above a certain height (commonly 12–18 inches above grade) or over a certain size (often 200 sq ft). Permit requirements vary significantly by municipality. Always check with your local building department before starting — unpermitted decks can cause problems at resale and may need to be removed.

Detached, ground-level platforms under 200 sq ft often don't require permits, but setback requirements from property lines still apply. Your local building department can tell you the exact rules for your area in about 5 minutes.

Frequently Asked Questions

How much lumber do I need for a 12x16 deck?

For a deck 16 ft along the house and 12 ft out, attached with a ledger and joists at 16 in on center in Southern pine, you need 13 2x10 joists at 12 ft, one 16 ft rim and one 16 ft ledger, two 2x10 boards for a 2-ply beam, 3 posts and footings, 28 decking boards at 16 ft, and about 676 deck screws. The joists step up to 2x10 because a 2x8 at 16 in only spans 11 ft 10 in.

What size joists do I need for my deck?

It depends on the distance from the ledger to the beam and the joist spacing. For No. 2 Southern pine at 16 in on center, 2x6 reaches 9 ft, 2x8 reaches 11 ft 10 in, 2x10 reaches 14 ft and 2x12 reaches 16 ft 6 in. Douglas fir, hem-fir and SPF span less. Past 16 ft 6 in you need either tighter spacing or an intermediate beam.

How many deck posts do I need?

Enough to keep the beam within its allowable span. A 2-ply 2x10 Southern pine beam can span about 9 ft 9 in between posts when it carries 4 ft of joists, falling to about 6 ft 11 in at 8 ft and 5 ft 8 in at 12 ft. So a 16 ft deck 12 ft deep typically needs 3 posts on its beam line, and a freestanding deck needs a second beam line with its own posts.

How many bags of concrete for deck footings?

A 12 in diameter hole 36 in deep holds about 2.4 cubic feet, which is four 80 lb bags. Multiply by the number of posts. Deeper frost lines need proportionally more, and many jurisdictions require a wider footing pad at the bottom, so confirm the footing size with your permit office before buying.

How many screws do I need for decking?

Two screws per board at every joist it crosses. A 16 ft by 12 ft deck with 5.5 in boards has 26 rows crossing 13 joists, which is 676 screws. Buy a few percent over for dropped and stripped screws, and use hidden fasteners instead if your composite decking specifies them.