Deck Calculator

A deck is a stack of parts, and each one is sized by the one above it. Deck boards sit on joists. The joists hang from a ledger bolted to the house and run out to a beam. The beam sits on posts, and every post stands on a concrete footing. The calculator follows the American Wood Council's DCA 6, the prescriptive deck guide most building departments accept, so the sizes come from its tables instead of guesswork.

It picks the smallest joist (2x8 and up, since ledgers and guard-post joists must be at least 2x8) whose span covers the distance to the beam. With a cantilever, the joists run past the beam by up to the table overhang or a quarter of the span, whichever is less, so the beam sits in from the edge. The beam span then sets how many posts you need, and the beam and joist spans together set the footing size.

Southern pine 2x8s at 16 in on center span 11 ft 10 in; 2x10s span 14 ft; 2x12s span 16 ft 6 in. Tighter spacing spans farther. Lumber yards in the West mostly sell Douglas fir or hem-fir, which span a little less.

The beam span is the distance between posts. Longer joists put more weight on the beam, so the allowed beam span drops as the joists get longer. A bigger beam means fewer posts, and fewer holes to dig. The calculator lists every beam size with its post count so you can trade one for the other.

DCA 6 puts every footing on undisturbed soil at least 12 in below grade, or below the frost line, whichever is deeper. Footings within 5 ft of the house foundation must bear as deep as the house footing. Enter your ZIP for a frost-depth reminder, then ask your building department for the local number.

The calculator adds a pier tube from the footing to just above the ground and counts the concrete for both. The concrete calculator does a single tube or footing.

Divide the deck's depth in inches by one board plus one gap (5-1/2 + 1/8 in for wood), round up for the rows, and buy boards long enough to run each row in one piece when you can. DCA 6 calls for two screws or nails at every joist. Composite boards need a wider gap, about 3/16 in in one maker's guide.

Enter your lumber price per board foot, your decking price per foot, and your concrete and hardware prices, and the ticket totals the order and works out the cost per square foot. The defaults are examples only. Railings, stairs, flashing, and labor are not included; use the stair calculator for the steps.

Fresh pressure-treated lumber can still be wet from treatment and much heavier than dry wood. A wet 2x8 x 12 ft weighs about 56 lb and a wet 6x6 x 10 ft about 129 lb. The 16 x 12 example above is about 3,600 lb with the concrete, several times a half-ton's 1,150 lb payload.

Sixteen-foot deck boards stick out more than 4 ft from most beds. On a commercial vehicle that needs a red or orange flag at least 18 in square, and many states require one on any vehicle. Most lumber yards deliver a deck package for a fee, which is usually the answer.

• Joist size comes from the span, the spacing, and the species. Southern pine 2x8s at 16 in on center span 11 ft 10 in in DCA 6. (American Wood Council)
• Bolt the ledger to the house band joist with 1/2 in lags or through-bolts, staggered. Lead anchors and nails are not allowed. (American Wood Council)
• Every post sits on a footing at least 12 in below grade, or below the frost line where the ground freezes. (American Wood Council)
• Composite decking needs a gap on every side for drainage and heat. Read the maker's chart, not the wood rule. (Trex)

• Measuring the joist span to the outside of the deck instead of to the beam. The overhang past the beam does not count as span.
• Hanging the beam off the side of the posts on bolts. DCA 6 calls for notched posts or a post cap.
• Fastening the ledger with nails or lead anchors, or over siding without flashing.
• Footings too shallow for the frost line, which lets the deck heave.
• Forgetting that composite decking often needs closer joists for diagonal patterns, per the maker's guide.