Getting a decking order right

Board counts are arithmetic. These are the things that make the arithmetic wrong before you ever pick up a tape.

• Measuring the frame, not the decking. Decking usually overhangs the rim joist by an inch or so on open sides. Measure the surface you intend to cover.
• Buying nominal widths. A "6 inch" board is 5.5 in once milled. Sizing off 6 in leaves you roughly 9% short — on a big deck that is a second trip.
• Skipping the gap. Boards need the 0.125 in gap to drain and to move with humidity. Butting them tight traps water and cups the boards.
• Ordering exact. Ten percent covers bowed boards, knots and cut-offs. It is cheaper than a second delivery.

Joists, beams and footings

Joist and footing counts vary with span tables, beam placement and local frost depth. Treat any number you get from a calculator as a shopping estimate, not a structural design, and check your local code before you build.

Why a "6 inch" board covers 5.625 inches

Nominal lumber sizes describe the rough sawn dimension before the board is dried and planed. A 2x6 or 5/4x6 leaves the mill at 5.5 in wide, and that is what actually sits on your joists. Add the 0.125 in drainage gap between boards and each row covers 5.625 in of deck.

Size a deck off 6 in and you are 6% short on every row. On a 16 ft wide deck that is nearly a foot of uncovered joist at the far side — which is why the mistake is usually discovered at the end, with the lumber paid for.

rows = (short span in inches) ÷ 5.625, rounded up

Which way to run the boards

Decking normally runs the LONG way, so the short dimension sets the number of rows and the long one sets the board length. Turning it 90 degrees swaps those two numbers, and it usually changes the order — sometimes for the better, if the new length lands on a stock board and the old one did not.

It also changes the framing. Joists must run perpendicular to the decking, so reversing the boards means reversing the joists, which changes their span and possibly their size. That is a framing decision, not a finish one, and it has to be made before anything is cut.

Stock lengths, and the offcut nobody budgets for

Decking is sold in 8, 10, 12, 16 and 20 ft lengths. A run that lands exactly on one of those is the cheapest deck you can build: no butt joints in the field, no offcuts. A run that does not — 14 ft, say — takes a 16 ft board and bins 2 ft from every row.

Where a run is half a stock length, one board makes two rows: a 10 ft deck cuts from 20 ft stock, an 8 ft deck from 16 ft. Fewer pieces to carry, and long stock is often slightly cheaper by the foot. Against that, it needs a truck and one bad cut wastes two rows instead of one.

What the 10% is actually for

It is not padding. Yards sell what they have, and a bunk of decking contains boards with crook, split ends and knots that fall where a cut has to be. The cull rate on pressure treated lumber is high enough that professionals order the extra without thinking about it. On a straight deck 10% is right; a diagonal layout or a picture-frame border wants 15%, because both cut every board at both ends.

How these numbers are worked out

Every figure on this site is computed from the dimensions you give, not looked up in a table. Rows come from the span divided by 5.625 in and rounded up, because you cannot lay part of a board. Joists assume 16 in on centre, which is the common residential spacing; footings assume a beam every 8 ft. Board counts assume decking runs the long way.

Where a real deck differs — 12 in joist spacing under a hot tub, a cantilevered edge, composite boards with a different face width — the arithmetic changes and the figures here will be close rather than exact. They are a material order, not a structural design, and anything carrying unusual load should be sized by someone looking at it.