Structures · Guide
Deck Board Gaps and Joist Spacing: The Numbers Behind a Solid Deck
Two small measurements, the gap between boards and the spacing between joists, decide how a deck feels underfoot and how much material goes into it.
Walk across a well-built deck and it feels solid. Walk across a poorly built one and the boards flex, the surface looks wavy, and water sits in the joints. The difference usually comes down to two numbers chosen before the first board was cut: the gap between the boards, and the distance between the joists underneath them.
A note on scope before going further. This guide covers the decking surface and the spacing of the joists under it. The size of the joists and beams, the footings, and the connection to the house are structural questions answered by your local building code and its span tables. Most places require a permit for a deck. Start there.
Actual sizes, not nominal
Lumber is sold by a nominal size that is larger than the real thing. Every calculation on a deck uses the real size.
| Sold as | Actual thickness | Actual width |
|---|---|---|
| 5/4 × 6 deck board | 1 in | 5½ in |
| 2 × 4 | 1½ in | 3½ in |
| 2 × 6 | 1½ in | 5½ in |
| 2 × 8 | 1½ in | 7¼ in |
| 2 × 10 | 1½ in | 9¼ in |
| Composite deck board, typical | about 1 in | about 5½ in |
A “six inch” deck board covers five and a half inches. Over a 12 foot deck, that missing half inch per board adds up to two extra rows.
The gap between boards
Deck boards need gaps so that water drains, debris falls through, and the boards have room to swell and shrink.
For wood: the target is a gap of about ⅛ inch (3 mm) once the wood has dried and settled. How you get there depends on how wet the lumber is when you lay it.
- Kiln-dried lumber, including boards marked KDAT, is already dry. Lay it with a ⅛ inch gap. A 16-penny nail or a ⅛ inch spacer between boards does the job.
- Wet pressure-treated lumber, which is most of what is sold, still has a lot of water in it. It will shrink across its width as it dries, by ⅛ to ¼ inch per board. Lay these boards tight together, or with the smallest possible gap, and the gaps will open by themselves over the first few months.
If you gap wet boards at ⅛ inch, you can end up with ⅜ inch gaps by the end of summer, wide enough to catch a heel.
For composite and PVC: follow the maker’s instructions exactly. These boards do not shrink as they dry. They expand and contract along their length with temperature. Makers specify a side gap, usually ⅛ to ¼ inch, and a separate, temperature-dependent gap at butt joints and where boards meet a wall. Getting it wrong can void the warranty.
How the gap changes the board count
Each row takes up the width of one board plus one gap.
For a deck 12 feet wide, which is 144 inches, with 5½ inch boards:
| Gap | Board + gap | Rows needed |
|---|---|---|
| None, boards tight | 5.5 in | 27 |
| ⅛ in | 5.625 in | 26 |
| ¼ in | 5.75 in | 26 |
The gap makes a difference of about one row in this case. It matters more for how the last board fits. The final row almost never lands on a full board, so plan to rip the last board to width, or adjust the gaps very slightly across the last few rows to finish on a full one. Measure the remaining distance when you are five or six rows from the end and decide then.
Joist spacing
Joist spacing is measured “on center”, from the middle of one joist to the middle of the next.
| Decking | Laid straight across the joists | Laid diagonally at 45° |
|---|---|---|
| 5/4 wood deck boards | 16 in | 12 in |
| 2 × 6 wood decking | 16 in, or 24 in where code allows | 16 in |
| Composite or PVC, most products | 16 in | 12 in |
| Composite, some products and all stairs | 12 in | Check the maker |
These are the common figures. The decking maker’s installation guide and your local code have the final say.
Why diagonal needs closer joists. A board laid at 45 degrees crosses the gap between joists at an angle, so the distance it has to span is 1.41 times the joist spacing. With joists at 16 inches, a diagonal board spans nearly 23 inches. Closing the joists up to 12 inches brings the diagonal span back to 17 inches.
Why composite often needs closer joists. Composite boards are more flexible than wood of the same thickness, and they soften a little in hot sun. At too wide a spacing they sag between joists and feel spongy.
How joist spacing changes the materials
Closer joists mean more joists, more hangers and more screws. For a deck 16 feet long, with joists running across the 12 foot width:
| Joist spacing | Joists | Screws, at 2 per crossing with 26 rows |
|---|---|---|
| 24 in | 9 | 468 |
| 16 in | 13 | 676 |
| 12 in | 17 | 884 |
Going from 16 inch to 12 inch spacing adds four joists and about 200 screws on this deck. That is a real cost, and a much smaller one than a bouncy deck that has to be rebuilt.
Fasteners
Face screws. Two per board at every joist, set about ¾ inch in from each edge. Use screws rated for exterior use and for contact with treated lumber, which corrodes ordinary steel. Stainless steel is the best choice near salt water.
Hidden fasteners. Clips that fit into grooved board edges, or plugs that cover countersunk screws. One clip per board at each joist, with the first and last boards face-screwed or fixed with special starter clips. They cost more and take longer, and they leave a clean surface.
How many. As a rule of thumb, about 350 screws per 100 square feet of deck with 5½ inch boards on 16 inch joists. Buy a box more than you calculate. Screws get dropped through the gaps.
Pre-drilling. Near the ends of boards, drill a pilot hole first. Screws driven close to an end split the wood.
Board direction and length
Deck boards run at right angles to the joists. On most decks the joists run out from the house, so the boards run parallel to it.
Buy boards as long as the run, if you can. A deck 16 feet long laid with 16 foot boards has no end joints at all. Joints are where decks look worst and rot first.
If the deck is longer than the boards, every row has a butt joint. Each joint must land on the center of a joist, and joints in neighboring rows should be staggered by at least two joist spaces. Many builders double the joist under a line of joints, or use a contrasting divider board across the deck so that all joints fall in one tidy line.
Trim after laying. Let the boards run a little long over the edge of the frame, then snap a chalk line and cut them all at once with a circular saw. The edge comes out perfectly straight.
Which side up
For boards with visible growth rings on the end, lay them bark side up, with the rings curving downward like a frown. Boards tend to cup away from the bark side as they dry, and laid this way, any cupping sheds water instead of holding it.
Not every board will cooperate, and the better face should usually go up regardless. But where the choice is free, bark side up is the safer rule.
Keeping the lines straight
Boards are rarely perfectly straight. Over 26 rows, small errors add up to a visible wander.
- Start with a straight board against the house, set to a chalk line, with a small gap to the wall for drainage.
- Every four or five rows, measure from the last board to the far edge of the frame, at both ends and in the middle. The three measurements should match.
- If they do not, correct gradually over the next few rows by opening or closing the gaps slightly. Nobody will see a sixteenth of an inch.
- Bent boards can be pulled straight as you fasten them. Fix one end, then lever the board into line with a chisel driven into the joist, working along its length.
Putting numbers on your own deck
Enter the deck size, the real board width, your gap, the board length and the joist spacing in the deck calculator. It gives the rows, the boards to buy with waste, the joist count and the fasteners. Change the joist spacing or the board direction and watch what moves. On most decks, the direction of the boards and the spacing of the joists change the order far more than the gap does.
Written and checked by Bertha
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