Deck board spacing and layout: gaps, direction, and joints

Deck boards look simple, but the spacing between them, the direction they run, and where their ends land decide how well the deck drains, how it ages, and how finished it looks. The numbers are set by the board maker and by the deck construction guidance your inspector uses, not by rule of thumb. This guide uses two documents: the Trex decking installation guide for composite boards, and the American Wood Council's DCA 6 deck construction guide for lumber decking. Then it shows how the choices change the board count in the deck calculator.

Why gaps matter

Trex explains that gapping “is necessary for drainage and the slight thermal expansion and contraction” of its boards, and also allows for shrinkage of the wood joist system. Water, leaves, and dirt need a path through the deck, and air needs to circulate underneath to keep everything dry. There are three different gaps to plan for:

Side-to-side gaps

• Trex composite: “The minimum required width-to-width gapping is 3/16" (5 mm),” for both hot and cold weather. For docks and heavily wooded areas Trex recommends 3/8 in, and “No gapping should ever exceed 1/2" (13 mm).” When you use its recommended hidden fasteners, the fastener sets the gap.
• Lumber decking (AWC DCA 6): “Space decking boards approximately 1/8" apart.”
• Other products: composite and PVC brands each publish their own gaps. Use the guide for the exact board you buy.

The gap changes how many rows fit. For a 12 ft (144 in) wide deck with 5½ in boards (the actual width of a nominal 6 in board under the softwood lumber standard, and the listed width of Trex square-edge boards):

Rows = the smallest whole number n where n × board width + (n − 1) × gap covers the width. Same formula as the deck calculator.

End gaps and gaps at walls

Boards also move along their length. Trex sets end-to-end gaps by the temperature on the day you install:

Source: Trex Decking Installation Guide (2024). Other manufacturers publish their own tables.

Trex also requires board ends to be cut clean and square, trimming at least 3/16 in from each factory end, and limits the overhang past the rim joist to 3/4 in.

Board direction and joist spacing

AWC's DCA 6 says decking placement “may range from an angle perpendicular to the joists to an angle of 45 degrees to the joists,” and that “Each segment of decking must bear on a minimum of 3 joists.” Trex likewise requires boards to span at least three joists.

Perpendicular boards

Running boards at 90° to the joists is the simplest layout. In Trex's span chart, its 1 in boards need joists at 16 in on center for residential decks, and its 2×6 boards allow 24 in.

Diagonal boards

Angled boards span farther between joists, so the joists move closer together. Trex reduces the chart spacing by 4 in for boards at 45° (16 in becomes 12 in for residential 1 in boards) and halves it for boards at 30°. Diagonal layouts also mean more angled cuts at the edges, so plan a larger waste allowance.

Butt joints and breaker boards

When a run is longer than your boards, boards meet end to end. Every joint has to land on framing. Trex says that where boards meet over a single joist, use its butt joint clip, or add a 2×4 “nailer” at the joist so each board end gets its own fastening. Screws near ends should be at least 1 in from the board's end and edge.

• Stagger the joints so seams in neighboring rows land on different joists instead of forming one line across the deck.
• Breaker board: a board running across the deck that the field boards butt into. Trex shows it over a single or doubled joist. It turns a row of scattered seams into one deliberate line, and lets you buy boards that fit each section.
• Picture frame: a border around the edge that hides cut ends. Trex notes patterns like this need framing adjusted to support them.

Planning the last board

If you start at the house with a full board and work outward, the last row is whatever is left over, which can be a thin sliver. With the default layout (a 12 ft deck, 5½ in boards, 3/16 in gaps) that leftover is only 1-13/16 in. Ripping the first and last rows to equal widths looks better:

Rows needed      = (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.352 → 26 rows
Full rows        = 26 − 2 ripped rows = 24
Gaps             = 26 − 1 = 25 × 0.1875 in = 4.688 in
Left for 2 rips  = 144 − 24 × 5.5 − 4.688 = 7.313 in
Each ripped row  = 7.313 ÷ 2 = 3.656 in (about 3-11/16 in)

Other ways to fix a sliver: change the overhang at the outer edge (within the manufacturer's limit), or use a slightly different gap within the allowed range. Lay a few dry rows across the joists before fastening to check the math.

How many boards to order

For the default deck, the deck calculator finds 26 rows, 416 linear feet of decking, and 29 boards of 16 ft with a 10% allowance. Ripping the first and last rows does not change the count, because each ripped row still uses a full-length board.

• Measure your boards' actual width and use it, not the nominal size.
• Enter the gap from your board's installation guide.
• Pick a board length that divides evenly into the run where you can. It reduces offcuts and joints.
• Raise the allowance for diagonal layouts, picture frames, and breaker boards, which add cuts.

Measuring an L-shaped or angled deck? Split it into rectangles with the methods in measuring irregular areas. Footings that use bagged concrete can be sized with the concrete calculator.