Deck Board Spacing

The gap between deck boards does three jobs. It lets rain run straight through instead of sitting on the deck, lets air reach the boards and joists so they dry, and leaves room for the boards to move. Wood swells and shrinks with moisture, and composites expand and contract with temperature. Too little gap and water and debris collect. Too much and the deck feels open and catches things.

There are three gaps to get right: side to side between parallel boards, end to end where two boards butt on a joist, and the gap where boards meet the house, posts or other fixed objects.

Side-to-side gaps by brand

Hidden fastener systems for grooved boards set the side gap for you. Trex's guidance notes they are self-gapping, so the gap is whatever the clip makes. Check that it falls within your board's range.

End-to-end gaps depend on temperature

Composite boards grow and shrink along their length as the temperature changes, so the gap at butt joints is set by the temperature on the day you install. Install cold and the boards will expand later, so the gap is wider.

The brands' temperature bands don't line up exactly, so the rows above are grouped approximately. The band each maker actually uses is in brackets. Use your own brand's figure. Wood is different: the Western Red Cedar Lumber Association recommends butting cedar boards between double joists with a 1/8 in space at the ends, and TimberTech's wood guidance is to butt wood ends with no gap.

Gaps at the house, posts and other fixed edges

• Trex: 1/4–1/2 in where boards abut a wall, depending on temperature, and 1/4 in (above 40°F) or 1/2 in (below 40°F) where a board end meets the side of another board.
• TimberTech: at least 3/16 in at structures and posts.
• Fiberon: at least 1/4 in from permanent structures.

What the gap does to your board count

Every gap adds width, so the gap changes how many rows of boards you need. For a deck 16 ft along the house and 12 ft out, with 5½ in boards and 16 ft lengths (10% waste), thedeck material calculator gives:

Going from tight to 3/8 in saves a couple of rows on a 12 ft deck. The difference grows with width. Enter your real gap in the calculator, not a rounded one.

Why wood and composite rules differ

The two materials move for different reasons, and the guidance follows that. Wood boards shrink and swell across their width as they dry out or take on water. That's why Simpson Strong-Tie's advice turns on whether treated boards are wet or kiln-dried, not on the weather. A wet board laid tight will open its own gap as it dries. A dry board laid tight has nowhere to go when it gets wet.

Composite boards are much less affected by moisture, but they change length with temperature. So the composite makers publish one fixed side gap and a sliding scale of end gaps tied to the temperature on installation day. Their charts all point the same way: the colder it is when you install, the bigger the end gap, because the boards will grow when it warms up.

Worked example: gapping a 16 × 12 ft composite deck

You're laying Trex boards (5½ in wide, 16 ft long) on a 16 × 12 ft deck on a 55°F spring day. Boards run along the house.

1. Side gap: Trex's minimum is 3/16 in, so use 3/16 in spacers.
2. Gap at the house wall: 1/4–1/2 in, depending on temperature, per Trex. At 55°F, start at the low end.
3. End gaps: above 40°F, Trex calls for 1/8 in between butted ends. Land each butt joint on a joist.
4. How many rows? Each row covers 5½ + 3/16 = 5 11/16 in. Across 144 in that's 25.35, so 26 rows. But 26 boards and 25 gaps measure 26 × 5.5 + 25 × 0.1875 = 147.7 in, about 3¾ in more than the deck.
5. Fit the last board. Trex limits the overhang past the rim to 3/4 in, so the extra width has to come out of the last board. Rip it to roughly 2 in, or plan a wider fascia gap. Dry-laying the last three rows before fastening shows which works.

The deck material calculator counts the full 26th row, so its 29-board total already covers the ripped board.

How to keep gaps even

• Use spacers. A purpose-made deck spacer, or anything of the right thickness, held between boards while you fasten.
• Check every few rows. Measure from the house or rim to the last board at both ends of the deck. If one end is gaining, close or open the next few gaps slightly to bring it back.
• Fasten consistently. Two fasteners at every joist, as R507.7 and the makers specify. TimberTech asks for screws no closer than 3/4 in to the board edge, and Fiberon at least 1 in from ends and sides with pre-drilling.
• Plan the last board. Dry-lay the last few rows so you don't end with a sliver at the rim. Adjusting a gap by 1/32 in over many rows is usually enough to make it fit.

What to look for in your board's install guide

Every composite brand publishes an install guide, usually as a PDF on its website, and some differ between product lines. Fiberon's PVC Paramount boards, for example, use different end gaps (1/8 in at 50°F and above, 3/16 in below) from its composite boards. Before you buy spacers, find these figures for your exact product:

• Side-to-side gap, a minimum and sometimes a maximum.
• End-to-end gap by installation temperature, and whether butt joints need a double joist or blocking.
• Gap at walls, posts and other fixed objects.
• Maximum overhang past the rim. Trex allows 3/4 in.
• Minimum number of joists each board must cross. Trex and the AWC's DCA 6 both say three.
• Fastener edge distance and pre-drilling, e.g. TimberTech's 3/4 in from the edge and Fiberon's 1 in with pre-drilling.
• Joist spacing for square and angled layouts. This sets the framing, not the gap, but it's on the same page.

Following the guide matters for the warranty as well as the look, so keep a copy with your plans.

Spacing mistakes

• Gapping wet treated boards. They shrink as they dry, so a 1/8 in gap on a wet board can open well past that.
• One end gap all year. Composite end gaps depend on the install temperature. Use the brand's chart for the day you lay them.
• Butting boards against the house. Leave the fixed-edge gap so water drains and boards can move.
• Ignoring joist spacing. Gaps don't change how far a board can span. That's set by the joists. See how to plan deck framing.

Planning a roof over the deck too? The roof pitch calculator is the place to start.