Deck Board Spacing: Material-by-Material Gap Guide

The gap between deck boards is not a finish detail you pick by eye. It is the expansion joint that lets the surface shed water, dry from underneath, and survive the season it was installed in. Set it wrong and the deck either cups and traps water (too tight) or looks like a ladder and grows weeds (too wide).

The right number depends on what the board is made of, not on a single universal figure. Here is the short answer, then the reasoning and the math.

How much gap between deck boards?

For softwood, the field rule is 1/8″ in a dry climate and 1/4″ where it is humid. For composite, the manufacturer's spec governs — and it is not the same for every brand. Pressure-treated lumber sold wet is the one case where you deliberately install it almost tight and let the gap open as the board dries.

Treat the wood rows as a planning range, not a code requirement. The International Residential Code governs how far the joists may be spaced under the decking; it never specifies the gap between the boards themselves. That number is manufacturer spec and trade practice.

Why the gap changes with the material

Wood moves almost entirely across its width — the length barely changes. A 5.5″ board that measures its full width in dry June air is wider in an August humidity spell and narrower again in January. The gap is the room you leave for that swing. A board installed tight in a humid month has nowhere to go when it swells, so it pushes against its neighbor, lifts at the edges, and pops fasteners.

Composite moves for a different reason: heat, not moisture. That is why the composite gap is a fixed expansion allowance set by the temperature range the board will see, and why two brands can publish different numbers for boards that look identical.

Pressure-treated lumber: the wet-board exception

Treated boards leave the lumberyard soaked from the treating process. They shrink noticeably as they dry over the first season, so the accepted practice is to install them nearly tight or with a narrow 1/8″ gap and let the spacing open on its own. Install them at a full 1/4″ while wet and many will end the summer with gaps near 3/8″.

If you are working with boards that have already dried and acclimated on site — or kiln-dried after treatment — they behave like ordinary softwood: 1/8″ in the dry, up to 1/4″ in a humid climate. The moisture content of the board in your hand, not the label on the tag, is what decides the gap.

Composite: read the manufacturer, not the wood rule

Composite decking is where the "1/8″ or 1/4″" rule breaks down, because each manufacturer sets its own expansion allowance:

• Trex publishes 3/16″ to 3/8″ between boards — wider than wood, because the board grows and shrinks with temperature.
• TimberTech and AZEK publish 1/8″ to 1/4″, and their hidden-clip systems install boards at a consistent ≈3/16″.

Two consequences. First, "composite wants a bigger gap than wood" is not universally true — it depends on the brand, and ignoring the spec can void the warranty. Second, if you use a hidden clip system, the clip is your spacer, and the gap it produces is the gap the manufacturer designed for. Do not try to widen it.

Hardwood and cedar

Tropical hardwoods such as ipe and cumaru are dense and dimensionally stable, and the installation guidance calls for a tight, uniform gap of about 1/16″ — enough for air and water to move, not enough to see through. Cedar sits in the wood camp: dealers recommend 1/8″ to 1/4″, with 3/16″ a common choice, because cedar still takes on moisture and moves even though it resists rot.

Worked example: make the gaps come out even

Say the deck is 12 ft wide across the joists (144″) and you are using 5.5″ boards at a 1/4″ target gap. The pitch — one board plus one gap — is 5.5″ + 0.25″ = 5.75″. Divide: 144 ÷ 5.75 = 25.04, so use 25 boards.

Twenty-five boards do not fill 144″ exactly, so let the last gap absorb the difference. The honest gap is:

gap = (deck width − total board width) ÷ (boards − 1)

• Total board width: 25 × 5.5″ = 137.5″.
• Space left for joints: 144″ − 137.5″ = 6.5″.
• Joints: 25 − 1 = 24, so gap = 6.5″ ÷ 24 = 0.271″ — a hair over 1/4″.

Now every gap is 0.271″, the last board lands flush with the far rim, and there is no half-inch of drift to hide at the edge. The deck board spacing calculator runs the board count and linear feet for you; the pitch formula above is what it is doing under the hood.

Gap and joist spacing are two different numbers

Do not confuse the gap between boards with the spacing of the joists under them. IRC 2021 R507.7 sets the joist spacing the decking may span — it says nothing about the gap:

• 1¼″ (5/4) wood decking, perpendicular to the joists: 12″ single span, 16″ multiple span. Diagonally: 8″ and 12″.
• 2″ (2×) wood decking, perpendicular: 24″. Diagonally: 18″ and 24″.
• Fasteners: at least two 8d threaded nails or No. 8 screws per board at every joist.

Stairways are excluded from R507.7. If you are laying diagonal decking, the joists need to be closer together — check the deck joist span table and the beam and joist span calculator before you frame.

How to keep gaps even

Eyeballing the gap across 20-plus boards produces a wavy deck. The mistake is chaining: set a board, drop in a spacer, set the next against it, repeat. Land each gap just 1/16″ off and twenty of them compound into an inch and a quarter of drift at the far rim.

Instead, mark every course from one fixed reference edge. Lay out the pitch from the formula above, snap a chalk line for each board's near edge, and set the board on its mark. A bad mark then stays in its own course instead of pushing every later board off target. Reliable spacing tools for the actual install:

• Spacer nails: two 8d nails (each ≈ 1/8″) laid across the joist as you set each board — the classic trick, and it gives you 1/4″ when you use both.
• Rip strips: cut a strip of plywood to your exact gap width and use it as a gauge that cannot compress.
• Hidden clip systems: Camo, Concealoc, Trex Hideaway and similar set the gap automatically and hide the fastener — but only at the gap that system is built for.

Common mistakes

• Zero gap on dry boards — fine for wet pressure-treated lumber, a cupped mess for anything already dried. A tight surface holds water on top and has no room to swell.
• Using the nominal width. A 2×6 is 5.5″ wide, not 6″. At a 6.25″ pitch, 120″ ÷ 6.25″ = 19.2, so you order 20 boards and come up short of the rim.
• Measuring the width in feet. The pitch is in inches, so the deck width must be in inches too — 12 ft is 144″.
• Ignoring the manufacturer on composite. A gap outside the published range can void the warranty, and every brand prints a different one.
• Butt joints without support. Every end-to-end joint needs a joist or a block directly under it, or the tips curl and that joint opens wide. Stagger the joints so a seam does not run straight down the deck.

Deck board spacing FAQ

Where to go next

This page sets the surface gap; the framing has its own math. The deck board spacing calculator counts boards and linear feet, the deck joist span table gives the R507.6 spans that R507.7 assumes, and the beam and joist span calculator checks a specific size. For the rest of the deck, the deck footing calculator sizes the piers, the deck stairs guide covers the steps, and the concrete calculator handles the pour.