For most composite and capped decking, the side-to-side gap is about 3/16”, set by the manufacturer’s installation guide, and end gaps change with the temperature on the day you install. Wood is different. Wet pressure-treated boards go down tight or nearly tight because they shrink as they dry, while kiln-dried wood usually needs about 1/8”. Below are the published numbers, the reasons behind them, and how the gap changes your board count.
Why deck boards need a gap at all
A gap between boards has three jobs:
- Drainage. Rain needs to fall through the deck instead of pooling on it. Standing water on wood leads to rot and mildew. On composite it leaves stains and slippery patches.
- Airflow. Air moving up between the boards dries the tops of the joists and the undersides of the boards.
- Movement. Wood swells and shrinks with moisture. Composite and PVC grow and shrink with temperature, mostly along their length. A gap gives that movement somewhere to go so boards don’t buckle against each other.
Debris is the other half of the story. If the gap is too narrow, leaves, pine needles and grit pack into it and stop the drainage you wanted. If it’s too wide, it catches heels and chair legs, and small items fall through. The published specs try to balance the two.
Composite and PVC: follow the installation guide
Composite manufacturers publish exact gaps, and some tie their warranty to them. Here is what three major brands say in their current guides:
| Brand (guide) | Side-to-side gap | End-to-end (butt) gap | Gap at house or posts |
|---|---|---|---|
| Trex | 3/16” (5 mm) | 3 mm above 40°F (4°C), 5 mm below | See guide |
| TimberTech | 1/8” minimum, 3/16” maximum | 3/16” at 32°F and below, 1/8” from 33-74°F, 1/32” at 75°F and above | 3/16” minimum |
| Fiberon | 3/16” minimum | Varies by product line and temperature | 1/4” minimum |
Some details are worth knowing:
- Trex’s guide also calls for a wider 3/8” side gap on docks and in wooded areas where debris builds up, and says no gap should exceed 1/2”.
- Fiberon’s chart splits its products into groups. For some lines, the end gap runs from 1/4” at 35°F and under down to 1/32” at 110°F and up. Other lines in the same chart call for 1/16” or less at every temperature. Fiberon also says its spacing requirements must be met for full warranty coverage.
- TimberTech’s 1/8” to 3/16” range gives you a little room to even out a layout. Trex and Fiberon give a single 3/16” figure.
If you use hidden clips, the clip usually sets the side gap for you. Fiberon notes that its Phantom Universal clip produces a 3/16” gap. That’s one of the practical reasons people choose clips. The trade-offs are covered in hidden fasteners vs deck screws.
Why end gaps depend on temperature
Composite boards change length with temperature far more than they change width. A board cut and fastened on a 95°F afternoon is near its longest, so it needs almost no end gap. That same board fastened at 30°F is near its shortest and will grow in summer, so it needs a bigger gap. That’s why every chart above is organized by install temperature.
Two practical habits help here:
- Measure board temperature, not just air temperature. Fiberon specifically warns that dark colors swing more, and it recommends keeping boards shaded until you cut and fasten them.
- Cut and fasten in one go. A board that sits in the sun after cutting will be a different length by the time you screw it down.
Pinning boards so the gap stays even
Over many seasons a hidden-fastened board can creep toward one end, so one butt joint closes up and the other opens. Fiberon’s guide recommends two face screws at the joist closest to the middle of each board. That pins the board, so expansion and contraction go equally in both directions.
Pressure-treated and other wood decking
Wood moves across its width as its moisture content changes. That’s the opposite of composite, which moves mostly along its length. So the right gap for wood depends on how wet the board is when you install it.
The Southern Forest Products Association notes that the width of pressure-treated Southern Pine decking varies with moisture content, and it advises adjusting spacing at installation to allow for shrinkage as the wood dries. In practice:
- Wet treated lumber. This is most big-box stock, which often feels heavy and damp. Installers commonly butt these boards tight or leave only a very small gap, because they shrink as they dry and open up a gap on their own.
- Kiln-dried after treatment (KDAT) or air-dried wood. These boards have already shrunk. Gap them about 1/8” so they have room to take on a little moisture without pressing against each other.
- Cedar, redwood and tropical hardwoods. Follow the supplier’s guidance. Many hardwoods are sold dry and need a set gap, and some are sold with their own spacer recommendations.
Check one board before you decide. If it’s noticeably wider than its nominal 5-1/2”, it’s carrying moisture and will shrink. If it measures 5-1/2” or a hair under, treat it as dry. The pressure-treated lumber guide covers how to tell treated stock apart and how to store it while it dries.
How the gap changes your board count
The gap adds to every row. Each row takes up one board width plus one gap, sometimes called the “pitch.” Here’s the arithmetic:
- Rows = (deck width across the boards + gap) ÷ (board width + gap), rounded up.
Worked example 1. A 20 × 12 ft deck has boards running the 20 ft direction, so the boards stack across 12 ft (144”). With 5-1/2” boards and a 3/16” gap, the pitch is 5.6875”:
- (144 + 0.1875) ÷ 5.6875 = 25.35, so you need 26 rows.
- With 20 ft boards, that’s 26 boards before waste.
At this size the gap doesn’t change the answer. A 1/8” or 1/4” gap also gives 26 rows. Wider decks are where it starts to matter.
Worked example 2. A deck 16 ft (192”) across the boards:
| Board width | Gap | Pitch | Rows (rounded up) |
|---|---|---|---|
| 5-1/2” | 1/8” | 5.625” | 35 |
| 5-1/2” | 3/16” | 5.6875” | 34 |
| 5-1/2” | 1/4” | 5.75” | 34 |
| 5.36” | 1/8” | 5.485” | 36 |
| 5.36” | 3/16” | 5.5475” | 35 |
A narrower gap or a narrower board can add a whole row. On a 20 ft run that’s an extra 20 ft board, so it pays to run the numbers with the actual board you’re buying. The deck board spacing calculator does this for any width, and the composite deck calculator uses brand-specific board widths. Board widths by brand are covered in deck board sizes explained.
Avoiding a sliver at the last row
In example 1, 25 full rows plus their gaps take up about 142.2”, which leaves roughly 1.8” for the 26th row. A 1.8” strip looks wrong and is hard to fasten. Before you start, decide how to handle it:
- Rip both the first and last boards so they’re equal and wider than half a board.
- Adjust the overhang at the rim slightly. Keep within the manufacturer’s limit: TimberTech, for example, allows a maximum 1” cantilever for many of its boards.
- With a manufacturer’s gap range like TimberTech’s, nudge the gap within that range over the whole deck.
Snap chalk lines every few rows to keep the layout straight. Boards that aren’t perfectly straight will wander, and a creeping gap is easier to correct early than at the last row.
Tools for keeping the gap consistent
- Spacers. Plastic deck spacers in fixed sizes are cheap and accurate. On wood decks, a common nail is the traditional spacer. Measure the shank with calipers first, because diameters vary by nail type and maker.
- Hidden clips. These set the gap automatically on grooved boards.
- Board-straightening tools. A cam- or lever-style straightener pulls a bowed board into line so you can hold the gap while fastening.
Pull spacers out as you go. A spacer left in overnight on wet wood can get stuck when the board swells.
Common spacing mistakes
- Gapping wet lumber like dry lumber. A 1/8” gap on soaking-wet treated boards can turn into a gap well over 1/4” once they dry.
- Ignoring the temperature on end joints. Composite fastened tight on a cold morning will push against its neighbor in July.
- No gap at the house. Boards fastened hard against siding or a post trap water and have nowhere to expand.
- Mixing gap sizes. Changing spacers halfway across looks obvious from the yard.
Before you buy, it’s worth putting the whole plan together with the main deck board calculator. It counts rows from your board width and gap, and builds a cut list.
Frequently asked questions
What is the standard gap between composite deck boards?
For most major brands it’s about 3/16” side to side. Trex and Fiberon both publish 3/16”. TimberTech allows 1/8” to 3/16”. End gaps vary with install temperature, so check the chart in the guide for your specific product line.
Should pressure-treated deck boards be installed tight?
If the boards are wet from treatment, many installers butt them tight or nearly tight, because they shrink as they dry and open a gap naturally. Kiln-dried-after-treatment boards have already shrunk and usually get about 1/8”. Measure a board’s width to judge its moisture before you decide.
Can deck board gaps be too wide?
Yes. A wide gap catches heels and chair legs, and small objects fall through. Trex, for example, says no gap should exceed 1/2”. Unusually wide gaps on a wood deck usually mean wet boards were installed with spacers and then shrank.
How big should the gap be next to the house?
Leave a gap where decking meets siding, posts or masonry so water can drain and boards can move. TimberTech calls for at least 3/16” and Fiberon for at least 1/4”. Wood decks also need the ledger flashed correctly, whatever the board gap.