Composite deck boards need a 3/16-inch side-to-side gap for standard installations per Trex and TimberTech installation guides. Wood deck boards go in at 1/8 inch — or tight if the treated lumber is still wet. End-to-end gaps at butt joints depend on the temperature at installation.
Side-to-Side Spacing (Width Gap)
This is the gap between the long edges of adjacent boards running parallel to each other.
| Material | Side-to-side gap | Source |
|---|---|---|
| Trex composite (most lines) | 3/16 in. | Trex installation guide |
| TimberTech Advanced PVC | 1/8–1/4 in. depending on fastener | TimberTech installation guide |
| TimberTech capped composite | 1/8–7/32 in. depending on fastener | TimberTech installation guide |
| Fiberon composite | 3/16 in. standard | Fiberon installation guide |
| Pressure-treated wood (dry, below 15% MC) | 1/8 in. | Standard practice |
| Pressure-treated wood (wet, fresh treatment) | 0 in. (tight) | Install tight; gap opens as wood dries |
| Hardwood (cedar, redwood, ipe) | 1/8–1/4 in. | Depends on species and MC at installation |
Wood expands primarily across its width (perpendicular to the grain). Composite and PVC boards expand primarily lengthwise. This is why the gap rules differ — wood needs side clearance, composite needs end clearance.
Check your specific product’s installation guide. Gap recommendations on the same brand can vary by product line.
End-to-End Spacing (Butt Joints)
The gap at the end of two boards that meet over a joist (a butt joint) depends on material and temperature:
Composite Decking End Gaps (Trex example)
| Installation temperature | End-to-end gap |
|---|---|
| Below 40°F | 3/16 in. |
| 40°F and above | 1/8 in. |
| Abutting house or fixed structure, above 40°F | 1/4 in. |
| Abutting house or fixed structure, below 40°F | 1/2 in. |
Source: Trex deck board spacing guide
Wood Decking End Gaps
Traditional pressure-treated wood does not require an end gap because wood expansion occurs across the width, not the length. Butt joints on wood decking can be tight (zero gap). This is the opposite of composite behavior.
Worked Example: 16 x 20 ft Deck, Trex Composite
Deck is 20 feet wide with boards running the full 20-foot length (or using butt joints over intermediate joists).
At 3/16-inch side gaps, how many 5.5-inch (actual width) composite boards fit across 16 feet?
- 16 ft = 192 inches
- Each board unit = 5.5 in. (board) + 0.1875 in. (gap) = 5.6875 in.
- Number of full boards: 192 / 5.6875 = 33.75 — so 33 boards plus a ripped partial board
- Remaining space for last board: 192 - (33 x 5.6875) = 192 - 187.7 = 4.3 inches — rip to width
Use the deck calculator to handle the board count and waste factor automatically.
Gap from the House
Where boards run perpendicular to the house and end near the siding or ledger:
- Leave 1/4 inch minimum between board end and any fixed structure
- Keep at least 1/2 inch between the edge of the deck surface and any siding or flashing
- These clearances allow the deck to breathe, prevent moisture trapping, and allow board movement
Tree Canopy Consideration
If the deck sits under trees, organic debris (leaves, seeds, twigs) collects in the gaps. Trex recommends widening the side gap from 3/16 to 3/8 inch under heavy canopy so debris passes through rather than packing in the joint. Organic material trapped in tight gaps accelerates surface staining and is harder to clean out.
Checking Alignment as You Go
Run a chalk line or string line across several boards periodically to confirm the run stays parallel to the house. As boards are pressed into hidden fasteners, small cumulative errors can pull the run off-square. Catch it every 4–5 boards rather than at the far edge where the correction is a ripped board.
Board Expansion: Understanding the Math
Composite boards expand primarily lengthwise (along the board) as temperature rises. Wood boards expand primarily across the width (perpendicular to the grain). This difference drives the gap rules.
For a 20-foot-long composite board installed at 40°F that heats to 120°F on a summer day:
- Trex boards have a thermal expansion rate of approximately 0.000036 in/in/°F
- Temperature rise: 80°F
- Expansion per inch of length: 0.000036 x 80 = 0.00288 in/in
- Total expansion for 240-inch board: 240 x 0.00288 = 0.69 in — approximately 11/16 in.
That 11/16 in. total expansion across one 20-foot run explains why end gaps of 1/4 to 1/2 in. are required at walls and fixed structures.
Fastener Types and Gap Implications
| Fastener type | Gap control | Notes |
|---|---|---|
| Face screws (visible) | Manual — spacer required | Use composite spacer clips or cut-wood spacer |
| Hidden clip fasteners (T-clip) | Creates ~1/4 in. gap automatically | No gap measurement needed; consistent |
| Grooved-edge hidden clips (Trex Hideaway) | Creates 1/4 in. gap by design | Requires grooved-edge boards; fastest install |
| Ipe-specific hidden clips | Tighter than composite — ~1/8 in. | Dense hardwood expands less |
Face screwing composite decking is acceptable with most brands but requires drilling a pilot hole and using composite-specific screws to avoid surface dimpling. Some manufacturers require color-matched plugs over screw heads to maintain warranty.
Avoiding Common Spacing Mistakes
- Starting with zero gap at the house: If the first board is installed tight against the ledger band, you have no adjustment room when cumulative errors push boards off-square over 20 feet. Leave 1/4 to 1/2 in. at the house from the start.
- Using metal as a spacer: Thermal transfer from a metal spacer can compress adjacent composite boards if left in summer heat. Use composite or plastic spacer clips.
- Ignoring product-specific rules: A 3/16 in. Trex gap is not universal. TimberTech Advanced PVC uses different gaps for its clip types. Always read the current installation guide for your specific product — gap requirements can change between product years.
- Incorrect gap on butt joints over double joists: Where two board ends meet over a doubled joist, each board end still needs its temperature-appropriate end gap. Plan the joist layout so butt joints fall centered on the doubled joist, leaving room for both board ends.
For joist layout that supports correct board installation, see deck joist spacing. For a material comparison, see composite vs. wood decking.