5-6mm for pressure-treated timber boards. This allows rainwater drainage and seasonal movement. Boards fitted tight in dry weather will still open up as they dry.
| Board type | Gap | Notes |
|---|---|---|
| Pressure-treated softwood | 5-6mm | Side gap, boards laid in normal moisture |
| Hardwood (e.g. balau) | 5-6mm | Pre-drill ends |
| Composite (grooved) | 5-6mm sides, 3mm ends | Manufacturer-dependent |
| Composite (solid) | 3-8mm sides, 3mm ends | Thermal expansion is the driver |
| PVC/other synthetics | Per manufacturer | Usually 3mm+ |
3-8mm side gaps depending on the brand (most specify 5-6mm), plus 3mm minimum at board ends and against walls. Composite expands along its length with temperature — end gaps matter more than side gaps.
Not for softwood boards near the edge with quality screws. Hardwood boards and board ends both benefit from pre-drilling to prevent splitting.
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