How much gap should I leave between decking boards?

Leave 5–8 mm gaps between decking boards when installed at the expected service moisture content (16–20% for external timber). If boards are installed dry (12–14% MC, e.g.

Leave 5–8 mm gaps between decking boards when installed at the expected service moisture content (16–20% for external timber). If boards are installed dry (12–14% MC, e.g. thermowood), reduce gaps to 3–5 mm as boards will swell slightly in service. Gaps allow water drainage, air circulation, and accommodate seasonal expansion. Insufficient gaps cause boards to buckle when they swell; excessive gaps create trip hazards and drop-through issues.

Why this matters

This question relates to Timber Decking Specification: Durability Classes & Species Selection, which covers the broader topic in detail. Understanding this specific aspect helps architects, builders, and timber dealers make informed specification decisions.

For full context and additional technical detail, see our comprehensive guide:

→ Timber Decking Specification: Durability Classes & Species Selection

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About this answer

This answer is part of the PuurModulair Knowledge Base — comprehensive timber specification guidance for architects, builders, and dealers across the UK, EU, and Scandinavia. All technical content is reviewed by qualified timber technologists before publication.