What happens if timber is installed too wet?

Timber installed above its equilibrium MC will shrink as it dries in service. A 47 × 200 mm joist at 20% MC will shrink approximately 3 mm in depth when it reaches 12% MC.

Timber installed above its equilibrium MC will shrink as it dries in service. A 47 × 200 mm joist at 20% MC will shrink approximately 3 mm in depth when it reaches 12% MC. Across a floor with 12 joists, this creates cumulative movement that cracks plasterboard, opens gaps in flooring, causes nail pops, and can compromise structural connections. In severe cases, trapped moisture leads to fungal decay (rot) requiring replacement.

Why this matters

This question relates to Structural Timber Moisture Content: Why 12% Matters, 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:

→ Structural Timber Moisture Content: Why 12% Matters

Related questions

• What moisture content should structural timber be?
• How do you measure timber moisture content?
• What is equilibrium moisture content?

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.