What is the zero-strength layer in timber fire design?

The zero-strength layer (d₀) is an additional 7 mm depth below the char line where timber has been heated above ambient but below charring temperature. This heated zone has reduced strength and stiffness and is therefore excluded from the effective residual cross-section in fire design calculations.

The zero-strength layer (d₀) is an additional 7 mm depth below the char line where timber has been heated above ambient but below charring temperature. This heated zone has reduced strength and stiffness and is therefore excluded from the effective residual cross-section in fire design calculations. The total sacrificial depth per exposed face is: char depth (βn × t) + zero-strength layer (7 mm).

Why this matters

This question relates to Fire Resistance of Structural Timber: Charring Rates & Eurocode 5, 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:

→ Fire Resistance of Structural Timber: Charring Rates & Eurocode 5

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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.