What size joists do I need for a timber deck subframe?

Subframe joist size depends on span and spacing. For typical domestic decking with joists at 400–500 mm centres: 47 × 100 mm joists span up to 1.2 m between bearers; 47 × 150 mm joists span up to 1.8 m; 47 × 200 mm joists span up to 2.4 m.

Subframe joist size depends on span and spacing. For typical domestic decking with joists at 400–500 mm centres: 47 × 100 mm joists span up to 1.2 m between bearers; 47 × 150 mm joists span up to 1.8 m; 47 × 200 mm joists span up to 2.4 m. All subframe timber should be preservative-treated to Use Class 4 (ground contact) or Use Class 3 (above ground with ventilation). Use C24 grade for structural reliability.

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.