What This Requirement Covers
Timber frame construction is the second most common structural system for UK housing after masonry. Timber frame buildings use engineered timber panels (walls) and floor cassettes manufactured off-site and assembled on site. The structural design must comply with BS EN 1995 (Eurocode 5: Design of Timber Structures).
Key Requirements
Design Standards
- BS EN 1995-1-1: General rules and rules for buildings
- BS EN 1995-1-2: Structural fire design
- BS EN 14081: Grading of structural timber (machine or visual grading)
- BS EN 14250: Requirements for trussed rafters
Timber Grades and Species
- Common grades: C16 and C24 (strength classes to BS EN 338)
- C16: Used for general studwork, joists, and rafters in lower-load applications
- C24: Used for higher-load applications, engineered elements, and where smaller sections are needed
- Engineered timber products: glulam (BS EN 14080), LVL (BS EN 14374), and I-joists offer higher strength and consistency
Moisture Content
- Structural timber must be dried to a moisture content of 18% or below at the time of installation
- Timber frame panels are typically manufactured at 12-15% moisture content
- Excessive moisture causes shrinkage, distortion, and can promote fungal decay
Durability and Preservative Treatment
- Timber in contact with masonry or below DPC level must be treated with preservative to Use Class 2 minimum (BS EN 335)
- External timber (cladding, decking) requires Use Class 3 treatment
- Timber in ground contact requires Use Class 4 treatment
- Preservative treatment is not required for naturally durable species (e.g., European oak, western red cedar) in appropriate use classes
Breather Membranes and Vapour Control
- A breather membrane (Type 1F to BS 4016 or equivalent) must be applied to the outside of the timber frame panels to resist wind and rain penetration while allowing moisture vapour to pass outwards
- A vapour control layer (VCL) must be applied to the inside face of the insulation to prevent warm moist air penetrating the frame and causing interstitial condensation
- The VCL must be sealed at all joints and penetrations
Practical Compliance Tips
- Source timber frame panels from manufacturers operating under a quality management system with third-party certification
- Protect timber frames from rain during construction; prolonged wetting before the weathertight envelope is complete can cause problems
- Install cavity barriers at the time of frame erection, not as a follow-on trade
- Ensure the VCL is continuous and sealed at all joints; tape and seal around service penetrations
- Use appropriate fixings for the timber species and treatment (stainless steel or hot-dip galvanised for treated timber)
- Coordinate with the timber frame manufacturer for all structural opening locations, beam positions, and connection details
- Keep the manufacturer's design certificate and erection records for the building file