TB/04 – Trench Fill Foundation, Cavity Wall and Suspended Timber Floor

This suspended timber floor detail illustrates a typical junction with a trench fill foundation and insulated masonry cavity wall.

The drawing is intended for architectural designers, building professionals, contractors, developers and self-builders preparing a coordinated foundation and timber-floor specification.

For further background, see our Timber Ground Floors Explained guide. Guidance on ground moisture and subfloor ventilation is available in Approved Document C.

The detail shows:

• Typical 600 mm wide trench fill concrete foundation
• Minimum foundation depth and relationship to finished ground level
• External brickwork and Celcon Solar aircrete block inner leaf
• Partial-fill cavity wall insulation with a 50 mm clear cavity
• Cavity tray and DPC/radon-barrier continuity
• Suspended timber floor joists
• Insulation fitted between the floor joists
• Insulation support netting
• Moisture-resistant tongue-and-groove flooring
• Insulation between the perimeter wall and joist end
• Continuity of wall and floor insulation at the junction
• Minimum ventilated subfloor void
• Concrete oversite, DPM/basic radon barrier, sand blinding and hardcore
• Airbrick and telescopic-vent requirements for cross-ventilation
• Guidance covering foundation construction and adverse ground
• Typical wall and floor thermal-performance targets

The accompanying notes cover trench fill foundations, concrete requirements, suspended timber floor construction, floor joists, subfloor ventilation, insulation, floor decking, damp and radon protection, wall construction, mortar, wall ties and cavity control.

What This Suspended Timber Floor Detail Includes

• One downloadable PDF construction drawing
• A3 landscape sheet
• Drawn at 1:10 scale when printed at 100% on A3 paper
• Drawing reference: TB/04
• Detail title: Trench Fill Foundation, Cavity Wall and Suspended Timber Floor

Important Information

This is a standard construction detail intended to provide general design and specification guidance. It must be reviewed and adapted to suit the individual building, site conditions, structural loads, ground conditions, ventilation requirements, radon or ground-gas risk, materials and applicable Building Regulations.

Foundation dimensions and depths must be confirmed by Building Control and, where required, a suitably qualified structural engineer. Timber joist sizes, spans, bearings, supports, strutting and connections must be designed for the particular project. The location and number of airbricks or telescopic vents must be coordinated to provide effective cross-ventilation beneath the completed floor.

The stated U-values apply only where the illustrated construction, dimensions, materials and installation standards are followed and should be confirmed through the project’s thermal assessment.

The drawing does not include project-specific structural calculations, timber joist calculations, ground investigation, drainage design, SAP assessment or Building Regulations approval. Purchasing or using the detail does not guarantee approval by Building Control.

The PDF is supplied for use by the purchaser on their own projects. It may not be resold, redistributed, shared as a standalone product or reproduced for commercial resale without written permission.