Building a flat roof involves several layers that work together to provide structural support, thermal performance, and weatherproofing. The specification differs between cold roof and warm roof construction, and choosing the right approach from the outset is important for long-term performance.

Cold Roof vs Warm Roof

In a cold roof, insulation is placed between and below the structural joists, and the roof deck above the joists is in the cold zone. A ventilated air gap must be maintained between the insulation and the underside of the deck to prevent condensation forming on the cold deck surface. Cold roof construction is simpler to build but more prone to condensation problems if ventilation is inadequate.

In a warm roof, insulation is placed above the structural deck, keeping all structural elements in the warm zone. No ventilation gap is required and the insulation can be optimised for thermal performance without ventilation constraints. Warm roof construction is the recommended approach for new flat roofs and most re-roofing projects.

The Structural Deck

Flat roof joists are typically C24-grade structural timber, sized to span the required width with adequate depth for the imposed load and insulation. The deck boards, typically 18mm or 22mm exterior-grade plywood or OSB3, are fixed to the joists with the face grain direction running across the joists. Deck joints should be staggered and a 2 to 3mm gap left between boards to allow for thermal expansion.

Warm Roof Construction Layers

From below upward: structural joists, plywood or OSB deck, vapour control layer (polyethylene or reinforced foil), rigid PIR or phenolic insulation boards at the required thickness for the target U-value, waterproof covering (EPDM, GRP, or single-ply membrane). The vapour control layer on the warm side of the insulation is critical: it prevents warm moist interior air from reaching the cold covering and condensing.

Falls and Drainage

A flat roof must have a minimum fall of 1 in 80 (approximately 12mm per metre) to drain rainwater toward the outlet. Falls are created using tapered insulation boards or by building the falls into the timber framework with firring pieces. Adequate drainage outlets must be installed to handle the design rainfall rate for the building's location; a single outlet is usually sufficient for a domestic garage roof but must be appropriately sized and kept clear of debris.

Summary

Build a flat roof using warm roof construction: structural deck, vapour control layer, rigid insulation, waterproof membrane. Ensure minimum 1 in 80 falls toward drainage outlets. Cold roof construction requires a ventilated air gap above the insulation. Building Regulations approval is required for flat roofs on habitable extensions, with a minimum U-value of 0.18 W/m²K.