What This Requirement Covers
Flat roofs lose heat rapidly if not properly insulated. When a flat roof is constructed or renovated, it must meet the Part L U-value requirements. The insulation method depends on whether the roof is a warm roof (insulation above the deck) or a cold roof (insulation between or below the joists).
Key Requirements
Warm Roof (Preferred)
- Insulation is placed above the structural deck, below the waterproof membrane
- Minimum U-value: 0.18 W/m2K (renovation) or 0.16 W/m2K (new build)
- Typical construction: structural deck, vapour control layer, rigid insulation (PIR or phenolic), waterproof membrane, ballast or paving
- No ventilation of the roof void is required (the insulation is on the warm side)
- The insulation must be tapered to provide falls for drainage (minimum 1:60)
Cold Roof (Less Common)
- Insulation is placed between or below the ceiling joists
- A ventilated void must be maintained above the insulation (minimum 50 mm cross-ventilated air space)
- Cold roofs are prone to condensation problems and are generally not recommended for new construction
- If a cold roof is used, a vapour control layer must be installed below the insulation on the warm side
Inverted Roof
- The waterproof membrane is laid directly on the structural deck, with insulation placed on top
- Extruded polystyrene (XPS) insulation is used (it is resistant to moisture absorption)
- Ballast (paving slabs or gravel) is placed on top to hold the insulation in place
- Suitable for roof terraces and green roofs
Practical Compliance Tips
- Always use a warm roof construction for new flat roofs; it eliminates condensation risk and is the preferred method
- Ensure the insulation is continuous with no gaps at edges or around penetrations
- Taper the insulation to provide adequate falls; ponding water on a flat roof causes deterioration of the membrane
- Use a vapour control layer below the insulation in warm roof construction
- For green roofs, ensure the structural design accounts for the additional weight of soil and planting
- When re-roofing a flat roof, upgrade the insulation to current standards; this is a Building Regulations requirement
- Keep the insulation specification and membrane warranty for the building file