Thermal Bridging in Flat Roofs: Causes, Consequences, and Solutions

A thermal bridge is a point in a building's envelope where heat can pass through more easily than in surrounding areas. In flat roofs, thermal bridges reduce energy efficiency and can cause localised condensation, which leads to mould and structural damage.

Common Thermal Bridges in Flat Roofs

• Joists in cold roof construction (heat bypasses the insulation at each joist)
• Fixings that penetrate the insulation layer (screws, bolts)
• Roof lights and skylights with thermally unbroken frames
• Parapet wall junctions where masonry connects inside and outside
• Roof edge details where the insulation is thinner or discontinuous

The Consequences

Thermal bridges increase heat loss, raising energy bills. More seriously, they create cold surfaces inside the building at which warm, moist air condenses. This condensation can cause mould growth on internal surfaces and, over time, damage to the roof structure itself.

How Warm Roof Construction Eliminates Most Bridging

Warm roof construction — with insulation above the structural deck — eliminates the joist bridging problem entirely. The structural deck is kept warm and above the dew point, virtually eliminating condensation risk within the structure.

At the Design Stage

Thermal bridge-free detailing requires attention at the design stage. Specify thermally broken roof light frames, use insulated edge details at perimeters, and ensure continuous insulation across the roof area. We're happy to advise on specification for new builds and extensions.