Modern Roof Design

Posted 5th September 2018


We inspected a school roof recently, 10 years old, which was leaking. Typical for modern roofs, it used 100mm of rigid polyurethane insulation boards wedged in between solid timber rafters. As the building was curved in plan, it required every board to be cut to a shape, a real challenge. These boards are known to shrink a little as they age, so any gaps will increase. Thermal bridging through the solid rafters can reduce thermal performance by 25%.

This construction requires an effective vapour control layer to prevent interstitial condensation, a loose sheet of plastic can be seen in the photograph. This is not sealed at the perimeters, allowing water vapour to enter the roof fabric, and is totally ineffective against air leakage (draughts).

The photo also shows hopelessly long flexible ventilation pipes, as a result the WC’s are not ventilated correctly.

Flexible insulation is much better between rafters, with an additional layer on top to prevent bridging through the rafters.