Inverted roofs are designed with the waterproofing layer, usually a liquid-applied membrane, beneath the insulation. Insulation boards are loose-laid on top of the membrane and then weighted down with paving slabs or gravel ballast.
The great benefit of inverted roofs is that the insulation protects the membrane from extremes of weather, such as frost, UV exposure and the expansion and contraction that comes about from summer/winter temperature cycles, which can be as much as 80°C on some roofs.
Rain that falls on the roof percolates through the ballast and between the insulation boards until it reaches the membrane. It then drains away through rainwater outlets. The build-up of the roof means that some water is almost always trapped beneath the insulation boards by capillary action. Consequently, the insulation is expected to be wet for the majority of its lifetime. This is taken into consideration when determining the U-value of the roof.
Condensation risk calculations are carried out assuming that the membrane is on the warm side of the insulation. This is generally fine, but during heavy rainfall the membrane can become ‘chilled’ by the water that percolates through the ballast and insulation and then sits on top of the membrane. Eventually, the standing water will be warmed up by the roof deck, but until this happens, the temperature of the membrane may be below the dew point. This creates a risk that condensation could occur within the build-up of the roof.
It is likely that, in the majority of cases, any condensation that occurs will evaporate once the membrane has warmed up, with no accumulation of condensate. However, in some circumstances it is possible for condensation to occur and to accumulate, leading to problems with the roof.
