Flat Roof Condensation Risk

There are two forms of flat roof construction which can reduce the risk of condensation forming.

Flat roof condensation risk.

The cold deck solution. A flat roof should be designed to minimise condensation and a condensation risk analysis should be undertaken taking into account positioning of insulating materials vapour control layers ventilation thermal insulation and the choice of materials. However doing so increases the risk of condensation forming on the elements not surrounded by insulation such as cold roof decks. This type of roof has normally been used over small extensions to dwellings using timber joists and plywood deck.

Cold roof warm roof inverted roof. Designers should therefore take a cautious approach and adopt fail safe solutions. Because the waterproofing layer constitutes a vapour control layer on the cold side of the insulation there is a high risk of harmful interstitial and surface condensation on the underside of the deck or the upper surface of the. To that end the standard lists three acceptable scenarios for the placement of flat roof insulation.

For a flat roof construction be it a cold roof a warm roof or a warm inverted roof the calculation method for year round condensation risk set out in bs en iso 13788 known as the glaser method is almost always sufficient. Check it isn t interstitial condensation. As long as the roof accords with good practice then a more complex calculation method should not be necessary. This problem may become more common as existing buildings are altered to become eco friendly.