In standard building construction, there are two methods we normally use to control condensation build up in the roof space; ie we prevent water vapour from reaching the loft space, or remove it once it gets there, before it builds up to harmful levels.

Much has been written previously about roof space ventilation and the control of condensation, though not often from the roofing contractor’s viewpoint. Using a series of examples, I will explain what the roofer needs to consider when installing a roof to prevent harmful condensation, or, in other words, how to follow the guidance given in BS 5250: the British Standard Code of practice for control of condensation in buildings.

It is important to make the distinction between vapour-permeable and air-permeable underlays. In this article, I will deal with vapour-permeable underlays, as defined in BS 5250. Air-permeable underlays are currently not defined in BS 5250; therefore, it is important to refer to their Agrément Certificate and manufacturer’s installation instructions. They generally require less, or no additional roof space ventilation.

Download the full guide here.