The average home in the UK has a pitched roof comprising several different components. Everything is built around a skeletal frame, usually made from timber. On top of this is placed a series of interlocking tiles, which distribute rainwater down into the guttering.
Beneath this is a waterproof roof membrane for added protection – a sheet of plastic acting as a barrier between the air inside the roof and the air outside.
Roof membranes come in several different types. There are breathable membranes and non-breathable ones. The former is favourable in most settings, as there is a perception that they eliminate the need for separate ventilation elsewhere. This is not always entirely accurate, and it’s still often necessary to include additional ventilation around the ridge and eaves.
The most breathable types of membrane today are ‘air open’ and ‘vapour permeable’. These represent the cutting edge when it comes to breathability, and are a result of extensive R&D on the part of the manufacturers. They’re especially useful in complex roofs, such as those found in extensions, skylight-equipped loft conversions, and heavily partitioned apartment buildings where there are many breaks in the roof, and reduced opportunity for ventilation.
Up to date guidelines can be found in British Standard BS 5250, which deals with the control of condensation in buildings.
The membrane’s job is to exclude external moisture from the underlying structure. While rainwater might not be able to directly pass through the tiles and into the roof, airborne droplets can find their way through. The membrane is placed on the ‘cold’ side of the insulation, and will allow moisture to pass from the inside to the outside, and still allow air to circulate below, limiting the likelihood of damp and other moisture-related problems.
If a membrane doesn’t incur direct damage, it should last for several decades with minimal maintenance. It is, after all, just a large sheet of plastic. Therefore, it’s important to ensure the right sort of membrane is chosen at the outset.
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