Choosing a roof membrane is not about picking the thickest roll, but about matching its parameters to the specific roof build-up. Check where the membrane will sit, its Sd value, how watertightness and durability are declared, and whether the manufacturer specifies a permitted UV exposure time. Also compare the requirements for tapes and joints. Grammage alone or the word ‘breathable’ is not enough, because suitability depends on conformity with the design and installation conditions.

The choice of roofing membrane is determined above all by its position and role within the layer build-up, not by the weight of the roll alone. First establish whether you need an underlay membrane, a vapour barrier or both, then check how they fit the roof design. Take into account the substrate, permitted contact with thermal insulation, exposure time and how laps and junctions at walls, chimneys and services are sealed. Compare the declared properties and intended use of the product, since the term roofing membrane covers materials with different functions.

First limit damage inside and have the roof inspected, rather than immediately looking for the culprit at the wet patch. Water may travel along the membrane or structure, and a hole visible above a stain is not always the source of the leak. It is worth establishing whether the problem occurs with every rain or only with rain from a particular wind direction, and distinguishing precipitation from water-vapour condensation. The check should cover valleys, chimney flashings, wall junctions and service penetrations. An effective repair removes the cause instead of covering an arbitrary section of the roof slope.