Begin by establishing whether the damage was visible on delivery. Material, transport, storage, installation or later use can all be the source, and a leak or a change in appearance alone does not identify who is responsible. Reconstruct the cause and gather documentation: product markings, photographs and the conditions in which the defect appeared. A crushed edge, scratch or deformed sheet may stem from transport and unloading; inspect the packaging and available components, and record any discrepancies found. Check colour and coating type before most of the roof is in place.

A roofing membrane forms the first barrier beneath the covering, shielding insulation and timber from water and helping to remove water vapour within its capacity. It does not, however, remove the need for ventilation or for airtightness on the inside. When choosing, compare water resistance, diffusion resistance, strength and conditions of use, including temperature resistance and exposure time. Grammage alone does not guarantee quality. Also check tape and seal compatibility and the installation method, as the membrane will not make up for careless metal laying or leaky flashing.