The quality of a roofer is shown above all by the way they approach preparation and execution. Rather than relying solely on price or years in business, check whether their experience matches your task – a small canopy and a complex house roof require partly different skills. It is also worth noting whether the conversation covers the entire layer build-up, not just the colour of the covering. A professional should identify the points that need to be agreed with the designer and explain what data is needed for a reliable quote.

First limit damage inside and ensure safety, then determine whether the water comes from precipitation or condensation. Check whether the leak appears only with rain from a particular direction, during thaw or at low temperature – this helps narrow the search. The damp patch need not lie beneath the damaged covering, as water can travel through roof layers. The inspection should cover flashings, penetrations, laps, fixings and drainage, rather than random application of sealant.

A roof membrane is selected primarily according to the layer build-up and the type of covering, not by grammage alone. It is worth checking the product's intended use, the tightness of joints and the permitted exposure time. The underlay membrane on the outer side drains water from beneath the covering, while the inner layer limits the passage of moist air and vapour in line with the design. These are different tasks and working conditions, so casually calling both of them foil leads to mistakes. The selection decision is left to the designer, who assesses the entire build-up.

First decide which side of the roof build-up the membrane will serve, rather than relying on a trade name. Check its diffusion resistance, watertightness, strength and the permitted substrate and gaps. Grammage alone does not confirm suitability. For a habitable loft, pay attention to the airtightness of the inner layer; for metal cladding, to drainage and ventilation. Compare the requirements with the cladding instructions and the build-up design. Only then ask about accessories and the compatibility of the whole system.