A roof valley needs stable support, a continuous underlay and a sufficiently wide drainage path so that water from both slopes can flow freely to the eaves. The most common problems arise from screws placed in the water course, cut pieces that are too narrow, and leaves blocking the valley. Before covering, it is worth checking the geometry where the slopes meet, the evenness of the cut line and the placement of fixings outside the main flow. Also make sure the lower end of the valley discharges water correctly into the gutter and that the membrane beneath the covering has no gaps.

A roofer is responsible above all for the roof covering and the details that protect the building from precipitation, not merely for fixing the sheets. Before work begins, check whether the contract clearly separates carpentry work on the roof structure from roofing work, that is, preparation of the substrate, protective layers and the covering itself. The scope depends on the contract and the team's competence, so one company may carry out both stages, but this should not be assumed without agreement. It is also worth asking who is responsible for assessing the substrate and any consultation with the designer.