A roofer is responsible for the covering and details that help protect the building from water, and their work begins before the first sheet is screwed down. Before starting, it is worth checking that the roof matches the documentation and the instructions for the chosen covering, especially the pitch, diagonals, eaves line and plane of the battens. The contractor should report any discrepancies found, and in a renovation assess the available parts of the old roof; doubts about the load-bearing capacity of the truss require a structural engineer's assessment. The scope of the contract should clearly define the layers, flashings, ventilation and acceptance of works.

Metal flashings determine whether water from the roof ends up where it should, so when choosing them it is worth starting by checking how a given detail works with the covering and the layer build-up beneath it. The sheet alone will not close edges, changes of direction or penetrations through the slope. At the eaves and verges, the relative arrangement of components and their fixing are key, while at the valley and ridge it is the drainage path and ventilation. It is also worth establishing whether the shape and material match the actual roof section before ordering folding.

A roof valley collects water from two adjoining roof slopes. Contaminants are carried there along with it. That is why the condition of this flashing should be assessed differently from an incidental section of a flat roof surface. What matters is free drainage, protection of the sheet metal and the quality of the joints. A well-chosen material helps maintain durability, but it requires proper workmanship and the removal of what the rain alone will not wash away. Leaves, small twigs and dust can accumulate along the valley line. Their presence impedes drainage and changes the conditions on the surface. It is worth observing where deposits appear regularly. Such a place requires assessment during an inspection, even if the rest of the roof looks clean.