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.

The watertightness of a chimney flashing depends above all on correctly shaping the detail, not on the sealant alone. It is worth checking whether each of the three critical points — the ridge side, the sides and the junction with the chimney surface — directs water in the right direction and allows for the relative movement of roof and chimney. These issues, together with the choice of materials and fixings, are covered below, with attention to how a particular solution affects the durability of the whole junction.