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.