A valley gutter carries water from two roof slopes that meet in a concave junction. The whole detail, not just the sheet width, determines watertightness. Before installation, check the valley line, continuity of support and underlay, and whether water can flow smoothly to the eaves. Consider the valley length, roof pitch and covering type rather than the appearance of the channel alone. Fixings must not interrupt the drainage path, and laps should lead water from the higher element to the lower one. Also remember that leaves, needles and snow collect there.

A fresh layer of sealant at the chimney sometimes holds back water only briefly. This happens when the repair covers the visible gap rather than the actual cause. Chimney flashings form a system of several connected parts. Each is meant to take water from a specific direction and pass it on. A leak may involve the sheet metal, the junction with the wall or the layer beneath the covering. An accurate diagnosis requires separating these locations. First, it is worth checking when the sign of moisture appears. The intensity of the rainfall, the wind and the time since the thaw all matter. The inspection should also cover the condition of the chimney's external surface. Water may enter the masonry above the flashing and show up lower down.

Whether a repair will suffice or the covering must be replaced depends on the cause and extent of the damage, not on the roof's age. Before deciding, the owner should inspect the slope from outside and from the loft and establish whether the planned work will remove the source of the problem rather than only the visible symptom. It is worth checking the condition of flashings, fixings and service penetrations, as well as hidden layers such as the membrane or insulation. Recording after which weather a mark appears helps diagnosis.

Before winter arrives, the owner should establish which drainage and fixing faults need repairing ahead of frost rather than leaving them until later. It is worth checking that gutters and downpipes flow freely, the condition of flashings and the completeness of snow guards, and removing leftovers from earlier work. For sandwich panels, the tightness of joints and the condition of the core are assessed. A fault list makes it easier to plan actions before the weather hampers access and the use of repair materials.