Flashing is the general term for the weatherproofing material used to seal the junctions in a roof where water could otherwise penetrate. It is one of the most critical details in roofing, as more roof leaks originate at junction points than anywhere else in a roof covering.

Where Flashing Is Used

Flashing is required wherever the roof covering meets a vertical surface, another roof element, or a penetration through the roof. Common locations include the base of chimney stacks, where step flashing and apron flashing seal the junction between the tiles and the masonry. Abutment flashings seal where a roof slope meets a wall, such as where an extension roof meets the wall of the main house. Valley flashings seal the junction between two sloping roof sections meeting at a V-shaped trough. Soaker flashings are used behind tile courses at abutments to direct water outward. Roof light and Velux window flashings seal around the frame of roof windows.

Materials Used for Flashing

Lead is the traditional and most durable flashing material, used in UK roofing for centuries. It is malleable and can be shaped and dressed to fit complex junctions, and when properly installed it lasts as long as the roof itself, typically 50 to 100 years or more. Its high cost and the skill required to work it correctly are its main disadvantages. Aluminium is a lighter, less expensive alternative that is used where lead is unavailable or too costly. Zinc and copper are used in premium and heritage roofing applications. For lower-budget applications, self-adhesive lead-effect flashing tape provides a quick and effective solution for straightforward junctions, though it has a shorter lifespan than cut lead.

Why Flashing Fails

Flashing fails most commonly due to the mortar bed or pointing that secures it to the wall face cracking or falling out, allowing water to track behind the flashing and into the structure. Thermal movement causes lead to crack if it is laid in sections that are too long without expansion joints. Self-adhesive tapes can lift if they are applied to a dusty, damp, or cold surface. Regular inspection of flashings as part of roof maintenance allows early detection of deterioration before leaks develop.

Summary

Roof flashing seals junctions at chimneys, abutments, valleys, and roof lights where the roof covering meets another surface. Lead is the most durable and traditional material. Aluminium, zinc, and self-adhesive tapes are alternatives at lower cost or for simpler applications. Flashing failure is a leading cause of roof leaks and is most commonly due to mortar deterioration or thermal cracking. Regular inspection identifies deterioration before leaks develop.