Flashing describes strips of sheet metal or other impervious material used to prevent rainwater penetrating into joints between different parts of a building.
The most common situations where flashing is used are:
- In the valleys where different sections of a roof meet
- Where the roof of part of a building abuts a wall, e.g. where a single storey extension has been built against the rear wall of a two-storey house.
- Where a chimney stack or pipe passes through a roof
- Around window frames and sills
- Around roof lights
Flashing may be visible or concealed, or a combination of the two, depending on the type of joint. Sheet metal is widely used, especially where flashing is to be visible. Lead and copper were widely used for this purpose in the past, and more recently zinc alloy, aluminium, galvanized steel and stainless steel have been used. Modern synthetic materials are also now available and widely used in areas where the flashing will not be visible.
Whatever material is used should have a long life, as it can be difficult and expensive to replace flashing.
It can be difficult to spot if flashing fails, but if this does happen water can penetrate into timber and brickwork in inaccessible areas, causing extensive damage.
Flashing should direct water away from the structure of a building, and should be designed so that rain cannot be blown in by the wind. It is also important to ensure that water cannot penetrate between joints in the flashing material, e.g. by capillary action. Installing or replacing flashing is therefore a job best left to experts.