The purpose of flashing is to prevent water from entering the building envelope and to intercept any that passes the first plane of protection and direct it to the exterior. Flashing is usually required wherever there is a discontinuity on exterior surfaces (for example, above windows), where there is a change in cladding materials (for example, vinyl siding above brick cladding), and at roof valleys. Carefully plan the location of flashing to fit with roofing materials, brick joints, sheathing membranes, windows, skylights and doors

Source : Canada Mortgage and Housing Corporation (CMHC).

Note : Flashings are components of the exterior building envelope used to intercept and direct the flow of water to designed drainage paths. They are required to minimize the entry of water through joints and between materials. Flashings are usually installed wherever there is a discontinuity on exterior surfaces, such as:

  • at the top of exposed walls;
  • at roof-wall junctions;
  • at roof-chimney junctions;around penetrations through roofing;
  • at valleys in roofs;
  • within walls above and below doors, windows and other wall penetration;
  • at foundation level to lead water out of the wall cavity; and
  • at locations where water might enter the building through a juncture between two materials.

Flashings are also used to direct water away from building elements susceptible to deterioration, staining, erosion, frost damage or corrosion when exposed to excessive wetting. As discussed in the previous chapter, flashings are part of the primary and secondary lines of defence against rain penetration.