Counter-Flashing

Counter-flashing, also called cap flashing or (in the UK) cover flashing, is the upper half of a two-part flashing system. The base flashing, such as step flashing, an apron or a membrane upstand, is attached to the roof and turned up the wall. The counter-flashing is attached to the wall and turned down over the top of the base flashing, overlapping it so that water running down the wall is carried out over the base flashing rather than behind it.

Why two parts

Roofs and walls move differently with temperature, moisture and settlement, especially where a timber roof meets a masonry chimney. Separating the base flashing (fixed to the roof) from the counter-flashing (fixed to the wall) lets each move without tearing the joint open. Neither part is normally sealed rigidly to the other.

Fixing methods

• Reglet: a groove is cut into a mortar joint or masonry, the bent top edge of the counter-flashing is inserted, and the joint is sealed or repointed.
• Stepped flashing: on a sloping junction with brickwork, the counter-flashing is cut in steps so each step is let into a horizontal mortar joint. UK practice uses lead cover flashings in this way over soakers.
• Surface-mounted: a termination bar or a profiled strip is fixed to the face of the wall and sealed at the top; generally considered less durable than a reglet.
• Through-wall flashing: built into the wall during construction.

Materials

Lead is traditional in Britain and Europe; copper, aluminium, stainless and coated steel are common elsewhere. UK guidance commonly cites Code 4 lead as a minimum for cover flashings. Metal compatibility with the base flashing and the masonry needs to be considered.

Common faults

Surface-applied counter-flashing that relies only on sealant, counter-flashing that is too short to lap the base flashing, and mortar joints that have eroded out of a reglet are common causes of leaks at chimneys and parapets.