counterflashing

Counterflashing is a secondary flashing material installed above and overlapping the top edge of base flashing, creating a protective cap that prevents water from entering the joint between a roof and an adjoining vertical surface such as a wall, chimney, or curb. While base flashing runs up the wall and bends onto the roof, counterflashing is driven or set into a cut made in the wall itself, then bent down and over the base flashing to shed water outward and downward. This overlap and concealment is what distinguishes counterflashing from the base flashing it protects.

Counterflashing is typically formed from galvanized steel, aluminum, copper, or elastomeric membranes such as rubber or modified bitumen, chosen to match the roof system and base flashing materials. Galvanized steel counterflashing is common in commercial work and offers strength and longevity; aluminum is lighter and resists corrosion; copper develops a protective patina and lasts fifty years or more; elastomeric options are used where the substrate is itself membrane-based. The material is bent into an L or Z profile, with the upper leg inserted into a reglet (a slot cut into the wall) and the lower leg bent down over the base flashing.

The reglet into which counterflashing is seated must be cut cleanly into masonry, concrete, or other wall material, typically one-half inch deep and wide. Poor installation of the reglet, such as cutting it too shallow or at the wrong angle, allows water to bypass the counterflashing entirely. The gap between counterflashing and reglet is sealed with sealant or elastic material to prevent water entry. Fasteners for counterflashing (typically corrosion-resistant screws or nails driven through the upper leg) must be covered by the reglet itself, not left exposed, to prevent rust and water penetration through fastener holes.

Why it fails and where it sits in the assembly

Counterflashing protects the most vulnerable part of a roof assembly: the transition from horizontal to vertical surfaces, where water naturally seeks entry. Without it, water wicks under the base flashing edge and into the building wall. Movement caused by thermal expansion and contraction, settling, or wind can open gaps between counterflashing and wall, and poor sealant maintenance allows water to penetrate. Counterflashing is not typically a working part of the roof drainage system itself; rather it is a guard that keeps the base flashing dry and functional. In a well-designed assembly, the base flashing sheds water onto the roof, and the counterflashing shelters the fasteners and upper edge of the base from direct weather exposure.

The term reflects its role: it opposes or counters the base flashing by running in the opposite direction (down the wall and over the base) to intercept water that would otherwise find its way into the base flashing's joint. On pitched roofs meeting brick or block walls, counterflashing is essential. Parapets, skylights, roof penetrations, and chimney saddles all require counterflashing as part of their weatherproofing strategy. Failure to install counterflashing or to maintain its sealant is one of the most common causes of roof leaks at wall junctions.