Counterflashing is metal flashing installed above and overlapping base flashing at a masonry or vertical surface, like a chimney or parapet wall, covering the top edge of the base flashing to prevent water from getting behind it and completing a fully watertight seal at that intersection.
This two-part flashing system, base flashing plus counterflashing, is particularly common at chimneys, where the base flashing integrates with the roofing material while the counterflashing is embedded into or against the masonry above it.
Because counterflashing often needs to be embedded into a mortar joint or reglet cut into masonry, its installation typically requires more specialized skill than simpler flashing applications elsewhere on the roof.
What Does Counterflashing Involve?
- Metal flashing overlapping the top of base flashing
- Embedding into a mortar joint or cut reglet in masonry
- Sealant at the embedded edge to prevent water entry behind it
- Coordination with the base flashing beneath
Why Is Counterflashing Important?
Without counterflashing properly covering and sealing the top edge of base flashing, water can work its way behind the base flashing over time, defeating its purpose even if the base flashing itself remains intact and properly installed against the roof surface. This two-layer system provides redundant protection at chimneys and similar masonry intersections, locations that experience some of the highest leak risk on any roof.
Deteriorated mortar or sealant at the counterflashing’s embedded edge is a common maintenance issue, particularly on older chimneys, and periodic inspection and resealing of this joint can prevent water intrusion that might otherwise go unnoticed until interior damage becomes apparent.
Counterflashing vs. Step Flashing
Counterflashing works with base flashing at masonry surfaces like chimneys, covering the base flashing’s top edge. Step flashing addresses the different condition of a sloped shingled roof meeting a vertical wall, using individual pieces integrated with each shingle course rather than a two-part masonry system.