Counter-flashing | Chalkline

A counter-flashing is a secondary flashing fixed to the higher building element (wall, chimney, parapet) so that its lower edge laps over the upstand of the primary flashing of the lower element (the roof sheet, membrane or gutter). Together the two flashings create a two-lap weather barrier at any junction where the roof or waterproofing meets a vertical surface. Water arriving at the junction cannot track behind the primary flashing because the counter-flashing’s lap covers the upstand from above.

The counter-flashing is required wherever the primary flashing alone is not enough to keep water out under wind-driven rain. Common residential applications:

Parapet wall to flat or low-pitch metal roof: roof sheet upstand finishes against the parapet face; the counter-flashing chase-cut or fixed into the parapet caps the upstand.
Chimney to tile or metal roof: side-step flashings (apron, soaker, back gutter) are the primary; the counter-flashing chase-cut into the masonry overlaps the side-steps.
Wall abutment, roof to brick or rendered wall: roof sheet upstand laps onto the wall; counter-flashing fixed to the wall covers the upstand.
Membrane roof to parapet: membrane turn-up is the primary upstand; metal counter-flashing covers and protects the membrane edge.

Material and lap rules:

Same metal family as primary flashing. Mixed metals at the junction (e.g. zincalume primary, copper counter) cause galvanic corrosion. Match the metal.
Lap downwards over the upstand by at least 50 mm. Less and wind-driven rain blows water under the counter-flashing.
Fix the counter-flashing only to the higher element. Mechanically fixing it to the primary flashing locks the two together and removes the differential-movement allowance that two-flashing junctions need to last.
Chase-cut into masonry where possible. Mortar-bedded counter-flashings deliver the longest service life. Surface-mounted counter-flashings (sealed with polyurethane) are common on retrofit work but need re-sealing every 5 to 10 years.

Common defects:

Counter-flashing missed entirely. Builder relied on primary flashing plus sealant. Sealant fails at 5 to 7 years, water finds its way behind.
Counter-flashing lap too short, water pushed up under in wind-driven rain.
Counter-flashing locked to primary flashing by an over-zealous roofer; thermal expansion deflects the joint and breaks the sealant line.
Mixed metals at the junction. Pinhole corrosion in 3 to 5 years.

Also known as: cover flashing; chase flashing (when chase-cut into masonry); apron flashing (loose usage, technically a different element).

Category: Roofing.

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