Counter flashing is the upper layer of flashing that covers step flashing where a roof meets masonry or a wall. Around chimneys, it is often set into mortar joints so water cannot run behind the roof flashing.
Counter flashing works with step flashing
Step flashing is woven with shingles along a wall or chimney side. Counter flashing covers the top of that step flashing so water cannot run behind it.
At a chimney, proper counter flashing is cut into the mortar joint, bent over the step or apron flashing, and sealed with the right masonry sealant.
Why counter flashing leaks
- Mortar joints crack and release the flashing.
- Old sealant dries out and pulls away.
- A prior repair used caulk instead of real metal counter flashing.
- The flashing was surface-mounted instead of cut into masonry.
- Shingles were replaced but old chimney flashing was reused.
How a real repair is done
The right repair depends on the existing detail. Often the roof around the chimney must be opened, old metal removed, damaged decking checked, new step flashing installed, and fresh counter flashing set into clean mortar joints.
Smearing roof cement over the chimney may slow water temporarily, but it rarely fixes the failure point for long.