How to Counterflash a Chimney

The point at which a chimney exits the roof creates a natural seam that, if left exposed, can allow water to leak into the home. To guard against water leaks, two pieces of flashing are used. Step flashing is mounted to the roof and sealed to the side of the chimney. Counterflashing is a second piece installed within a mortar seam within the chimney and covers the step flashing. Step flashing is straightforward to replace, but replacing the counterflashing takes a bit more effort as you must dig the top of the flashing piece out of the chimney mortar.

Things You'll Need

Hammer
Chisel
Eye protection
Heavy leather gloves
Aviation snips

Tape measure
Bench vice
Brush
Premixed mortar
Caulking gun

Instructions

- 1
  Chip away the chimney mortar that surrounds the top of the counterflashing with a hammer and chisel while wearing eye protection.
- 2
  Pull the counterflashing piece out of the chimney.
- 3
  Cut a new piece of counterflashing with aviation snips, using the old flashing piece as a template.
- 4
  Measure the length of the old counterflashing piece that fit within the chimney’s mortar joint with a tape measure.
- 5
  Mark the measured distance on the replacement counterflashing piece.
- 6
  Secure the new counterflashing piece into the jaws of a bench vice, then bend the piece to conform to the old flashing piece.
- 7
  Sweep the loose mortar deposits out of the chimney’s mortar joint with a brush.
- 8
  Insert the bent end of the new counterflashing into the chimney’s mortar seam.
- 9
  Apply a bead of premixed mortar across the top and bottom edges of the portion of the counterflashing that fits within the chimney’s mortar joint with a caulking gun.
- 10
  Slide the edge of the counterflashing into the chimney’s mortar joint and then apply a bead of mortar across the chimney’s mortar joint to seal the flashing in place.

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