How is roof repair done around a chimney?
Short answer. A proper repair around a chimney removes the shingles beside it, replaces the step flashing on the sides and the apron across the front, adds a cricket on the uphill side if the chimney is wider than 30 inches, and cuts new counterflashing into the mortar joints to cover it all. Any rotted decking is replaced while it is exposed, and the shingles are woven back in.
The repair, step by step
- Diagnose first. Confirm the water is coming through the roof joint and not a cracked crown, missing cap or porous brick, which are masonry repairs.
- Remove shingles and old flashing. Usually a few courses out from the chimney on each side, plus the old counterflashing.
- Check the decking. Chimney leaks often rot the plywood or OSB right at the corners, and soft wood is cut out and replaced.
- Add a membrane. Self-adhered ice and water membrane is run onto the deck and a few inches up the chimney as a backup layer.
- Install the apron. One continuous piece across the downhill face, lapped over the shingles below.
- Weave step flashing. One L-shaped piece per shingle course up each side, each lapping the one below it.
- Build or reflash the cricket. The code calls for a cricket or saddle on the uphill side of any chimney wider than 30 inches measured across the slope, so water splits around the chimney instead of pooling.
- Cut in counterflashing. A slot is ground into a mortar joint, and the counterflashing is inserted, stepped to follow the brick on the sides, and sealed.
- Shingle back in. Shingles are woven back over the flashing and the work is photographed.
Material and timing notes
Use coated aluminum, painted steel, copper or lead wherever metal touches mortar, since bare aluminum corrodes against it. On older roofs the shingles near the chimney may crack when lifted, which widens the repair. If the roof is close to replacement age, reflashing the chimney during the replacement costs less overall. See chimney leaks and crickets and roof flashing explained.