A chimney is an area of the roof particularly vulnerable to water damage. Chimney flashing creates a water-resistant shell around the chimney, but flashing can be damaged, resulting in leaks. Repairing chimney flashing is dicey. Once the system of overlapping metal plates is compromised, sealing provides only a temporary fix. The best tactic for repairing damaged chimney flashing is to remove the old material and replace it. This will cost a little more and take some time, but in the long run, your house will be better protected from severe damage caused by leaky flashing.
Cut away sealant from along the edges of the flashing, where the cap flashing meets the chimney mortar.
Pry up the shingles around the edges of the chimney flashing. Do not remove the shingles completely. Lift the shingles with your pry bar enough to gain access to the flashing edges.
Drill out plastic expanding anchors with a 3/16-inch drill bit.
Pry the flashing away from the chimney and rooftop with a pry bar. Remove all the flashing from around the chimney.
Scrape away any leftover caulking or sealant from the rooftop and chimney with a paint scraper.
Mark and cut your front base flashing. This piece of flashing measures about 8 inches longer than the front of the chimney and is bent in half lengthwise. Measure in 4 inches from each side and make a mark on the bend in the centre of the piece. Cut from each inside corner to the mark with tin snips. Your cuts should follow approximately 45-degree angles.
Shape and position the front base flashing. Set the piece in place on the front of the chimney, and hand bend the cut corners so they wrap around the sides of the chimney.
Cut and position your front step flashing pieces. At the prefabricated bend, cut in about 1 inch using tin snips. Place a single dollop of caulk on the inside corner of the front base flashing and set the front step flashing in place. The corner of the step flashing piece will wrap around to the front of the chimney.
Place each piece of step flashing along the sides of the chimney and refasten the shingles in place to secure them. Nail the shingles with galvanised roofing nails.
Position the chimney saddle behind the chimney. This specialised piece of sheet metal is designed to redirect water away from the chimney. Fasten the chimney saddle with galvanised nails driven every 6 inches along the edges of the piece.
Cut and bend your front cap flashing to fit around the corners of the chimney and into the grooves cut into the chimney mortar. Knock the front cap into place using a wood block and hammer. The grooves in the mortar were cut when the original flashing was installed.
Drill a hole into the top corner of the front cap flashing. Use a 3/16-inch masonry bit and drill the holes about 1 ½ inches deep, through the flashing and into the brick behind it. Insert an expanding plastic anchor into the hole to secure the flashing.
Cut and band side cap flashing to fit into the grooves cut into the mortar. The prefabricated flange at the top of the cap flashing will fit into the mortar grooves.
Drill holes into each piece of side capping with the 3/16-inch masonry bit, and fit expanding plastic anchors to hold them in place.
Install the back cap flashing onto the backside of the chimney. Knock the flange into the groove with a wood block, then drill and fasten with expanding plastic anchors.
Caulk along the top seam of the cap flashing, where the flashing flanges have been knocking into the grooves in the mortar.
Always be careful when working on a rooftop. Fasten and wear a safety harness to avoid injuries resulting from falls.