How to Flash a Cement Chimney

Place a section of L flashing tight against the base of the chimney and nail the horizontal section on top of the roof shingles with the hammer and galvanized roofing nails.

Place your step flashing up the sides of the chimney in the same fashion. Overlap the lower sections on top of the bottom L flashing, then overlap each subsequent piece higher up the incline of the roof on top of the lower piece of step flashing so that you have a waterfall effect where the flashing runs off each higher piece onto the lower, eventually down to the base flashing and then onto the shingles.

Install your saddle flashing section on the upper side of the chimney. Use the same nailing methods, but tuck the upper side of the saddle flashing underneath the shingles or roofing material so that the water doesn’t pour under its edge. Butt the lower edge of the saddle section up against the chimney and on top of the topmost piece of step flashing.

Hold a piece of your L-shaped cap flashing up against the chimney on top of the already-in-place flashing so that it overlaps the vertical section of the bottom L flashing and step flashing by at least 2 inches, with the horizontal section of this piece sliding into a grout joint. Find the best grout joint by holding the cap flashing up to determine the best placement so that you not only have a joint to slide into, but also the 2 inches of overlap required.

Find the closest grout joint in the concrete blocks above your bottom L flashing and use the grinder to grind out a slot in the grout joint. Repeat the process on the sides, keeping in mind that you have to “step” the cap flashing up the wall, so you will need to cut into different joints for each piece of step flashing as you work your way up. Repeat this around the entire perimeter.

Apply urethane caulk to the back side of the vertical sections of cap flashing. Slide the horizontal sections into the slots you ground out of the grout joints. Press them firmly in place to bond the caulking with the first layer of flashing. Mount all of the cap flashing pieces and then caulk the joints where the flashing is slotted into the concrete chimney. Run another layer of roofing material on top of the flashing around the base of the chimney to completely lock it in place with the roof.