How to Lead a Chimney
Flashing a chimney, sometimes referred to as leading, can keep your chimney sound and prevent expensive problems down the line. Without good flashing, a chimney can develop leaks at the join between the bricks and the roof surface. Over time, these can rot your roof sheathing and cause mold growth and leaks inside the house. You might even need a new roof. Installing lead flashing on your own takes a little expertise and a few specialized tools but doesn't have to feel impossible.
Prepare the chimney by removing old flashing and roofing cement with a chisel and prybar. Clean the chimney's surface of debris that might interfere with the new flashing. Seal around the base of the chimney with strips of self-adhering roof membrane, overlapping them onto the bricks about 4 inches.
Cut the pre-bent sheet lead base flashing with straight tin snips to fit across the front of the chimney to length, then cut a 45-degree angle at one end to accommodate the bend. Secure the base flashing with roofing nails and place a dot of polyurethane caulk at the corner where the angle begins, to keep water from seeping under the flashing.
Attach the first piece of step flashing by first cutting a piece of pre-bent 8-inch-square step flashing so that the end bends about 1 1/2 inches around the front corner of the chimney. Set this end along one side of the chimney, so that the front corner overlaps the base flashing and covers the dot of caulk. Secure the flashing with roofing nails.
Complete the step flashing, omitting the caulk, with additional pieces of step flashing until you have covered the entire side of the chimney, placing the bottom of the flashing under each course of shingles. Overlap each piece of step flashing on the one below it, diverting water onto the shingles and away from the chimney. Repeat the process for the other side.
Place the cricket, also called a saddle, at the uphill side of the chimney, overlapping the last pieces of step flashing. This piece keeps water from running down the roof and into the chimney Nail the saddle down firmly, then seal it with more pieces of roof membrane. Shingle over the part of the saddle that rests on the roof and cut the shingles along the join, leaving about 2 inches of lead exposed.
Cut grooves 1 inch deep in the chimney mortar joints immediately above the base flashing with a grinder fitted with a 3/16-inch masonry bit to apply the cap flashing. Cut a piece of cap flashing to the width of the front of the chimney. Push the bent end into the groove. Repeat this step for each piece of step flashing and for the saddle. Fill the remaining space in the groove with polyurethane caulk.
Things You Will Need
- Self-adhering roof membrane
- Straight tin snips
- Roofing nails
- Polyurethane caulk
- 26 gauge pre-bent lead base flashing
- 26 gauge pre-bent lead step flashing
- 26 gauge pre-bent lead saddle
- Cap flashing
- Utility knife
- Grinder with masonry bit
- Wear a safety harness while working on the roof to prevent falls.
- Wear goggles, a dust mask and earplugs while working with the grinder.
- Have your sheet lead pre-bent to the correct dimensions by a sheet metal specialist.
- Paint your flashing to extend its lifespan.