Chimney flashing is the protective layer of metal that stops water leaks from seeping through where the chimney meets the roof. Chimney flashing is extremely important in roof construction and must be installed correctly in order to avoid expensive roof and ceiling repairs that are the result of water damage.
Since you will be installing chimney flashing on the roof, it is a good idea to have someone to help you or wear a safety harness to avoid injuries that might occur if you accidentally fall off the roof. Also wear safety glasses and protective gloves, as you will be cutting very sharp galvanized steel flashing materials.
Step 1 - Measure
Measure the width of the chimney. Use the tin snips to cut a piece of galvanized flashing 10 inches longer than the width of the chimney. Start at the front side and lowest side of the chimney, bend the flashing horizontally, and follow the roof angle where it meets the chimney line. This will be the starter strip for the flashing.
Step 2 - Cut
Using the tin snips, cut the flashing precisely 5 inches from the edge on each side. Do not cut deeper than halfway down from the top to the bend in the flashing. The flashing will need to be able to lie flat against the roof, so do not cut the bottom portion of the flashing.
Step 3 - Bend
Place the starter strip along the edge of the chimney. Bend the cut sides down to the roof on each side of the chimney. Nail the sides of the flashing into the chimney masonry to attach it. Do not nail the flashing directly into the roof since this will damage the roof.
Step 4 - Place Flashing
Place the step flashing along the roof, setting the preformed bend in between the roof and the chimney. Use the step method of overlapping the shingles, using a shingle, then flashing, and so forth. You should not be able to see the flashing on the roof, since it will be covered by shingles. Do not nail the step flashing into place on the roof because this will damage the roof and create a hole for water to seep through (which you are attempting to prevent). Nail the flashing to the chimney.
Step 5 - Repeat
When you reach the top edge of the chimney, repeat the same process as earlier with the starter strip. However, instead of cutting halfway from the bottom of the flashing, you will cut halfway from the top.
Place the flashing against the chimney and roof in the same manner as before and nail it into place on the chimney, avoiding nailing into the roof.
Step 6 - Caulk
Apply silicone caulk under the edge of the shingles as adhesive. Also apply silicone caulk any place where the flashing is exposed along the side of the chimney.