Chimney flashing prevents water leaks from seeping through the roof and creating water damage in the roof and down into the ceiling. If you discover that you have water leaks occurring near or around your chimney, then it may be time to replace the roof flashing and reseal around the chimney.
Using the flat bar and hammer, gently loosen the shingle adhesive and pull back the existing shingles approximately 18 inches from the base of the chimney. Remove the roof nails, but do not remove the shingles.
Pull off the existing flashing, along with the nails. Discard the flashing and the nails.
Scrape away and remove the adhesive and roof cement from the roof and chimney where the flashing was installed.
Using tin snips, cut a piece of galvanized flashing 10 inches longer than the width of the chimney. This will be the starting strip for the flashing. Start at the front edge of the chimney and on the lowest side of the chimney, bend the flashing horizontally following the roof angle where it meets the chimney.
Using the tin snips, cut the flashing on each side precisely 5 inches from the edge. Cut no more than halfway down from the top to the line of the bend in the flashing. Do not cut the bottom of the flashing because it needs to lie flat against the roof.
Apply the starter strip of flashing 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. Do not nail the flashing directly into the roof.
Pull the existing shingles back over the front of the flashing. As you progress up the sides of the chimney, periodically pull and smooth the shingles back into place. Smooth from the bottom upward in order to create a leak-proof barrier on the roof.
Install the step flashing along the sides of the roof, placing the preformed bend on the seam between the roof and the chimney. Overlap the shingles in steps, using a shingle, then flashing, then shingle and so forth up the sides. As you progress, fold the layer of shingles back over the top. As you replace the shingles, you should not be able to see the flashing. Do not nail the step flashing into place on the roof. Only nail the flashing to the chimney.
Repeat the same process when you reach the top edge of the chimney as you did with the starter strip. The only difference will be that instead of cutting halfway from the bottom of the flashing, you will cut halfway from the top. Place the piece of flashing against the chimney and roof in the same manner as before and carefully nail it into place, not nailing to the roof, but only the chimney.
Apply silicone caulking under the edge of the shingles as adhesive. Also apply silicone caulking any place where the flashing is exposed and is not overlapped with shingles.