Installing metal roofing flashing around chimneys and vent stacks prolongs the life of your roofing material and prevents water damage to your building. Sheet metal flashing is used to divert precipitation away from potential weak points in the roof seal, such as junctions with plumbing and ventilation ducts. Flashing can be made from many materials, including copper, aluminum, lead and stainless steel.
Step 1 – Work Safely
Choose a day with calm weather to work on the roof. Doing roofing work on windy or rainy days is dangerous and exposes your home to the elements. Use of a rope and harness is recommended.
Step 2 – Take Measurements
Measure the perimeter of the chimney and the slope of the roof. For front base flashing, use a piece of sheet metal 1 foot longer than the width of the chimney. For step flashing, use 8 x 8 inch squares. For the saddle, measure the front of the chimney from the roof to the third mortar joint. Obtain flashing pieces from a sheet metal shop or cut and bend them yourself.
Step 3 – Remove Old Roofing Material
Scrape off and discard the old shingles and roofing cement from around the perimeter of the chimney.
Step 4 – Apply Water and Ice Barrier
Seal the area around the chimney with a masonry primer or vapor barrier before installing the flashing.
Step 5 – Install Base Flashing
Coat the bottom side of the base flashing with roofing cement. Stick the piece firmly against the front of the chimney, over top of the shingles. Nail the flashing into place.
Step 6 – Install Step Flashing
Begin installing step flashing at the low end of the junction between the roof line and the chimney. Bend and place the squares against the side of the chimney. Overlap the pieces as you work your way to the top. The pieces of flashing should resemble the profile of a staircase when viewed from the side. Cover the pieces of step flashing with shingles and then nail both into place.
Step 7 – Install Saddle or Backer Flashing
You can make a saddle to connect the roof line to the chimney using two plywood triangles. The saddle redirects water from the back of the chimney towards the front and side. If using a saddle, wrap step flashing up to its tip. Otherwise, install the backer flashing the same way you installed the base flashing.
Step 8 – Cut Grooves
Cut 1-inch grooves into the masonry joints with a diamond bit saw.
Step 9 – Install Cap Flashing
Insert the cap flashing into the grooves and bend it down over the step flashing.
Step 10 – Wash and Paint Flashing
If you used galvanized stainless steel flashing, wash it with lacquer thinner prior to finishing it. Then, paint the flashing in the color of your choice. This prevents corrosion and helps you match the flashing to the style of your building.