How to Install a Chimney Roof Flashing

A chimney roof flashing prevents rain water from seeping into the ceiling. While roof flashing can be fabricated using plastic, rubber, or roofing felt, a chimney roof flashing is mostly made from rust-resistant metal such as galvanized steel, aluminum, or copper. If you are looking to do it yourself, aluminum is the best option.

Chimney Flashing Parts

A chimney roof consists of many different flashing parts. The first part is the flashing that goes around the base of the chimney, also called base flashing. A single sheet is wrapped around the base and extended to the sides. The second component is the step flashing with runs along the sides of the chimney. Typically, step flashing continues till under the roof shingles along the chimney. Saddle flashing is placed at the top of the chimney if the top is wide (more than 24 inches). For a narrow chimney, instead of saddle flashing, backer flashing is installed at the top.

Step 1 - Plan

Before you begin installing a chimney flashing, figure out whether you want to cut the metal sheets yourself or get it done professionally. If you are new to working with sheet metal, it is better to get the flashing parts and saddle (if needed) cut by a professional.

Step 2 - Measure

Before you cut the metal sheet or give it to a professional for cutting, take the measurements. Do measure the chimney for each separate flashing component.

Step 3 - Install

Begin by applying masonry primer to the base of the chimney. Then, spread the plastic roofing cement along the base flashing sheet (underside) and simply place the flashing around the base of the chimney and over the shingles. Use mortar nails to secure the base flashing to the chimney. Make sure you leave a gap of two inches at the base for the water to drain.

Now you need to install the step flashing. Apply the plastic roofing cement to both sides of the chimney, starting at the base. Then, simply wrap the step flashing over the chimney. Keep the lower end in place by using a roofing nail.

Once the step flashing is in place, apply some more plastic roofing cement to the lower half of the step flashing. On this, place a shingle and secure it with a roofing nail. Follow this procedure for all sections of the step flashing. Make sure that you overlap the previous section with the new one.

Once you reach the top, if you are using a saddle, place the saddle on the top of the chimney and attach the last section of the step flashing sheet such that the point where the saddle meets the chimney is covered. If you are not using a saddle, simply attach the backer flashing with the help of plastic roofing cement and mortar nails. Finally, place shingles over the upslope part of the backer flashing with shingles.