How To Replace Chimney Flashing

Roof flashings are an integral part of any roof system. There are many different types of flashing designs such as step flashing, saddle flashing, drip flashing, vent flashing, continuous flashing, etc. Each flashing design has a particular function in protecting the home envelope from water damage. Chimney flashing is a combination of step flashings and counter flashings. Replacing chimney flashing begins with a detailed inspection of the roof surfaces. Look for rotted wood, deteriorated shingles, or shingles that have lifted up, corroded flashings, flashings that have pulled away from the roof plane or the vertical surface, and deteriorated chimney brick or mortar joints. Make all the necessary repairs prior to replacing the chimney flashings.

Preparation  

Properly installed chimney flashings contribute to the integrity of the roof.

 

1. Remove the shingles at least 18″ from the chimney vertical surfaces all the way around. Lift the lower edge of each row of shingles with the aid of a thin crowbar. Pull the roofing nails that hold each row of shingles down. 
2. If the chimney has counter flashing, rake the mortar out of the joints at the very top edge of the counter flashing all the way around until you hit the metal flashing inside the mortar joints.
3. Remove any nails holding the counter flashing in place and remove the flashing.
4. Remove the nails in the side of the chimney that hold the step flashings in place. 
5. Remove the step flashings.

Installing the new base flashing

1. Cut a piece of flashing material that is longer than the width of the chimney. 
2. Fold it in the middle lengthwise.
3. Mark the width of the chimney on the flashing material starting from the middle of it.
4. Cut in from the top edge half way down to the bend at the edges of the chimney width markings.
5. Lay the flashing in place across the lower edge of the chimney. 
6. Secure it in place with nails installed into the chimney, not the roof.
7. Bend the tabs that were cut out down against the roof.

Installing the step flashings

1. Place the first step flashing in place at the lower edge of the chimney along the side overlapping the base flashing tabs.
2. Replace the lowest course of shingles. 
3. Install the next step flashing, followed by the next course of shingles which should overlap the step flashing at that point.
4. Follow this pattern, installing the step flashings up the side of the roof.

Installing the top flashing

1. Bend and cut the top chimney flashing to the same dimensions as you did the base flashing except cut from the bottom edge instead of the top edge.
2. Lay the top flashing in place and fasten it to the side of the chimney with nails.
3. Bend the tabs down against the roof. 
4. Finish installing the step flashings and the final courses of shingles.

Installing the counter flashing

1. Cut the counter flashings to match the length of the sides of the chimney. Cut the ends at an angle to match the angle between the side of the chimney and the slope of the roof.
2. Place the L angle side of the counter flashing into the empty mortar joint so that the lower edge of the counter flashing overlaps over the top of the step flashings.
3. Remortar the joints of the chimney where the counter flashing fits to seal each joint watertight.
4. Examine the flashings carefully and seal each joint where flashings meet each other, the chimney, or the roof surfaces with a good quality roof cement or silicone caulk. Force the cement or caulk into the joints with your fingers then add a generous amount over the top of the sealant and trim it with a putty knife.