Chimney Flashing

Chimney flashing is a crucial component of your chimney for preventing water intrusion. IRC code 903.2.1 states that “Flashings shall be installed at wall and roof intersections, wherever there is a change in roof slope or direction and around roof openings.

Chimney flashing shall be installed to divert the water away from where the eave of a sloped roof intersects a vertical sidewall. Where flashing is of metal, the metal shall be corrosion resistant with a thickness of not less than 0.019 inches (0.5 mm) (No. 26 galvanized sheet)”.

What exactly is it, though? We definitely will not be installing a strobe light on top of your chimney. Flashing is a metal used to seal off openings in your home from water intrusion. There are two main components to the flashing around your chimney. First, you have step flashing. This is a series of small overlapping L-shaped pieces of metal placed underneath each course of shingles.

The piece of metal is nailed on top of an extended portion of each course of shingles above the strip of shingles you see in each row. The next course of shingles then covers this piece of metal, and the process continues along the whole length of the chimney.

Next, is the piece you see. A large piece of metal called the counter flashing is placed over the step flashing on the side of the chimney. Finally, the edges of this counter flashing are sealed with a roofing sealant.

Flashing is exposed to sun, rain, wind, and constant temperature swings, so even a good installation won’t last forever. The sealant along the top edge of the counter flashing is usually the first thing to go. It dries out, shrinks, and cracks, leaving a gap that lets water run down behind the metal.

Installation shortcuts are another common cause. Counter flashing should be set into a groove (called a reglet) cut into the mortar joints of the chimney, then sealed. When it is simply glued or caulked to the face of the brick, it tends to pull away within a few years. We also see problems after roof replacements, when old, bent, or rusted flashing is reused instead of replaced, or when step flashing is skipped and the roofer relies on a bead of sealant.

Wide chimneys add another challenge. Water and leaves pile up behind them, and the extra load can overwhelm the flashing on the uphill side. That is why the code requires a chimney cricket on chimneys more than 30 inches wide.

Every flashing job starts with an inspection. We look at the flashing from the roof, check the sealant, metal, and surrounding shingles, and, when we can, check the attic for signs of water getting in. That tells us whether a repair will hold or whether the flashing needs to be replaced.

Minor repairs can include resealing the counter flashing with a high-quality, flexible sealant, refastening loose sections, and replacing a few damaged pieces.

Full replacement means removing the shingles around the chimney and installing new step flashing woven into each course of shingles. We then cut a reglet into the mortar joints, set new counter flashing into it, and seal the edges. All metal we use is corrosion-resistant and meets the code requirements listed above. If your chimney is wide enough to need a cricket, we can build one at the same time.

While we are on the roof, we also check the chimney cap, crown, and brick joints, since a leak blamed on flashing sometimes starts somewhere else.

A roof replacement is the best time to deal with chimney flashing, because the shingles around the chimney are already coming off. Reusing old flashing on a new roof is a common shortcut, and it often leads to leaks within a few years. Ask your roofer how they plan to handle the chimney, or call us to coordinate flashing work with your roofing project so the job is done right the first time.