Chimney Leaks and Flashing Repair: What Massachusetts Homeowners Should Know

Most chimney leaks come from one of four places. The first is the flashing, the metal that seals the joint between the chimney and the roof. The second is the crown, the concrete or mortar cap on top of the chimney that sheds water away from the flues. The third is the brick and mortar itself, which can soak up water through cracked joints or porous brick. The fourth is a missing or damaged chimney cap, which lets rain fall straight down the flue.

Sometimes it is more than one. On older homes around Everett, Melrose and Arlington, it is common to find tired flashing and a cracked crown on the same chimney, because both have been through the same decades of freeze-thaw.

Proper chimney flashing has two layers. Step flashing is woven into the shingles along the sides of the chimney, one piece per course of shingles. Counter flashing (sometimes called cap flashing) is set into a cut in the mortar joints of the brick and laps down over the step flashing. Together they let water run off the chimney and onto the roof instead of behind it.

Flashing fails in a few predictable ways. The counter flashing pulls out of the mortar joint as the mortar ages. Sealant that was used in place of proper reglet cuts dries and cracks. Metal corrodes. Or the flashing was never done correctly in the first place, with roofing cement smeared over the joint instead of real step and counter flashing. Ice dams, which are common in Massachusetts winters, also push water up under shingles right at the chimney.

Look at when the leak shows up. If it appears only during wind-driven rain from one direction, the flashing or brick on that side is a strong suspect. If it shows up during any rain, even straight down, look at the crown and the cap. If it shows up mostly when snow is melting, ice damming near the chimney may be involved.

Look at where the stain is. A stain on the ceiling right beside the chimney often points to flashing. Water inside the firebox or a damp, musty smell from the fireplace suggests a cap or crown issue. White, chalky deposits on the brick inside the attic point to water moving through the masonry itself. A mason on the roof can usually confirm the source by looking at the condition of each part.

A real flashing repair is not a fresh bead of caulk. When the flashing is worn out, the right approach is to remove the old counter flashing, cut a clean reglet (a groove) into the mortar joints, install new step flashing woven with the shingles, and set new counter flashing into the reglet so it laps over the step flashing. The reglet is then sealed with mortar or a compatible sealant.

On many chimneys the mortar joints at the flashing line are already failing, so some repointing at the base of the chimney is part of the job. If the crown is cracked, it makes sense to repair it at the same time, because the scaffolding or roof setup is already in place.

As a rough guide, flashing repair or replacement on a typical residential chimney commonly runs somewhere around $500 to $2,000, more for large chimneys, steep roofs or when roofing work is needed around it. Ranges vary, and an on-site look is the only way to get a firm price.

Patching with roofing cement or caulk can slow a leak for a season, and sometimes that is the right short-term move in the middle of winter. The problem is that these products crack under sun and frost, and they trap water behind them once they start to fail. Many older chimneys have several layers of old patch stacked on top of each other, hiding the real damage underneath.

If your chimney has been patched more than once and still leaks, it is a sign the flashing needs to be redone properly, and possibly the brick and crown along with it.

Once the flashing is done properly, a few simple habits keep the chimney dry. A chimney cap with a screen keeps rain, leaves and animals out of the flue, and it protects the crown from the full force of the weather. A crown that overhangs the brick slightly, with a drip edge, keeps water from running straight down the face of the chimney.

Keep the roof around the chimney clear of debris that can trap water, and watch for ice dams in heavy snow years, since they push water up under the shingles right at the flashing line. Look at the chimney from the ground each spring and fall with binoculars if needed: loose counter flashing, missing mortar or a cracked crown are all easier to fix when they are small.

If your roof is due for replacement soon, coordinate the timing. Replacing flashing during a reroof is often simpler, and it avoids tearing into brand new shingles a year or two later to fix a leak.

Call sooner rather than later if the stain is growing, if you see loose brick or mortar on the roof, if there is any sign of rot in the ceiling or framing, or if the leak shows up in more than one storm. Water that gets into the house also soaks the brick, and soaked brick is what freeze-thaw tears apart over the winter.

Flashing sits right at the line between roofing and masonry, so look for someone who can handle both the metal and the brick joints. A free on-site estimate should explain where the water is coming from and what the fix involves before any work starts.