When a roof leaks around the chimney, the flashing is usually to blame. That's the metal that seals the joint where the brick meets the shingles. It leaks when it rusts, cracks or pulls away, when the counterflashing comes loose from the mortar, or when it was installed as a continuous piece instead of being woven into the shingles. Sometimes the water comes in through the top of the chimney instead. A good repair starts by working out which it is.
How a chimney is sealed to the roof
A chimney is a big box of brick sitting in the middle of a sloped roof, and water runs down the roof straight into it. The roof and the chimney also move independently. So the seal between them is built in layers.
There's an apron across the downhill face, step flashing up both sides, and on the uphill side, often a cricket. That's the arrangement Fine Homebuilding's chimney flashing guide lays out.
Step flashing is made of L-shaped pieces woven into the shingle courses and lapped up the side of the chimney. Each piece sends water back out on top of the shingle below it.
Over that goes the counterflashing: a second layer of metal set into the mortar joints of the chimney and folded down to cover the top of the step flashing. The layers aren't fastened together, and that's on purpose. Houses and chimneys settle at different rates, and separate pieces can move without tearing.
The cricket is a small peaked roof built behind the chimney. It's sloped like a tent to deflect water around the chimney instead of letting it pool against the brick. The residential code that UpCodes publishes for Pennsylvania requires a cricket or saddle on the ridge side of wider chimneys. It also requires flashing where a roof meets a wall, and says chimney flashing on a shingle roof has to follow the shingle maker's printed instructions.
Why the flashing fails
Old metal is the simple answer. When flashing rusts, cracks or pulls away, water runs in behind it.
Shortcuts are the other big cause. A continuous strip of metal bent along the side of the chimney looks tidy, but it lets water get behind the flashing and under the roof. The same goes for flashing made as a solid piece fixed to the roof and the brick together. When the house and chimney settle, it tears.
Then there's the counterflashing. It's held by a folded lip pushed into a mortar joint. When the mortar wears away, that lip works loose, and the counterflashing stops covering the step flashing behind it.
And there's caulk. A sound flashing job still leaves small spots at the corners that get sealed with urethane caulk, and that caulk can loosen over time. If the metal is in good shape, fresh caulk may stop the drip. But caulk isn't a substitute for properly lapped flashing. A chimney edge that's held together mostly by sealant will leak again.
When the water comes in from the top
Not every chimney leak starts at the roof line. The concrete or mortar cap on top of the chimney can crack, and those cracks let water run down over the brick and leak into the house. A chimney without a rain cap lets rain straight into the flue.
Those need a chimney or masonry repair. That's why it pays to find out where the water's getting in before anyone starts tearing off shingles.
How to narrow it down from the ground and the attic
You can do a lot of the detective work without climbing onto the roof.
- Look at where the stain is. A brown patch on the ceiling or wall right next to the chimney, or around the fireplace, points to the chimney. Water can run along the framing first, so the stain isn't always directly under the leak.
- Check the attic after rain. Shine a flashlight on the wood around the chimney where it passes through the roof. Dark, wet or soft wood on the uphill side often means water is pooling behind the chimney. Wet wood along the sides points to the step flashing.
- Look from the ground with binoculars. Watch for counterflashing pulled out of the mortar, metal that's rusted or bent, gaps between the metal and the brick, thick smears of tar or caulk along the joint, and crumbling mortar or cracks near the top of the chimney.
- Notice when it leaks. A drip that only shows up in wind-driven rain from a particular direction can point to the side of the chimney facing that wind.
If the stain is away from the chimney, near a bathroom vent, our guide to a roof leak around a plumbing vent pipe covers that leak. A stain along the inside corner where roof slopes meet points to a roof valley leak.
Leave the roof itself to a roofer. Working beside a chimney on a pitched roof is no place to improvise.
How a chimney flashing leak gets fixed
The repair matches what failed.
If the step flashing is sound and only the corner caulk or a loose bit of counterflashing is the problem, resealing and resetting it can be enough.
If the flashing is rusted, torn, installed as a continuous strip or held together with tar, it should come out and be replaced the right way. That usually looks like this:
- The shingles along the chimney are lifted, and the old flashing and sealant come off.
- The deck under it gets checked. Wet or soft wood is dealt with before anything new goes on.
- New step flashing is woven in with each course of shingles up both sides, with the apron across the front.
- A cricket is built behind the chimney where the code or the roof calls for it.
- Counterflashing is set into the mortar joints and folded down over the step flashing, with the top edges sealed.
If the chimney cap or the brickwork is letting water in, that needs a chimney or masonry repair as well. New flashing won't stop water that's coming in through the top.
If you're replacing the roof anyway, ask whether the chimney flashing will be replaced or reused. It's worth knowing when you compare estimates.
Get the leak traced before it's patched
At In The Light Roofing, we specialize in shingle roof repairs across the Lehigh Valley, and we focus on finding the real source of a leak before we fix anything. When flashing is the problem, we remove the old or corroded metal and check what's underneath for water damage.
If you're not sure where the water's coming from, our roof inspection covers the outside of the roof, the attic, including the insulation, the ventilation and any signs of water damage, and your ceilings. You get a detailed report of what we found and what we recommend.
Seeing a stain next to the chimney? Read more about our roof repair and leak repair service in Allentown and across the Lehigh Valley, or contact us to book a look at it. Hablamos español.