Leak-Proof Your Chimney

In a monthly column, Christopher W. Closs, an expert with Maine Preservation, answers your questions about maintaining antique and vintage homes.

Q: How can I prevent water from getting into my house through the chimney?

– Peggy Gierhan, Auburn

A: Historically, when fireplaces were used on a daily basis for cooking, rain was absorbed into a chimney’s bricks and evaporated rapidly. Now that we don’t heat up our chimneys as frequently, that water can puddle in the fireplace, drip through the joints in a stovepipe, and even damage the framing of your house, if the chimney’s liner, bricks, and flashing are not maintained properly. If you’ve got a leak, this three-step guide can help.

Take it from the top.

Start by engaging a chimney sweep or mason to inspect your chimney (ideally with a camera on a cable) to make sure it is lined and the lining is in good condition. If you live in an older house with a massive center chimney, you may have as many as six flues that require lining. Each flue vents a single heat source (such as a fireplace, stove, furnace, or water heater) and is a potential conduit for rainwater.

You or the professional should also use a flashlight to examine the chimney’s exterior, paying particular attention to the mortar joints, any corbeling (a projecting, decorative necklace of bricks that circumscribes the top and helps divert water away from the exterior bricks below) and the crown — a sloped cement sheet at the top that sheds rain outward and fills in gaps between the chimney’s liner and interior walls. If you notice loose or cracking mortar, out-of-place or missing bricks, and/or fissures in the crown, have a mason repair these immediately. Alternatively, you can apply a waterproof, brushable sealant, such as CrownCoat, to the crown. Waterproofing exterior bricks with a clear liquid sealant will be of marginal benefit because products sold for this purpose don’t seal cracks and gaps.

Next, carefully examine the flashing. Chimneys in the 19th century were usually fitted with lead flashing. In the early 20th century, copper was frequently used. And from the 1950s on, aluminum flashing became popular. No matter what material you find, you should observe what’s called “counterflashing” on the sidewalls of the chimney — pieces of metal mortared into the joints and folded down vertically to cover the bricks below. Just behind, you should see sections of L-shaped “step flashing” that start under the roof shingles and disappear beneath the counterflashing. These two types of flashing interlock and the assembly, along with the flashing at the upper and lower faces of the chimney, is essential to keeping rain out of your house. If you notice gaps or cracks anywhere in your flashing, contact a roofing professional asap.

Head inside.

You can also assess your flashing — or lack thereof — from the attic. Look up at all four sides of the chimney where it pierces the roof. Do you see slivers or pinpricks of daylight anywhere? If so, you likely don’t have counterflashing, or it has become pierced and needs to be replaced. Mark any areas of concern with chalk, then return to the attic when it’s raining to see if those spots are wet; also note if any creosote is seeping through leaky flue joints or cracks in the brick or mortar joints. If these issues are observed, enlist a professional, as above. Note: Be wary of roofing contractors who recommend quick repairs like an inexpensive coating of tar or caulking. These are three- to five-year fixes at best. Once cracks penetrate the tar, or the caulking dries and shrinks, you’ll face the same problems all over again.

Chimney with rain cap and flashing
This is a close-up of the chimney shown above. Note the corbeling, counterflashing on the sidewall (integrated with step flashing beneath the shingles), apron flashing in front, and stone rain cap. Photo by Jay Cox.

Consider a cap.

The best antidote to water penetrating your chimney is a “rain cap” in the form of sheet metal, a box-like fixture, or flat stone cover (left) elevated 10 inches or more above the crown. The cap permits smoke and heat to escape but keeps water out and is sometimes fitted with a steel mesh “spark arrestor” that deters birds and rodents. Another reliable fix is a “top damper”— a hinged steel or cast aluminum flap or cover that can be installed in lieu of a traditional throat damper and operated directly from the fireplace, thanks to a cable connected to a lever at the firebox. These dampers are water- (and pest!) proof, and can be custom-designed to seal off multiple flues. (Unfortunately, the standard metal throat damper, used to prevent heat loss and chilly downdrafts when the fireplace is not in use, does nothing to prevent water from entering and damaging the chimney above.)

One final DIY fix that’s only appropriate for seasonal camps and cottages: you can place a wooden cap (an orange crate works well) clad in galvanized sheet metal over the chimney when the house is not in use. Yes, you will have to get on the roof to install and remove your homemade cover, but this is an inexpensive and effective solution.

Have an old house question for Chris? Leave a note in the comments and you may be featured in an upcoming column!

Cover image: This circa 1793 Cape Cod home with a recently repaired chimney is located on the property of The Old Farm Christmas Place in Cape Elizabeth. Photo by Jay Cox.

Christopher W. Closs is field service advisor for Maine Preservation, the only nonprofit historic preservation group working to preserve and protect treasured places across the state of Maine. Closs holds a Master’s Degree in Historic Preservation from the University of Vermont and is skilled in restoration carpentry and stone masonry. You can email additional questions to Chris@mainepreservation.org and find more helpful information at mainepreservation.org.