Chimney leaks are hard to pinpoint. They can come from nearly anywhere on a chimney. Here are four common sources of leaks:
1. The mud cap, or mortared area around the top of the chimney, is often a culprit. Cracked or loose mortar can allow rainwater to get into the fireplace. Symptoms are usually water in the hearth, but impediments may cause rainwater to change course and appear elsewhere in the home.
Repair method: Replace the mortar cap and insert a flu cap. Or cover the entire chimney top with a full-sized metal chimney cap, not just a flu cap.
2. Mortar joints can deteriorate, leaving holes that admit water. In some cases, large sections of mortar in the joints may actually be missing, allowing water to enter during blowing rains. The water may enter a hole in the mortar joint, move to a hole in the bricks, then follow a zigzag course for a few inches or several feet. I have seen water enter through mortar joints, travel inside the brick holes to a point past the flashing, and then be directed onto a ceiling or continue all the way to the slab.
Repair method: Repoint mortar joints with new mortar.
3. Leaks around flashing can be particularly difficult to find since just one tiny hole in a corner during a hard rain can leak a sizable amount of water.
Repair method: Don't attempt to surface seal the metal. Have an expert sheet metal contractor or roofing contractor with specific credentials in sheet metal work redo the flashing properly. Many residential roofers lack the sheet metal expertise to handle reflashing a chimney.
4. The roof shingles around a chimney can also have problems, although shingles are one of the least common causes for a chimney leak. If the leak does show up in this area, have an expert check and repair the shingles.
Repair method: Don't surface seal. Replace missing, worn, or damaged shingles in the appropriate fashion.
How to Pinpoint the Leak Source
Finding a leak on the chimney is made easier with a minimum of two people. Three or four is better. Here are things you will need:
- Have two cell phones, or one cell phone and a cordless phone, or a couple little walkie-talkies to maintain contact between the person on the roof and the observer in the house. Sometimes yelling isn't very effective from inside an attic or under a ceiling.
- One or two water hoses with shutoff nozzles on the roof.
- A roll of clear plastic long enough to wrap around the chimney several times.
- Good shoes and safety harnesses for people on the roof.
- A yellow lumber crayon
Once you've assembled the items you'll need, follow these step-by-step instructions:
1. Starting from about 1/2 inch above where the chimney meets the roof, wrap the chimney with plastic all the way around to a point about 18 inches up. Cover the chimney bricks while leaving the flashings around the very base of the chimney exposed.
2. With a person stationed in the attic (or inside the house if the attic is inaccessible) watching the ceiling, run water from the hose down the roof along the lowest side of the chimney. Run the water for two minutes in the same spot before moving it. Don't splash the water up on the chimney or direct it up under the base flashing. Have the observer in the attic report the very first drop of water seen. As soon as water is reported, use the yellow lumber crayon to mark the spot where the hose was pointing at the time. Because it may be several days before the repairs are done, mark the spot firmly since water or rain will fade the lumber crayon quickly.
3. If there is no leak on the front, or low, side of the chimney, move the water one shingle at a time up along the side of the chimney. Always run the water downhill. Pause at least one minute at each shingle as you move up the roof shingles on that side of the chimney, but don't let the water get behind the chimney yet.
4. If there is no leak there, repeat the process on the other side of the chimney, one shingle at a time. If you do find a leak, stop and mark it, but continue to look for others above if you can do so without further interior damage. If water is gathering on the ceiling drywall, poke a hole in the center of the wet spot and let it drip into a bucket underneath. Let the drywall fully drain before moving along to another surface.
5. If neither side leaks, go to the back of the chimney and repeat everything. Do not get water on the vertical chimney surface yet.
6. If there are no leaks up to this point, it means the leak is higher than the base flashing or nearby roof shingles.
7. Remove the plastic wrap and reapply it so the bottom edge is just above the uppermost piece of counter flashing, which is the vertical pieces of metal. Wrap the chimney just above the point that flashing enters the bricks. The plastic is to make sure water is not accidentally sprayed higher than it should be, which would defeat the purpose of the careful search.
8. Starting at the front and then working the sides and finally the back section, run water around the chimney while the observer keeps watching for a leak. If none is found, the leak must be confined to the mortar joints, bricks, or mud cap.
9. Next, remove the plastic and begin working your way up the chimney, repeating the pattern of water application on each brick course until you find the leak. If there is no sign of a leak at this point, then you've traced it to the mortar cap. Apply water there to make sure.
Remember, since you know there is a leak, starting this process at the top would mean you wouldn't be able to pinpoint the source. What you are attempting to do is isolate the leak's location by covering each plane from the lowest point to the highest and crossing each nonleaking area off your list as you go.
This process is tedious and time-consuming but quite thorough. When you finish, you'll know where the leak is.
Author Profile: Greg Scott is the pseudonym for a veteran roofing contractor active in roofing industry safety and legislation.