The most common cause of chimney related roof leaks is the chimney flashing. In fact, if you have a roof leak in the vicinity of the chimney, it is most likely the flashing. Chimney flashing is a 6 to 8 inch metal strip (usually lead or aluminum) used to seal the transition joint between the shingles and the chimney.
In some cases the chimney flashing was never installed, installed improperly or over time has deteriorated. Seasonal heating and cooling can cause flashing joints to break and open the water tight seal. The “old technology” method of repairing chimney flashing used a tar based material over the suspected leaking areas.
Unfortunately, the tar material for flashing repair is only a short term patch. All tar based chimney flashing patching compounds are not UV stable, and over time they degrade and crack on the roof. This either opens the old leaking areas or allows new leaks to form.
The first step to determining if the flashing is the cause of the leak is to examine the chimney. The weakest points of the chimney are the top and sides. These are the areas that face the greatest UV and weather exposure, and therefore damage. Since is better protected, you will notice the side facing the ground does not have flashing. Inspect the base area for cracks or areas of water penetration.
The brick and mortar will expand and contract with seasonal change. Since the mortar is not elastomeric, this expansion and contraction can cause cracks. In addition, mortar is very porous and absorbs water that will freeze in cold climates. This freezing and thawing will destroy the mortar joint causing a potential leak and degrading the structural integrity of the chimney over time. Inspect the mortar joints for cracks.
If there are cracks in the chimney use a clear exterior elastomeric caulk to seal the cracks. You can also use an exterior masonary waterproofing sealant to coat the entire chimney. Be sure to use a clear elastomeric waterproofing sealant to maintain the color of the brick and mortar.
Repairing Chimney Flashing Easily
Once you have completed the inspection of the chimney, take a look at the flashing and shingles around the chimney. If the shingles around the chimney are cracked or have holes then repairing the chimney flashing area is easily accomplished by sealing the shingles and the chimney flashing with an elastomeric roof coating. If you do not want unsightly black patches on your roof use a clear elastomeric roof coating designed for asphalt shingles.
Before applying the elastomeric roof coating, fill the cracks and holes with clear exterior elastomeric caulk. Next, following the manufactures instructions apply the elastomeric roof coating.
Although the best time for repairing chimney flashing is in warmer weather, some elastomeric roof coatings and caulk can be applied in below freezing and even on a damp roof surface.