Treating a Roof

Q: I recently had new shingles installed on my roof. The workers removed the old roof and replaced damaged wood, then added a black tar paper and new shingles. My problem is the roof leaks at the chimney. After the first rain I noticed water marks on my ceiling. I had a man check the roof, and he said the roofer had sealed the roof to the chimney with a black tarlike material. He said the tar had several cracks and he could see where the rain might seep through. He added more tar.

Will I have to add tar all the time, or is there a better way to fix the chimney?

A: Roof shingles are basically water-resistant and do a good job of keeping rainwater and snow melt out of our homes. When a shingle has a hole in it or there is something penetrating the shingle, a seal has to be made to protect the home from water entry.

Chimneys, plumbing vent stacks, and roof vents have to be properly sealed. Roofing tar is not a good choice for a sealer. Tar is a petroleum product that can eat away the protective covering of the shingle.

The roofers probably used a roofing mastic that over time will harden, shrink and crack, exposing the roof to leaks.

The proper way to seal a chimney is with flashing and counter flashing. Flashings are generally made of thin pieces of lightweight aluminum or galvanized metal. The metal is bent in an "L" shape, with the upper part snug against the chimney and the lower part resting on a roof shingle. The flashing is installed in a stair-step fashion starting at the bottom or lower side of the chimney and working toward the top. The flashing is nailed or fastened to the roof only, not to the chimney.

Next, a single piece of counter flashing is installed over the flashing and fastened to the chimney only.

Proper installation would require cutting a groove in the covering of the chimney and bending a small lip on the counter flashing. The lip is then placed inside the groove and sealed in place. When the roof moves with winds or as the result of shrinkage, the two pieces of metal slide against each other to prevent bending and damage.

Although these instructions are printed on the wrapper of each bundle of shingles, only a few roofers are experienced in the application. A sheet metal worker or HVAC (heating venting air conditioning) contractor has the material and tools to bend and install the flashings.

Dwight Barnett is a certified master inspector with the American Society of Home Inspectors.