Ask the Carpenter: Tips for locating the source of a leak

Ask the Expert Home Improvement
A leak leaves brown stains on the ceiling and wall in this home.
A leak leaves brown stains on the ceiling and wall in this home in a file photo. Fotolia

Q. We have a condo on Cape Cod, and we try to spend as many long weekends there during the summer as possible. When it rains hard, there is a small leak in the ceiling of our master bedroom closet. For example, we got to the condo on a Saturday morning after there were very heavy rains the afternoon before, and we noticed a couple of items in the closet were wet. Leaks have left brown spots on the ceiling and wall. We know this has been an issue in the past, particularly during the severe winter two years ago. The condo association is responsible for taking care of the problem, but, as you know, finding the source of a leak is a trial-and-error process. So, how can we detect new water leaks in this closet when we are not there often enough to check it after it rains and a condo association representative is not always available to come into our unit? It doesn’t seem practical to re-paint this area each time we get a brown spot in an attempt to detect the next time there’s a leak.

R.S., Sharon

A. Finding roof leaks has often been associated with the idiom “It’s like finding a needle in a haystack.’’

Here are methods that have proved useful to me:

Inspect the roof and sidewall flashing:

Most of the roof leaks I have dealt with involved improperly installed or damaged flashing. Flashing is a transitional roofing material. It connects a roof to chimneys, roof valleys, house walls, skylights, plumbing stacks, etc.

Some roofs stop at a house wall. Metal flashing must be in place to direct water down the wall and away from the stopping point of the shingles. This “step’’ flashing may be behind wood siding or in a brick wall. The flashing should extend over the shingles at least 3 inches. If the wall is masonry, this flashing must bend and extend 1 inch into a mortar joint.

Use a garden hose

Start spraying from the lowest possible roof-leak location and work upward. Note: Never point the spray up the roof, which directs water under the shingles. Instead, try to simulate rain falling from above. Be patient, as tiny leaks often take a long time to start dripping.

Remember that water can travel sideways and present itself inside the house several feet or more away from the leak location. Water leaking into a roof can hit a seam and follow it sideways until it drips into the house. I’ve found roof leaks more than 10 feet away from where it shows on the interior.

Inspecting a low-pitched roof

Flat rubber roofs often have seams where standing water or snow can infiltrate. Low-sloped roofs are highly susceptible to leaks from wind-blown rain and ice dams.

Checking shingles and nails

Sometimes shingles wear down or are damaged by workers walking on them, making the nails holding them down pop up, and sometimes, a nail is not driven flush and cuts into the shingle. Either situation can cause a pinhole leak, so look for missing or damaged shingles, especially cracks.

Look at your chimney flashing

Chimneys have four types of flashing. All must be right or you will have a leak. Plus, the counter flashing that goes into the brick mortar joint must be right. A hairline crack above the flashing can allow vast amounts of water to run behind the flashing. Look for soldered corners of flashing that are broken or have holes. Sometimes installing a cricket behind the chimney is the answer. A cricket is designed to divert water away from the chimney.

Inspect plumbing vents and skylights

Many plumbing vents consist of a rubber seal with aluminum flashing. The rubber can fail in as little as 10 years. Look for cracked rubber around the plumbing pipe. The flashing should be installed up and under the shingles that extend up roof from the middle of the plumbing vent. The bottom half of the flashing should be exposed and overlap the lower shingles to shed water.

Check your plumbing

Finally, roof leaks may be coming from a leaky plumbing fixture or pipe. Sometimes it is necessary to open up a wall or ceiling to inspect the cavity and locate the leak.

Rob Robillard is a general contractor, carpenter, editor of, and principal of a carpentry and renovation business. Send your questions to or tweet them to @robertrobillard. Subscribe to the Globe’s free real estate newsletter at