If you made a list of things you don't want to hear after buttoning up a roof job or skylight installation, somewhere in the middle would be your customer's voice saying, “The skylight's leaking.” If you look out your truck or office window and see cats and dogs raining from the sky, the choice is pretty clear about what needs to happen first: get over there with a tarp, if only to provide the customer service your clients expect and to prevent expensive damage to their home — and to your reputation.

Leak Diagnosis To isolate the problem, you need to give the roof the Sherlock Holmes treatment. Mike Guertin, author of Roofing With Asphalt Shingles, says “Nine times out of 10 the problem is with the flashing around the skylight curb.” If you can isolate the leak there, then you need to re-flash the unit. “Very rarely,” he points out, “is the skylight itself defective, but it can happen.”

Doug Plotke, owner of Roof Services in Bay Shore, N.Y. — a 30-year veteran of the trade —frontloads his diagnostic energy for skylights (and all roof penetrations). “Roofing contractors are guilty until proven innocent if something goes wrong,” he points out. So in a rip-and-re-roof with existing skylights, Plotke performs a “condition assessment.” He says, “If you see lots of roof cement and caulk on the skylight unit, let your customer know in the contract — bold and underlined — that it may need replacing.” The key is to prevent the problem before it starts and to create a productive line of communication between contractor and customer. “I'm trying to prevent controversy and bad feelings ahead,” Plotke points out.

Look Around Skylight leaks, so-called, may actually be a failure somewhere else in the roof system, a failure that frames the skylight for a crime it didn't commit. If water gets under the shingles of, say, a poorly detailed dormer, defective vent boot, or improperly sealed flashing, it will pinball over lumps in the asphalt paper and around nails until it finds the easiest way out. Or in. So assuming the water coming down from the skylight may originate there might be a mistake. Walk the entire roof before you carve out the skylight, or your phone may just ring again the next time it rains.