Window in the roof

Capitol Hill resident Danielle Wiblemo loves her hall skylight. “We live in a row house with no side windows, so that extra natural light that comes from above makes a big difference,” Mrs. Wiblemo says.

It took just a few hours for roofers to install her new skylight — a replacement. That doesn’t necessarily mean installing a standard-size skylight (usually 2 feet by 4 feet) is an easy task, local contractors say.

“I would not recommend a homeowner to do it on their own,” says Scott Siegal, owner of Maggio Roofing in Takoma Park. “If it’s not done correctly, they can really leak a lot.”

Doing it correctly includes such tasks as finding a good spot for the skylight away from support beams and moving possible obstructions, such as heating and cooling ducts as well as electrical wiring.

Other tasks include building an 8-inch curb on the roof on which the skylight sits, adding flashing around the skylight to eliminate leaking and possibly redoing drywall or painting on the inside, depending on the interior conditions, Mr. Siegal says.

In Mrs. Wiblemo’s case, the work was easy because it just entailed replacing an old, leaky skylight with a brand-new one.

Many manufacturers say skylights should last at least 10 years, Mr. Siegal says, but in his experience, they actually last longer, up to 20 years.

Mrs. Wiblemo’s type of replacement job, which involved some minor roof work, such as building a curb (often made out of wood) may cost about $700, while installing a skylight from scratch, including the cost of the skylight itself, may cost about $1,200, Mr. Siegal says.

Starting from scratch includes cutting the hole in the roof, usually with a circular saw; building the curb; installing the skylight; and then adding flashing — covering the seams between roofing material, curb and skylight with a material such as aluminum or lead — to ensure there is no leakage.

One of the most important steps when installing a skylight is adding the flashing, says Fred Ess, a contractor based in Edgewater, Md.

“The flashing makes it air- and watertight,” Mr. Ess says. “To make sure we get a perfect fit, we break the aluminum on site,” he adds.

As a general rule, domed skylights should be used on flat roofs to avoid water and snow accumulation, while flat skylights can be used on slanted roofs.

Story Continues →