How to Install a Skylight
TOH general contractor Tom Silva shows how new designs and better flashing make for a watertight roof window
Q: I've heard that all skylights, eventually, leak. Is it possible to install one so that it stays watertight?
—Angelina Holcomb, Westwood, Mass.
Tom Silva replies: Although complaints about leaking skylights were once valid, new designs and better flashing methods have largely put an end to water-infiltration worries. In fact, the Velux skylight I'm installing here boasts a 10-year warranty against leakage.
A warranty that confident doesn't allow for shortcuts on installation, though; the flashing steps have to be followed to the letter.
On a roof with rafters, I frame the rough opening from below by doubling the rafters on the sides and nailing a doubled-up header and sill to the top and bottom of the opening, respectively. (Check with your local building inspector if you have a truss roof.) The rest of the steps take place on the roof, where a fall-protection rope and harness are a must. If you're intimidated by heights or reluctant to don the safety gear, leave the job to a pro.
Cut The Opening
From below, drive a deck screw up through the roof at each corner of the rough opening. On the roof, strip the shingles about 7 inches beyond those points, and snap a chalk line between each pair of screws. Use a circular saw to plunge-cut through the sheathing along the chalk lines, as shown, on three sides. Stay off the cutout.