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.
Flash The Sill
Have a helper inside carefully lower the cutout as you cut the fourth side. Nail the roof sheathing down around the perimeter of the opening. Stick a strip of self-adhesive membrane across the bottom edge of the opening, as shown. Cut and fold the top 1 inch or so of the strip over, onto the exposed sill.
Place The Skylight
With a helper standing inside with the skylight, lift it up through the opening, rest its bottom edge against the sill, and lower the unit onto the roof, as shown. Drive 1¼-inch galvanized roofing nails through the holes in the skylight's flange and into the roof.
Seal The Sides
Cover the bottom flange with a strip of membrane that extends 6 inches past the skylight's sides. Cut the extensions at a 45-degree angle out from both corners of the skylight. Wrap the top part around the skylight's frame, as shown, and press the other part against the roof. Cover the side flanges, then the top, in the same way.
Sill-Flash The Bottom Edge
Nail roofing shingles over the bottom membrane; the membrane will seal around the nails. Now place the U-shaped sill flashing over the shingles, as shown, and slip it over the skylight's bottom end. Nail the piece to the roof, at the top outside corners of the U, and cover the exposed sides with shingles.
Step-Flash The Sides
Overlap the top half of the shingles that cover the sill flashing with a piece of L-shaped step flashing. Drive one nail near the top outside corner and a second one a few inches below that. Cover the flashing with a shingle, add the next piece of flashing atop that, and so on, up both sides of the skylight.
Counterflash The Sides
Cover the vertical legs of the step flashing with strips of counterflashing—they snap right onto the rim of the skylight. Counterflashing is the key to sealing out wind-driven rain and snow.
Install The Saddle Flashing
Lift the first course of shingles above the skylight, slit the underlayment (the sheeting under the shingles), and slip the saddle flashing's top edge up through the slit. Fit the flashing against the skylight, lift the shingles, and nail the flashing at its top corners. Nail a final course of shingles over the flashing, leaving 4 inches of it exposed.