Choosing a Skylight

A skylight brightens interior rooms in a way no artificial source can match. Even on cloudy days, a skylight provides a surprising amount of light—and the operating cost is nil. Prefabricated kits—available in various domed, rectangular, and square shapes—include an acrylic window (or “light”) and a flanged metal frame that you nail to your roof deck. Unless your roof line is exposed inside, you will have to figure out how to make the light reach down through the attic space into the ceiling below. Often the greater part of a skylight installation is building the sbaft through which the light shines. You may need to build a frame and cover it with drywall. Some newer models have a flexible shaft that can be snaked down into a room below; this eliminates the need for framing.

Domed skylight. Domed skylights are suitable for installation on flat or pitched roofs. Some also include a cranking system so you can open them for ventilation. Some types allow you to see clouds and stars, but most are opaque and only allow light.

Dormer skylight. Dormer skylights make sense only for pitched roofs. Good skylights are double-glazed, with an airspace to reduce heat loss. An automatic opener raises the lid when things get hot and helps keep a room cool.

Installing a Skylight

Cutting a hole in the roof for a skylight requires only modest framing and flashing ability. You will need to construct a light shaft through your attic, unless your home has a flat roof or a ceiling that follows the slope of the roof. Plan the location of your skylight, then work from the inside out, framing the ceiling opening and building the shaft before you cut into the roof. Skylights are designed to span two or three rafters on typical 24-inch spacings; often it is necessary to cut and tie off rafters and ceiling joists to accommodate the shaft. NOTE: Be sure that you shore up the ceiling with temporary bracing before cutting into the ceiling.

For a flat-roof installation, frame the opening, paint the rafters and headers around it, then install the skylight. You may also need to raise the unit with nailers. Once you have built a skylight shaft, topping it off with a prefab skylight takes a day or less. First, you must cut and tie off the rafters with headers, as you did with the ceiling joists. Locate the opening by sliding the shaft into position and marking around its perimeter. When you set the unit in place, fit its flange under roofing at the top and sides, but let it overlap the shingles on the down-roof side.

Tools: Level, tape measure, drill, boring and screwdriver bits, square, hammer, utility knife, pry bar, caulking gun, saber saw.

Three ways to position a skylight. If you locate a skylight directly above the ceiling opening, you can get by with a simple straight shaft (above, left). A tilted shaft (middle) lets you offset the roof and ceiling openings. These are trickier to construct. The ceiling opening can be bigger than the skylight. To build a shaft, construct a sleeve with 1/2-inch plywood, check it for fit, then paint the inside white to ensure maximum reflective value. Alternatively, build 2x4 framing and cover it with drywall. A “sun tunnel” (right) is a flexible shaft that comes with the skylight. To install one, cut a hole in the ceiling and follow the manufacturer’s instructions.

1. Frame the ceiling and the roof. Work in the attic space. Use a level to reference down from the roof to the ceiling. When planning the opening, be sure to include the thickness of framing members as well as the thickness of the plywood or drywall. Install headers before cutting the roof sheathing or the ceiling drywall.

2. Mark the opening. To mark the opening, drive long nails through the roof at each corner, then chalk an outline on the top side.

3. Cut the opening. You may need to cut away an opening in the roofing larger than the hole in the sheathing. Cut through shingles and building felt with a utility knife and pry away the roofing. Cut the sheathing using a saber saw, a circular saw, or a reciprocating saw. When you saw the sheathing, don’t let it fall—it might damage the ceiling.

4. Install the shaft. Use nails to mark the ceiling below for the opening and cut the drywall with a saber saw. Carefully measure for the size of each plywood piece. Slide each piece up from below and nail it to the framing.

5. Roof around the skylight. Follow the manufacturer’s instructions carefully to prevent future leaks. Slide the top flange under the roofing and allow the bottom portion to rest on the roofing. Seal under the flange before you nail it, then cement the shingles to it with roofing cement.

6. Install a diffuser. A diffuser makes it easy to finish the opening and hides flaws in the shaft. If you have no diffuser, trim the opening neatly. Caulk the corners where pieces of plywood abut, then paint.

1 .To install a skylight on a flat roof, add nailers. On flat roofs take steps to ensure that water cannot work its way up and under the skylight during heavy rain. Install nailers around the opening. Set them in roofing cement before driving nails or screws; otherwise standing water may overflow the frame.

2. Roof around the opening. Seal around the nailers with roofing cement, then slip flashing under the roofing and up the sides of the nailer. The skylight should fit tightly around the roofing material.

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