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 -- 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.

A tilted shaft lets you offset the roof and ceiling openings somewhat. These are trickier to construct, of course. The third shaft design is a larger shaft than the skylight. A splayed shaft disperses daylight over a broad area.

Regardless of what you choose, construct a sleeve with 1/2-inch plywood, check it for fit, then paint the inside white to ensure maximum reflective value. For a flat roof installation, just frame the opening, paint the rafters and headers around it, then install the 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. Dormer skylights make sense only for pitched roofs. Good skylights are double glazed, with an air space to cut heat loss.

Going through the roof. Once you have built a skylight shaft, topping it off with a prefab skylight takes a day or less. Locate the opening by sliding the shaft into position and marking around it's perimeter. When you set the unit in place, fit the flange under roofing at the top and the sides, but let it overlap the shingles on the down-roof side.

Getting through the roof requires only modest framing and flashing expertise. Keep in mind, however, that you will need to construct a light shaft through your attic, unless your home has a flat roof or an integral roof-ceiling deck.

Work from the inside out, framing the ceiling opening and building the shaft before you cut into the roof. Skylights are designed to span 2 or 3 rafters on a typical 24-inch spacings; often it is necessary to cut and tie off rafters and ceiling joists to accommodate the shaft.

Caution: Be sure that you shore up the ceiling with T-braces before cutting into the ceiling. There are three basic designs for shafts depending on the situation. If you locate a skylight directly above the ceiling opening, you can get by with a simple straight shaft.