A window on the roof lets in significantly more light than a vertical window, so even a small skylight can make a room feel larger and more airy.

This is a difficult project, involving cutting the roof and installing new structural framing members, as well as drywalling and finishing the ceiling beneath the skylight. Most importantly, roofing paper and metal flashing must be installed correctly, or the roof will leak around the skylight. Follow the manufacturer's directions very carefully if you have good carpentry skills, or hire a professional carpenter.

The instructions presented here will give you an idea of the work involved. Some skylights use different installation methods and different types of flashing.

Bubble lights and plastic skylights are inexpensive, but you cannot see clearly out of them. A fixed skylight will produce heat in the summer, but the effect will be minimal if it uses low-E glass. Some companies sell remote-controlled shades or Venetian blinds for their skylights. At the high price end, a ventilating skylight can open to allow escape of hot air, significantly cooling a house even when the air conditioning is on.

Follow the manufacturer's specifications for cutting a hole in your roof. From the inside, drill a locator hole where you want the center of the skylight to be, and then work on the roof to cut the opening with a circular saw.

Frame the opening so it is strong, installing header joists that are perpendicular to the rest of the joists (see Figure 1). If you have a standard ceiling (one that does not follow the angle of the roof), cut and frame the ceiling as well, and frame a vertical passageway for the light to flow through.

Using a utility knife and a straight edge, cut the roof shingles about 3 inches back from the opening on all four sides. Set the skylight in place, centered on the opening, and attach it to the roof with nails or screws.

Cut strips of roofing paper about 8 inches wide and slip them under the shingles (see Figure 2). Install the bottom piece, then the sides, then the top piece. Sneaking the paper under the shingles will be tricky. Use a flat pry bar to lift up all the roofing nails that would get in the way, taking care not to damage the shingles.

Install the bottom flashing, a single piece that wraps partway around the skylight and sits on top of the roof shingles (see Figure 3). Drive roofing or flashing nails horizontally into the skylight, not vertically into the roof.

Next, slip individual pieces of step flashing under the shingles. On each side, start at the bottom and work up. The step flashing pieces must overlap each other by 4 inches or so. Attach the flashing pieces to the skylight, not to the roof.

These pieces are designed to fit tightly onto the skylight and ensure that water cannot enter between the step flashing and the skylight. Attach the bottom piece first, then the side pieces (see Figure 4).

Install the top piece (called the head flashing) by slipping it under the roofing and attaching it to the skylight.