If you understand basic carpentry and feel comfortable working on the roof, you can install a self-flashing skylight.

Installing a Skylight

Project Level: Advanced

If you understand basic carpentry and feel comfortable working on the roof, you can install a self-flashing skylight. Make sure you have a helper and good safety equipment. This project will be easiest if your roof is made of asphalt or wood shingles and has a moderate slope. For other situations, or if the skylight is particularly large, have it installed by a professional. Be sure to find out whether a building permit is required.

Before You Begin

If your house has an attic or crawl space between the ceiling and the roof, be aware that you must install a light shaft to direct the light through the attic to the room below. In the attic, protect your head with a hard hat, and step only on ceiling joists or on boards supported by joists.

When planning your skylight's size, consider that some light will be lost in the shaft; the longer the light shaft, the larger the skylight must be to achieve the same level of lighting. Splaying the light shaft will help minimize this problem.

Mark the roof opening.

Placement

1. Mark the four corners and the center of the proposed skylight opening on the ceiling. Then drive 16d nails through these five points.
   
   If any of them hit a ceiling joist, consider moving the proposed opening or adjusting its size to save framing work later. Ideally, two opposite sides will butt up against the facing sides of two joists.
   
    
2. In the attic, locate the five nails, clearing away any insulation material. Look for obstructions — wires, pipes, or heating or cooling ducts — within the area of the proposed opening. Also check the underside of the roof for any obstructions.
   
    
3. If the roof ridge or a major structural member crosses the opening, relocate the opening and either move the proposed ceiling opening or use an angled or splayed light shaft between the roof and the ceiling.
   
   You should never cut trusses. If you have trusses rather than rafters, install a skylight that fits between them, such as one of the newer systems that utilizes a "light tube" capped on the roof by a small dome.
   
    
4. Use a plumb bob to transfer the ceiling opening's center nail straight up to the roof. Mark the manufacturer's recommended roof-opening dimensions on the underside of the roof. Ideally, one edge will occur along a rafter to simplify framing.
   
   Drill a small hole at each corner and drive a 16d nail up through the holes to mark the corners on the roof's surface.

Cutting the Roof

Safety Note: Be alert while sawing — a blade that binds can throw you off the roof.

1. For wood shingles or shakes, use a circular saw with a combination blade. For asphalt shingles, use a circular saw with an old carbide-tipped blade—you will be cutting through nails, and the blade will be worn out after this job. Be sure to wear safety glasses.
   
   Another option is to use a utility knife to cut through shingles, then use a circular saw with a combination blade to cut through sheathing. For a built-up roof, buy a disposable saw blade. When using a power saw, avoid awkward positions and keep out of the line of the blade.
   
    
2. Adjust your saw so that the blade cuts through the roofing material and the sheathing but not into the rafters.
   
   Resting the front of the saw's base plate on the roof, align the saw blade with the chalk line, pencil line, or knife cut.
   
   Turn on the power and lower the saw until the base plate is resting on the roof.
   
   Saw slowly and steadily along the marked line until you reach a corner. Repeat for the other sides.
   
    
3. Carefully remove shingles and pry up the sheathing. The amount of roofing material you will have to remove depends on the type of skylight you are installing.
   
   Try not to damage the shingles, since you will have to replace some of them when you are flashing the unit. For self-flashing skylights, remove shingles in an area about 10" wide on the sides and top of the rough opening, but do not remove any shingles from the bottom of the opening.
   
   Do not worry if the shingles below the opening have been cut through, since they will be covered with the bottom flange of the skylight.

Mark and cut the rough opening.	Mark and cut the rafter.

Cutting the Rafter

1. If a roof rafter is in the opening, determine the angle to cut it, then mark the cuts, using a combination square or adjustable T-bevel. Also mark lines on the rafters at both sides of the opening to indicate placement of the headers.
   
    
2. Support the rafters with 2x4s nailed between the rafters and ceiling joists at each side of the opening before cutting. Leave these in place until after headers are installed. Finally, cut through the rafters along the marked lines with a reciprocating saw — but only after you are sure the structure is properly supported.

Install headers.

Installing the Headers

Headers carry and distribute the loads across the span where one or more rafters (or joists) are removed. The way you build and install headers will depend upon the roof's slope and the type of skylight shaft you are building.

1. Measure the distance between the rafters. If your headers will be set at a 90-degree angle to the roof surface, use lumber of the same thickness and width as the rafters.
   
    
2. Cut four pieces to the length measured. Nail them together in pairs to form two doubled headers. Place two nails at the ends, then nail along the length of the header, staggering the nails about 6" apart. Insert each header into double joist hangers. Nail the joist hanger to the rafter, using framing anchor nails, and to the header using 16d nails. Toenail the header to the cut rafter with 8d nails.
   
    
3. If your headers will not sit at a 90-degree angle to the roof surface, use lumber the same size as the rafters and attach the headers at an angle, or cut the headers from larger stock, angle-ripping (beveling) each piece to the appropriate angle. To attach headers at an angle, use framing anchors instead of joist hangers.
   
    
4. For single headers, cut just two pieces of lumber, and use single joist hangers. Otherwise, installation is the same.

Place and Secure the Buy Skylight

1. Make sure that the sheathing around the rough opening is covered with roofing felt. If needed, slide pieces under the existing felt above and trim them at the edges of the opening.
   
    
2. With a putty knife, spread roofing cement (or recommended sealant) in a band 4" wide and 1/4" thick around the rough opening.
   
    
3. Align the skylight over the opening and press the nailing flange down onto the roofing cement. At the bottom, the flange should rest on top of the shingles.
   
    
4. Fasten the skylight to the sheathing through the flange, using the type of fastener suggested by the manufacturer.
   
    
5. Apply more roofing cement on top of the flange, covering both the screws and the edge of the flange.
   
    
6. Press a shingle into the cement along the bottom of the skylight. Then replace the shingles at the sides and top, shingling right up to the sides of the skylight.

Place the skylight.	Secure skylight and replace roofing.

Cut the ceiling opening. Cutting the Ceiling In your attic, clear away any insulation and use a tape measure and carpenter's square to check the ceiling opening's position. Then, from below, stretch a chalk line around the corner nails and snap the line between each pair of nails to mark the opening.   Clear the area below the ceiling opening and protect furniture and floors with heavy drop-cloths. Wear a dust mask and goggles to protect yourself.   Cut through drywall with a reciprocating saw, saber saw, or compass saw. Cut lath and plaster with a reciprocating saw or saber saw fitted with a coarse wood-cutting blade.   When you come to a joist, cut through the wallboard but do not cut the joist.   After the opening is cut, break off the wallboard and remove the fasteners.

Support joists before cutting them. Supporting & Cutting the Joists If you have to cut one or more joists, you will need to add sister joists to beef up the structure. Cut two pieces of 2x4 lumber long enough to span both the opening and two joists on each side of the opening.   Position the pieces at least 12" from the edges of the opening and fasten them with wood screws to the joists as shown.   After supporting joists, use a reciprocating saw to cut the joists.

Safety Note: Removing more than one joist can seriously compromise the ceiling's structural integrity. Get advice from a professional.

Framing the Shaft

The frame for the light shaft not only provides a nailing surface for the walls of the shaft, but also joins the ceiling to the roof, giving support to both.

1. After installing the headers as shown above, measure the distance between the ceiling headers and the roof headers at every corner and at least every 16" in between.
   
    
2. Cut vertical studs from 2x4s to the measured lengths. Unless your roof is flat and your light shaft straight, you will need to cut one or both ends of the studs at an angle; mark the cuts with an adjustable T-bevel.
   
    
3. Toenail the studs to the ceiling and roof headers with 8d nails. Install two studs at each corner to provide nailing for the wallboard.
   
    
4. Insulate between the framing members and install drywall in the shaft area.
   
    
5. Around the ceiling opening, apply molding that is wide enough to cover the edges of the paneling and the wallboard. Also cover the joint between the wallboard and the ceiling.
   
    
6. To help distribute light more evenly, rest a translucent polycarbonate ceiling panel on narrow strips of molding around the inside perimeter of the shaft.

Install headers and frame shaft.	Drywall the shaft.