With a bit of basic carpentry skills and confidence 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 easier if your roof is made of asphalt or wood shingles and has a moderate slope. A skylight in the bathroom will help bring in more light into the dark areas and make your bathroom look larger. Natural lights also reduce the use of electricity to illuminate your bathroom.
If the skylight is particularly large, or you are unsure, have it installed by a professional. Be sure to find out whether a building permit is required.
Before you begin:
If you have an attic or crawl space between the ceiling and the roof, you must install a light shaft to direct the light through the attic to the room below. When working in the attic, protect your head with a hard hat, and step only on ceiling joists or on boards supported by joists.
The longer the light shaft, the larger the skylight must be in order to achieve the same level of lighting desired in your room as some light will be lost in the shaft. Splaying the light shaft will help minimize this problem.
- tape measure
- safety glasses
- circular saw
- framing nailer
- joist hangers
- framing square
- utility knife
- impact driver
- roofing underlayment
- roofing nails
- Mark the roof opening, 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. Two opposite sides will butt up against the facing sides of two joists.
- Clearing away any insulation material located within those 5 nails. Wires, pipes, or heating or cooling ducts within the area of the proposed opening need to be located. Also check the underside of the roof for any obstructions.
- If the roof ridge or a major structural member crosses the opening, relocate the opening and either moves the proposed ceiling opening or use an angled or splayed light shaft between the roof and the ceiling.
- DO NOT cut trusses. If you have trusses rather than rafters, install a skylight that fits between them, like the newer systems that utilizes a “light tube” capped on the roof by a small dome.
- 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
- Remove the shingles in area of roof to be removed. If required cut them using a straight edge and a utility knife. Fold back the shingles that will later be covered with flashing.
- 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.
- 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.
- Saw slowly and steadily along the marked line until you reach a corner. Repeat for the other sides.
- 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.
- Keep the shingles intact as 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 inch wide on the sides and top of the rough opening, but do not remove any shingles from the bottom of the opening.
Cutting the Rafter
- If a roof rafter is in the opening, determine the angle to cut it, then mark the cuts. Also mark lines on the rafters at both sides of the opening to indicate placement of the headers. You will need to support the rafters with 2 x 4s nailed between the rafters and ceiling joists at each side of the opening before cutting.
- Leave these in place until after headers are installed. Cut through the rafters along the marked lines with a reciprocating saw, AFTER you are sure the structure is properly supported.
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.
- 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.
- 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.
- 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.
- For single headers, cut just two pieces of lumber, and use single joist hangers. Otherwise, installation is the same.
Place and Secure the Skylight
- 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.
- With a putty knife, spread roofing cement (or recommended sealant) in a band 4″ wide and 1/4″ thick around the rough opening.
- 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. Fasten the skylight to the sheathing through the flange, using the type of fastener suggested by the manufacturer.
- Apply more roofing cement on top of the flange, covering both the screws and the edge of the flange.
- 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.
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.
Supporting & Cutting the Joists
- Support joists before cutting them. 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 2 x 4 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.
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.
- 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.
- 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.
- Toenail the studs to the ceiling and roof headers with 8d nails. Install two studs at each corner to provide nailing for the wallboard.
- Insulate between the framing members and install drywall in the shaft area.
- Around the ceiling opening, apply moulding that is wide enough to cover the edges of the paneling and the wallboard. Also cover the joint between the wallboard and the ceiling.
- To help distribute light more evenly, rest a translucent polycarbonate ceiling panel on narrow strips of moulding around the inside perimeter of the shaft.