| 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
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
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.
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.
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- Plumb bob
- Circular saw
with combination or carbide-tipped blade
- Utility Knife
- Chalk line
- Tape measure
- Combination square
or adjustable T-bevel
- Reciprocating saw
- Saber saw or compass saw
- Putty knife
- Hard hat
- Safety glasses or
- Dust mask
- Drop cloths
- Skylight unit with fasteners recommended by manufacturer
- 2x4 lumber
and 16d nails
- Framing anchor nails
- Framing anchors
- Joist hangers
- Roofing felt
- Roofing cement
- Translucent polycarbonate ceiling panel
| Mark the roof opening.
- Mark the four corners and the center of the proposed skylight
opening on the ceiling. Then drive 16d nails through these five
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
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.
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
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.
- 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.
- 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
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.
- 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.
- 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
| Mark and cut the rough opening.
|| Mark and cut the rafter.
Cutting the Rafter
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.
- 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
| 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.
- 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.
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
- 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
- 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.
| Cut the ceiling opening.
Cutting the Ceiling
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
| Support joists before cutting them.
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
Safety Note: Removing more than one joist can seriously
compromise the ceiling's structural integrity. Get advice
from a professional.
Framing the Shaft
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
- Toenail the studs to the ceiling and roof headers with 8d nails.
Install two studs at each corner to provide nailing for the
between the framing members and install
drywall in the shaft area.
- 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.
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.