Remodeling 7: How to Install a Skylight in an Existing Roof

"Location, location, location!", is what they say in real estate, but it's just as important with skylights.

In this home improvement article, we will talk about a 2'x4' skylight, so if you have a larger skylight, just adjust the numbers.


With most skylights there are two locations to consider, the roof and the ceiling. The skylight location you choose is dependent on the type of roof you have. The two types of roof common today in North America are the rafter and truss roofs. The best place to find out what kind of a roof you have is in the attic. Rafters are individual structural members that give the roof its shape and support. They are usually 2x6 on 16" centers. Trusses are units that support the roof as well as the ceiling. These are usually 2x4 construction with webs and gusset plates over the connection points and placed on 24" centers. Watch for the location of any hip or valley rafter, you don't want to cut through these or the roof trusses. Since your roof rafters will probably be on 16" centers we have to choose one to cut out for your 2'x4' skylight to fit. Another decision to make, once discovering the structure of your roof, is the flare of the skylight. Usually the ends of a skylight are flared a bit to allow extra light to enter. The ceiling opening and the roof opening do not have to be the same size. The roof opening size is dependent on the rough opening required by the chosen skylight. The rough opening for the ceiling is dependent on your imagination, but is restricted to width on a truss roof, unless of course a home improvement engineer is consulted to enable the trusses to be cut. That is a major change and not recommended at this level of home improvement skills.

Home Improvement: A Truss Roof Skylight

The 2'x4' skylight is designed to fit into the space created by two adjacent roof trusses. Since these roof trusses are placed on 24" centers, that means for 2" thick construction the distance between the roof trusses is 22 1/2" or 1 1/2" less than 24". This 22 1/2" is called the width of the rough opening. The skylight manufacturers did this for a reason, you simply don't cut roof trusses. In keeping with this design, the length of the rough opening would be 1 1/2" less than 48" or 46 1/2". Therefore, the rough opening for a 2'x4' skylight is 22 1/2"x46 1/2". For a 2'x2' skylight it would be 22 1/2"x22 1/2", a 4'x4' would be 46 1/2"x46 1/2", and so on. The rough opening for a home improvement truss system consists mainly of inserting two pieces of 2x4, each 22 1/2" long between the top chords and 46 1/2" apart on the inside. The ceiling opening is similar with two pieces of 2x4 cut 22 1/2" long inserted between the two bottom chords allowing for the chosen home improvement design of splay. Nailing these pieces in place through the truss with 3" nails is usually sufficient. I usually nail the ends in plumb, rather than on an angle with the ceiling. This eliminates home improvement problems with corner bead or wood trim around the opening later.

Home Improvement: A Rafter Roof Skylight

Since the rough opening of a 2' skylight is 22 1/2" and the rafters are on 16" centers or 14 1/2" apart, we need to do some home improvement renovating. In order to remove a rafter designed to hold up a portion of roof, we have to compensate. Try to use an existing rafter for one side of the rough opening. Since we are removing a portion of a rafter, we should strengthen the rafters on each side to compensate for this. The rafter we choose to use for the one side should be doubled up and the rafter two over should be doubled up as well. The purpose of doubling up each side rafter is because we are transferring the roof load from the middle rafter to the sides. The middle rafter is the one we are going to cut out. If this can't be done from the attic side, the entire ceiling finish will have to be removed along the run of these three rafters. Once the two rafters are doubled up and well nailed every 12" top and bottom, we can frame the ends of the rough opening. It doesn't matter which side of the rafters are doubled up, whichever is the easiest.

Nailing the rafters together is best done with an air nailer or a power screwdriver in order to disturb the system as little as possible. We don't want to loosen nails holding the sheathing on or drywall or plaster on the ceiling by too much banging. Okay, the two side rafters at 32" centers are doubled up and well nailed or screwed. Determine where you want the ends of your skylight in relation to the ridge and soffit. As discussed under Location, the roof opening is dependent on the skylight size, but the ceiling opening is dependent on your imagination. With a rafter system we can quite easily cut out rafters and ceiling joists and compensate for this. That is, you can splay the sides out as well as the ends.

Let's put in the rough opening ends in the rafters. Layout the upper rafter where you want the top end of the skylight to be. Go up the rafter another 1 1/2" and cut off the rafter at this mark. Cut a length of 2x4 (or 2x6 if your rafters are 2x6) the length of the inside between the two rafters. Nail this in place making sure it is square with the sides and against the rafter you just cut off. Do the same to the other end making the distance between the ends equal to our rough opening of 46 1/2". Remember to allow for another 1 1/2" down the rafter for the end piece and nail it into the rafter sides and end. Measure your rough opening to be sure it is 46 1/2" long on the inside. Put joist hangers around these six intersection points. Finish the opening by nailing a 2x6 into each end, forming the side of the rough opening, 22 1/2" from the inside of one of the doubled up rafters. You can put a joist hanger at these two points or just toe-nail the piece into the ends with 4 - 3" nails on each end, since the main support is only about 8" away.

Now, the ceiling rough opening is next. Frame that in a similar manner to the roof. The ceiling joists should also be doubled up on each side to support the ceiling load in the middle, this is not as critical as for the roof and is only needed on long spans of 10' or more. I should point out that there are other home improvement ways to support the rafters and ceiling joists besides doubling them up. These include adding collar ties to the rafters or supporting the rafters off of walls below them, then supporting the ceiling joists off the rafters. I only mention this in passing and to get the home improvement point across that when structural members are severed, these members must be supported in a manner consistent with good framing practices.

Home Improvement: The Skylight Tunnel

Now that the roof and ceiling openings are framed we can concentrate on the skylight tunnel framing. This is the section between the roof and ceiling. The shape of this tunnel depends on the splay you have chosen as well as the distance up the roof where you situated the skylight. The further up the roof, the deeper the tunnel. Of course, for those with a vaulted ceiling or roof joists the tunnel is the height of the framing. In this home improvement situation the roof opening and the ceiling opening would not be much different from each other. To frame the tunnel just use 2x4s on the flat and put 2 - 2x4s at each corner to supply backing for your inside finish, drywall, panelling, 1x4 V-joint cedar or whatever. The studs should be placed no more than 2' on centers. Cut them on angles to fit snug with the end 2x4s of the rough openings. Toe-nailing is the preferred home improvement way of fastening, with the inside corners nailed together, too.

Well done. Before we continue with the insulating and finishing and while we are in the skylight tunnel, mark the roof opening by taking 4 - 3" long nails. Put one in each corner and drive them up straight through the roofing material so you can be sure to see them when you go on the roof.

Home Improvement: The Curb

While you are still on the ground let's cut and assemble the curb for the skylight. The curb solves two home improvement problems; it is a structure to fasten the skylight down and a means of raising the skylight off the roof deck to be able to add flashing in order to weatherproof the roof. I prefer a curb of 2x6 rather than 2x4. It lifts the skylight a bit further off the deck so snow and ice won't build up too high and cause home improvement problems. The curb should be made of 2x6 for roofs with thick roofing such as shakes and tile. If the roof has a low pitch, below 3 in 12, you can increase the slope of the skylight by getting a wider board and tapering it down on the bottom of the two sides and matching the ends with the sides.

Okay, we know the rough opening, so the curb extends this rough opening above the roof. For a 2'x4' skylight the curb pieces should be 22 1/2" and 49 1/2". Nail the long pieces onto the short pieces creating an inside opening of 22 1/2"x46 1/2".

Home Improvement: Cutting the Roof Opening

Let's go up on the roof. Depending on the pitch of the roof make sure you are safe. Tie yourself off by using approved gear or at the very least a rope around your waist. Tie off to something on the ground over the ridge or around a chimney or such. Those home improvement people afraid of heights should simply stay on the ground, it's not worth the hassle.

Take your circular saw with you and power cord. It's handy to have your home improvement helper inside to hand you up things including the curb and the skylight through the opening to be cut. Find the four nails. Remove the shingles in the hole area and about 6" all around the skylight hole to allow for flashing. Try to be careful and save the shingles. A flat bar works good for this by sliding it up under the shingle to the nail and popping the nail up. Connect the four nail points and check the diagonals for square. Set the thickness of blade on your saw to just penetrate the sheathing and start cutting. There is your hole.

Now install the skylight curb, receive it from your home improvement helper patiently waiting to see light through the tunnel. If everything was done correctly, when the curb is placed on the roof it shouldn't fall through the hole but its inside should be flush with the inside of the hole. Nail one of the long sides of the curb down on top of the roof and check the diagonals again. This is very important with home improvement, you can't put a square skylight onto a curb that is out of square by much. Now nail the curb down into the truss or rafters and end blocks securely. Toe-nail down through the ends of the long pieces of curb both from the outside and toe-nail around the inside of the curb with 3" nails. For our home improvement friends in windy places a good trick is to take some tin flashing and nail it on the inside of the curb extending down over the rafter or truss. Use 1 1/4" galvanized roofing nails for this, so the nails won't penetrate the exterior of the curb. That will help hold it down in a severe wind.

Home Improvement: Roofing and Flashing

Purchase the flashing with the skylight. Some home improvement retailers have a flashing kit to go with the skylight. This is good for home improvement novices; just follow the instructions with the kit. For other home improvement people with more daring I'll tell you how to install your own flashing and save a bit of cash.

    Choose the colour of flashing to match the color of your shingles:
  • A back pan about 3' long which extends up the roof about 12" and up the curb.
  • A front apron about 3' long, which extends over the shingles in the front by about 4" and up the curb as well.
  • Some step flashing - about 18 pieces. They usually come in a bundle of 25.
The step flashing goes over the top of the second layer of shingle and under the second layer of the next course.

Okay, start replacing the shingles from the bottom up. When you come to the bottom line of the skylight, cut the shingles off and continue full size on the sides. When the final row is up to the bottom of the skylight, install the front apron over the shingles and up the curb. Extend the apron on each side of the curb the amount of step flashing that will lay down on the roof (it should be a minimum of 3"). Place the front apron against the bottom of the curb to extend the flashing over a piece of step flashing laid temporarily on each side and mark the length of flashing required. Before you remove it, scribe a pencil mark on the back side of the flashing down the outside of the curb on each end. Turn the flashing over and cut just above the bend of the flashing to the pencil marks on each side.

Now install the apron in position, bend the tabs up the curb on each side, leaving the front apron in place in front of the curb and past it on each side. Now nail the flashing on the extreme top of the curb. Keep those nails high. Do not nail the flashings into the roof deck at all. Use only 1 1/4" galvanized roofing nails so they won't penetrate through the curb.

Put your first step flashing in position on each side. Bring it right down to the front edge of the apron. Don't cut this first piece at all. Drive a nail at the top of the curb near the lower end and a nail through the outside edge of the flat part into the roof deck at the top. The next shingle will go over this step flashing as you progress up the roof.

Place another step flashing over the next course of shingles. The bottom edge of the flashing should be flush with the top of the exposure. Nail this flashing near the bottom and at the extreme top of the curb. Notice you are nailing now through the step flashing below it. If this is going correctly the shingles will cover the step flashing all the way up. The only shingle that this doesn't hold true for is the t-lock shingle. In this case where the flashing is not covered, just cut a piece of shingle to fit under the shingle above it to cover the flashing. Don't nail it on. Just stick it down with a bit of caulking or roof lap cement. Continue up the roof and the sides of the curb until you have come to the top.

When placing the last step flashing at the top, put it into position and scribe with a pencil on the back side along the top of the curb. Cut the bend of the flashing as done at the front apron. Bend the tab over against the top end of the curb and nail in position, keeping the nails high. When both sides are up to this same point, install the back pan. The wide part lays down on the deck above the curb. Lay it into position with one side flush with the outside of the step flashing and mark the other side at the outside of the step flashing. Cut the back pan square off at this line. Place it in position again and nail it at the top of the curb. Do not cut the tabs off this back pan. It should go across the top of the curb extending out past the curb as the step flashing came down the side extending out below the bottom of the curb. This forces the water to go about 3" past the side of the curb before it goes down the roof. Nail the back of the pan, laying on the deck down. Only nail it at the extreme top of the flashing when you've got it laying nice and flat. We don't want any wrinkles here.

Continue with the rows of shingles. Be careful to put in only those nails that are required to hold the shingle in place, preferably none near the bend of the flashing at the curb. If your nailing line is close to the bottom of the back pan, use caulking to cover the nail heads and place the next row on while the caulking is still wet. Before placing the skylight in position, cut the corners of the flashing down about 1 1/2" from the top of the curb and bend them over tight against the vertical piece towards the inside corner to form a triangle. Apply caulking to this inside corner.

Get your home improvement helper to hand you up the skylight and give it a try for fit. Make sure it goes all the way down resting on the top of the curb and not hitting the flashing corners. If you're happy with the fit, remove it and place a bead of caulking on the top of the curb. Set the skylight down into its final resting place.

There are two ways, generally, of securing the skylight to the curb. The inside or the outside. The inside is preferred. Just use some 1 1/4" screws and screw them into the curb through the channel of the skylight and holes provided on the inside. Some home improvement manufactures prefer the outside with enclosed hold down clips that go into a channel on the inside of the skylight and are screwed to the outside of the curb through the step flashing. If this home improvement method is chosen apply caulking liberally over the screw heads and around the clips.

Home Improvement: The Finish

Now that your home is weatherproof again, we can go inside and apply finish to the tunnel. The sides of the tunnel should be insulated with the same R factor as in your attic. You can insulate later if you have access to the attic. Bear in mind that it may be easier to insulate the soffit end of the tunnel now, since you may not have the room to crawl in there later.

Staple 6 mil poly onto the inside of the tunnel making sure to overlap any joints by at least 4". Install your choice of finish to the tunnel as well as to the ceiling. Apply molding as required around the perimeter of the skylight at the ceiling level. It is recommended, if drywall is used, to install plastic j molding on the top edge of the drywall against the underside of the skylight to prevent any condensation from dissolving the drywall. It finishes it off nicely, as well.

Enjoy your new skylight.


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