Dear Pat: A friend gave me a small (not curb-mounted or self-flashing) skylight for free. What is the proper method to build the curb and flash the skylight to avoid leaks in my cathedral ceiling? — Michelle C.
Dear Michelle: Normally, I would recommend to anyone to buy curb-mounted, self-flashing skylights. They are easy to install and very seldom leak unless you make some serious installation errors. In your case, it would be silly to turn down a free skylight, and you should be able to successfully build the curb and flash it yourself.
For those not familiar with skylights, the curb is a box frame upon which the actual skylight is mounted. The curb is usually a minimum of four inches above the roof surface, and it is often made from 2-by-6 lumber on shingle roofs. On some other roof surfaces, such as tile, the curb may need to be deeper than 4 inches.
The flashing is the sheet material used around the skylight to direct water away from it and over the shingles. Flashing is often made of sheet metal and installed as small pieces, which overlay one another in a similar fashion to roof shingles.
The skylight installation instructions indicate the size of the rectangular hole you must saw through the roof sheathing. Make the inside dimensions of the curb to the exact size of the roof opening. Mark the location of the cuts on the 2-by-6s, taking into account the width of the saw blade cut.
If the lumber has knots, make sure none of the saw cuts will be through a knot. Having knots at the mating ends of the curb is asking for water leaks. Also, drill pilot holes for the nails to reduce the possibility of small splits when you assemble the curb. The measurements across diagonal corners should be identical when the curb is assembled perfectly square.
Once you have the curb assembled, install temporary braces in two opposite corners. You will be handling the curb quite a lot, so you don't want it to get out of square. The braces can be made of 1/2-inch plywood and secured with nails driven only halfway in so they are easy to remove later.
Your small skylight should fit between the roof rafters without having to cut through any of them. Locate one side of the roof opening against a rafter. Install upper and lower header lumber at the top and bottom of the opening. Also install one jack rafter section between the two headers along the other side of the opening.
The flashing sheet metal is not difficult to install, but it can be tricky to make. It would be wise to have a sheet metal shop make the flashing pieces for you to the skylight manufacturer's specifications. If you really want to make the pieces yourself, use lead sheet because it is much easier to work with than aluminum or copper.
Slide the top flashing under the shingles and roofing felt. Be careful as you slide in the flashing not to puncture the felt. Place the curb over the opening, sliding it up from the bottom. Place step flashing along the sides of the curb under the shingles. Spread roofing cement on top of the curb, press the skylight into it and nail the skylight in place.