Gambrel roofs are broad roundish roofs that can be found on many houses and barns across the country. Although their look is quite different than that of normal flat roofs found on most modern houses, the process of installing windows in them is quite similar. The steps below are a guide to how the task can be accomplished.
Step 1 - Mark the Area of the Window
From the inside, plan out where the window is going to go. Using the dimensions of the window as a guide, mark the areas where the four corners of the window will be with a marker. Then, drive nails up through these four points so that they stick out of the shingling on the exterior side of the roof.
Step 2 - Cut the Hole for the Window
Climb up to the roof with a ladder or construct scaffolding up to where the window will be installed. Scaffolding is better for this job because gambrel roofs are generally more roundish than other roofs and often cannot be walked upon. If using a ladder, make sure it is very stable.
Using the points of the nails sticking up through the shingles, snap a chalk-line to indicate the straight cuts needed for the window opening. Remove the shingles from within this area with a utility knife and claw hammer. Then, with a circular saw, cut through the roof deck along the chalk-line marks to create the opening for the window.
Step 3 - Support and Cut Rafters if Needed
If the location of the window requires rafters to be cut, nail two-inch by four-inch boards across them perpendicularly in at the top and bottom of where the widow opening is in the roof deck. These will support the rafters while the window is being installed and should not be taken out until the headers are installed. Once supported, use a reciprocating saw to cut through the rafters.
Step 4 - Frame the Opening
Install boards that are two-inches thick between the rafters. The boards should be long enough to span between the rafters and as wide across their face as the window that is being installed is thick. For example, if the frame of the window casement being installed is six-inches thick and three-feet wide, the boards used should be two-inch by three-inch and cut to three-feet wide. Install these header boards with three-inch framing nails. If a rafter was cut for the opening, it is wise to double the header at the top of the window.
Step 5 - Set the Window in Place
Install metal flashing around the new window opening and use roof sealant to ensure it is watertight. The flashing at the bottom of the window should extend over the closest row of shingles below it, while the flashing at the top should slide under the shingles closest to it. Once this is done, place the base plate that came with the window into place.
Shims can be used to make minor adjustments to the spacing of the window to ensure the window is square and snug. Use a drill to install coach bolts that usually come with a roof window to the rafters on the sides and the headers at the top and bottom. Finally set the window casing itself into the base-plate opening and attach it to the base-plate.
Step 6 - Finish the Roof
On the exterior of the roof, finish the window by re-shingling any areas that were torn up so the shingles run up over the flashing and butt up against the base-plate at the top and along the sides. (If the shingles that were cut away to make the opening are in good condition they may be reused). Use an exterior caulking to finish waterproofing the area between the shingles and the new window.