How to Install a Loft Window

Select a pre-framed window or skylight of the desired dimensions for the space. Mark the outline of the window with chalk on the roof where you want it mounted. Go into the attic space to locate the rafters. Consider fitting a small window between rafters spaced 24 inches apart or plan on cutting at least one rafter for wider windows.

Remove the ceiling covering and insulation from inside the roof to expose the rafters. Make sure there are no electrical wires in the area. Drive nails through the roof deck to mark the corners for the window opening alongside the rafters. Brace the rafters on either side of the window opening with 2-by-4 boards placed underneath the rafters or boards fastened to the outside rafters and to corresponding rafters on the other side of the roof. Remove shingles and use a circular saw to cut roof decking around the window outline.

Make a rough frame for the window. Measure the top and bottom points for the frame. Place horizontal double 2-by-4 headers between the rafters at the top and bottom of the window area. Nail the boards together with 16d framing nails and a hammer then mount them in place with framing nails driven through the rafters into the header ends. Use a level to set these boards evenly.

Add 2-by-4s on each side of the window opening, between the top and bottom headers. Measure from corner to corner to make sure the opening is square. Cut the boards to fit with the circular saw. Nail the 2-by-4s to the rafters on each side. Make the dimensions of this rough opening conform to the space needed for the pre-built window.

Set the pre-built window into the rough frame opening and brace it in place with wood shims at the top and bottom. Use a level and square to make sure it is properly set and use shims to adjust it to fit. Fasten the window frame to the rough frame with screws; follow the instructions provided by the window manufacturer to secure it in place.

Apply roofing paper and metal flashing around the edges of the roof or loft window. Lift the shingles around the window frame with a pry bar just enough to slide the paper and flashing under them. Fasten the flashing with shingle nails. Run flashing up the sides of the window and over the roof to seal the junction. Replace any shingles that were removed.