How to Make a Half-Round Wood Window

Mark a rectangular outline the width and height of the round window. Remove the house siding above the door or in the gable end with a pry bar to expose the sheathing. Move inside the house or attic and remove any drywall from the rectangle area. Make a cardboard template of the half-round window at least 1/4-inch larger than the actual window and draw that outline onto the plywood or oriented strand board sheathing.

Put temporary braces on studs on both sides of the window opening to support the wall while the window is being framed. Make a header of 2-by-6 inch boards sandwiched around a 1/2-inch spacer of plywood or OSB and a bottom sill of 2-by-4 inch lumber with two boards fastened with the 4-inch faces up. Nail these between the outside studs at the top and bottom of the rough frame opening with 16d framing nails and a hammer. Use a level to set them level.

Install 2-by-4 inch trimmer studs on either side between the bottom of the header and the top of the sill plate. Nail these to the side studs and nail them into the header and sill at an angle. Make a cardboard template of the half-round window at least 1/4-inch larger than the actual window and draw that outline onto the plywood or OSB sheathing.

Drill a starter hole on the outline and use a reciprocal saw to cut through the sheathing and internal studs inside the half-round outline. Cut two 2-by-4s with 45-degree angles on each end and fasten them between the trimmer studs and the top header with framing nails, one on each side. Make sure the bottoms of these angled boards are just outside the curved outline.

Fasten house wrap around the outline with tape. Secure the new wrap to the existing wrap with tape. Nail flashing to the top of the sill plate that bends down the sides on the outside of the rough opening. Bend flashing around the curved top and nail it to the sheathing on the outside with shingle nails. Notch it with tin snips to bend it around the curve. Run a bead of caulk under it before nailing it in place.

Set the window in the opening. Use the level to ensure the bottom edge is even. Fasten it with installation tabs on the edges or however the manufacturer recommends. Apply a bead of caulk around all of the edges. Replace the siding and drywall; use a jigsaw to cut panels to fit around the rounded top of the window. Use the cardboard template to help determine these cuts. Caulk around the window after the siding is replaced.