Framing for a round, octagonal, oval, or half-round window starts the same as for a standard window. Cut and frame a rectangular opening to accommodate the window at its greatest dimensions—in this case the top of a half-round window's arc.
The installation of the round window shown here includes sheathing with self-adhesive flashing tape. There is no metal drip cap over the rounded top; the joint between the curved brick molding and the siding is simply caulked. For a more weatherproof installation, have a roofer or a sheet metal shop make up a curved drip cap to fit your window.
With a flanged window, it often makes sense to remove a section of siding so pieces that overlap the flange can be woven in. For curved cuts through heavy materials, a reciprocating saw is nearly indispensable. You can rent one if you don't own one.
You'll need roughly 3 hours to frame and install a half-round window in a rectangular opening. Before you begin, cut and frame the opening on the inside, but do not cut the sheathing.
Place the window onto a piece of cardboard and trace its outline. Cut the cardboard along the line with a utility knife.
Tape the template to the sheathing and mark for cutting the opening.
Bore an access hole and use a reciprocating saw to cut the opening. Cut to the outside of the line so the opening will be 1/4 inch larger than the window.
To measure for a short framing piece with 45-degree cuts on either end, hold a layout square against the stud and measure along its edge toward the header.
Use a power mitersaw or a circular saw to cut the piece with a 45-degree bevel at each end. Test the fit; it should be just outside the curved cutout. Drive nails to install it.
Remove the siding so you can weave it in. Add flashing, working from the bottom up. Install the window. To mark the new siding, scribe with a compass. To keep the desired overlap, set each piece on partially driven guide nails. Hold the compass level as you mark.
Cut the siding using a jigsaw. Try the fit after cutting, rescribing with the compass as needed. Once you are satisfied with the fit, hold the siding in place and mark its length for cutting.
Apply a bead of caulk where the piece will be applied and nail it in place using 6d galvanized nails. When all the siding is installed, caulk around the window.
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