Installing a Round Window

This story shows you how to install a round window.


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 like the one shown, 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.

Prestart Checklist

With a rectangular opening cut, about 3 hours to frame and install a half-round window

Tape measure, hammer, drill, scribing compass, reciprocating saw, circular saw or power miter box, stapler

Measuring; framing; using a circular saw, jigsaw, and reciprocating saw

Cut and frame the opening on the inside, but do not cut the sheathing.

Round-topped window, cardboard for a template, 2x4s for angled framing pieces, roofing felt or building paper, self-stick flashing tape, nails and screws

Installing a Round Window: Step 1

Place the window onto a piece of cardboard and trace its outline. Cut the cardboard along the line with a utility knife.

Installing a Round Window: Step 2

Tape the template to the sheathing and mark for cutting the opening.

Installing a Round Window: Step 3

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.

Installing a Round Window: Step 4

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.

Installing a Round Window: Step 5

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.

Installing a Round Window: Step 6

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.

Installing a Round Window: Step 7

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.

Installing a Round Window: Step 8

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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