Trimming a Bay Window

This story shows you how to install a bay window.

Installing a Bay Window

The farther a bay window protrudes from the house, the more dramatic the effect and the more it opens a room. You can choose among units that protrude at 45-, 30-, or 10-degree angles; the latter is sometimes called a bow window. Individual windows may be fixed, double-hung, or casement.

A bay unit attaches like a standard flanged window but with some important differences. Some units also require support from above using cables that attach to framing members in the eaves or the wall above. Some require support from below using brackets or a knee wall. Some need both types of support. A smaller bow window may not need this type of support and can be installed much like a standard flanged window.

Plan how you will finish the top and bottom of the bay window. You may be able to purchase a ready-made roof or you may need to custom-build one. The bottom is usually easy to trim out, but you may choose to build a wall down to the ground. In these the roof and skirt are built on the ground and then installed.

Prestart Checklist

A day or two depending on trimwork

Tape measure, drill, hammer, level, stapler, caulk gun, screwdriver, flat pry bar, circular saw

Good carpentry skills

Cut the opening and check for square. You will need one or two strong helpers.

Bay window, exterior casing, 2x4s, shims, caulk, roofing felt or building paper, drip cap, flashing, plywood, roofing, finishing nails, staples, roofing nails, interior trim, casing nails, insulation

Trimming the bay window: Step 1

Add facia to the skirt. Make the angled cuts with a table saw, power mitersaw, or a circular saw. These angles can be difficult; always try the joint before cutting the piece to final length so you can recut the angle if needed. Tack each piece in place using 6d galvanized box nails. Complete the final nailing only when you are satisfied with all the joints.

Trimming the bay window: Step 2

Complete the fastening of the jambs and seat. For a clean finish, bore countersinks to hide the screw heads. Use a Forstner bit to make a clean hole. Purchase wood plugs for the holes.

Trimming the bay window: Step 3

Make a final check for level and plumb all around. Adjust the shims accordingly. Check again that the windows operate smoothly and are aligned with the frames. Fasten the screws in place.

Trimming the bay window: Step 4

Apply carpenter's glue to the plugs and set them in the fastener holes. Gently tap the caps until they are flush with the surface; most are tapered for this purpose. Once the glue dries, lightly sand the caps.

Trimming the bay window: Step 5

Stuff insulation into the gaps between the side and top jambs and the house framing. Stuff gently but fill the spaces completely. Or use nonexpanding foam insulation.

Trimming the bay window: Step 6

Tack pieces of casing in place and measure for the length of the apron. Fasten the apron to the seat board with finishing nails. (If your unit has a seat with a finished edge that extends beyond the wall, place the apron under the seat.)

Trimming the bay window: Step 7

Install the casing. Fill all the nail holes with wood putty, sand smooth, and apply finish or paint.

Trimming the bay window: Step 8

Install the opener cranks or other hardware and the window screens.

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One Hour or Less

Three simple projects to cross off of your to-do list -- just print these instructions and begin!