Installing a Bay Window

This story shows you how to install 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

Attaching the window: Step 1

Cut and frame an opening. Flash the opening. The jambs of the window shown fasten directly to framing members. Some other bay windows have flanges; with such units you'll need to set the window in temporarily, shim it square, mark for cutting the siding to accommodate the casing, and cut the siding.

Attaching the window: Step 2

Following the manufacturer's instructions mark the fastener locations. Drill four or so holes in the unit to allow you to firmly fasten it in place for shimming and leveling.

Attaching the window: Step 3

Cut a pair of temporary supports. With a helper or two, raise the window into place and temporarily support it. Check for level and plumb all around and shim as needed. Drive screws at the top and sides to temporarily hold the window in place. Check that operable windows open smoothly.

Attaching the window: Step 4

Make a template or carefully measure so you can prefab the roof. It is much easier to finetune the angled cuts and install the pieces while working at ground level. Drill large holes for the support cables. Measure and cut the final sheathing pieces.

Attaching the window: Step 5

Set the roof in place and mark where you'll cut the siding to allow for the flashing. Use a spacer to mark for adequate flashing space. If needed mark the siding for cutting, remove the window, and cut the siding.

Comments (2)
Add your comment

You must be logged in to leave a comment. Register | Log In
One Hour or Less

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