How to Install a Bay Window

Cantilevered bay windows, box bay windows, and bow windows can be built in the same type of opening as conventional windows. The job is heavier but in essence it is only a little more involved than installing a conventional window. Be aware that the building-code requirements for the rough sill for a bay window may be different than for a conventional window, because it must support considerably more weight. To install one of these windows higher up than the first floor call a professional.

You can buy a bay window as a kit that includes the windows, head board, and seat board, plus various trim pieces you must assemble. Or you can buy a completely assembled unit ready to install. With the latter type, you just build the roof over the window(unless it reaches the soffit), install support brackets, and finish the inside. Some manufacturers supply a precut roof or roof framing and will also supply the support brackets or cable support system that they recommend. The installation method shown here is for a bay, but it’s applicable to a bow or box window as well.

Be sure you have all the necessary materials before you start. Then prepare both the window and the opening.

Assemble the window according to the manufacturer’s instructions, and check its outside dimensions against the rough opening. It should be about ½-inch shorter than the opening in both height and width. If you need to cut back the siding, follow Steps 3 and 4.

A bay window is heavy! To set it in place, you’ll need helpers, one for every 3 feet of window width. You’ll also need a sawhorse every 3 feet to support the window once it’s in the opening.

Before starting installation, read through the following directions and study the illustrations to become familiar with the various parts and steps.

To install support brackets, toenail a cripple stud to the rough sill and the sole plate under each mullion. Position one of the support brackets under one of the mullions with the long leg against the siding; the short leg goes against the underside of the window. Use 3/8-inch lag screws to fasten the long leg through the siding into the cripple. The other leg is fastened to the underside of the window with wood screws. Install the other bracket. For large windows, use more brackets.

Some manufacturers suggest installing brackets before the window is lifted into the rough opening; others suggest raising the window into the frame first and adding the brackets afterward. If your window requires a cable support system instead of(or in combination with) support brackets, follow the manufacturer’s instructions.

Positioning The Window Prop the window in the rough opening and have your helpers hold it there while you go inside to get it level and plumb. Nail the highest end to the sill, about 2 inches from the jamb, then drive shims from both inside and outside between the sill and the seat board, placing shims over each cripple stud, until the seat board is level. Nail through the shims to the rough sill.

While helpers move the window, drive shims between jamb and the trimmer stud until the jamb is plumb, nail it.

Unless your window comes with a preassembled roof, you’ll have to build one; design it to reflect the style of your house.

Cover the top of the head board with insulation, packing it loosely inside the frame. Attach the end pieces of sheathing, then the center. You can shingle the roof of the match your house or have a sheet-metal professional it with copper. (Some manufacturers provide a copper roof as an option.) If you shingle, you’ll need to flash it would the main roof.

To finish the unit, install exterior trim and caulk along it. If needed, install a skirt and bottom panel.