Find out what the building code requirements are in your area, and follow the window maker’s instruction on how to install your particular bay window. The bracket fasteners must penetrate the siding into the wall studs. The support cables brackets must line up properly with the window attachment.
Create a plan. You need to work through each step of your plan so that you can take out the existing window, prepare the opening and install the new window in a day. Purchase the materials, rent, borrow or purchase the tools and line up your help. Make sure that you prepare your work area.
Remove the existing header or studs. Use your reciprocating saw to cut through sheathing and siding. You’ll need to install new king studs on both sides. The king stud is the last stud connecting the top and bottom plates of the wall section. Add six inches to the manufacturer’s rough opening width. The additional six inches is the total width of two 2-by-4 trimmers to support the new header.
Install a new sill. The sill is the board that supports the window from the bottom. The height of the sill may be lower or higher than the previous window. If the new bow window is larger, install a double sill for added strength.
Once you have the window installed, the roof and bottom of the window will need to be finished. A bay roof is a small hip roof, with 45-degree angles. You can have a roof custom made or build the roof yourself to fit your window.
The roof is similar to building a standard roof system. Secure the 2-by-6 ledger board on the back wall. The ledger board and 2-by-6 sub fascia boards form the perimeter of the roof. Cut the rafters and install them. Before you enclose the framing, fill it with fiberglass batt insulation. Use 1/2-inch plywood for the roof's sheathing.
You’ll also need to install flashing and a drip edge and cover the roof with felt and roofing shingles.
Frame the bottom of the bay window with 2-by-4s. Secure 1/2-inch plywood to the bottom of the frame. Fill the cavity with fiberglass batt insulation. Install trim on the frame. If you decide to build a wall, you’ll need to pour a concrete pad.
A Finishing Touch
Knee braces may be necessary to support the window from the bottom, and some people install them for purely aesthetic reasons. If you are handy with a band saw, you can fashion knee braces out of 2-by-12 boards. Ready-made knee braces can also be purchased. Use lag screws to fasten the braces to the studs.