Building a cantilever is an important part of the overall installation of a bay window. Careful attention to this framing component is crucial because it will support the majority of the weight of the entire bay window. However, you don’t need to possess the skills of a master carpenter to complete this framing. This project can be tackled by a homeowner provided the right tools and materials are used.
Layout the Framing
The framing for your bay window cantilever will consist of 2x6 lumber that is angled at 45° away from the side of the house. You’ll need to layout the cleat along the side of the house first by using a 4-foot level to mark where the bottom of the cleat will land. Just angle the level so that one side rests on the bottom of the window and then mark where the other side of the level where it touches the exterior of the house. Using the 4-foot level again to draw a horizontal line that intersects the mark that you’ve just made.
Install the Framing
The first step in framing underneath the bay window is to fasten the cleat to the exterior wall. You’ll have to remove the siding and insulation first so that the cleat will lay flush. Next, cut the 2x6 to match the dimensions of the bay window using a circular saw. Make sure that you locate the studs when you're fastening the cleat to the wall. Before taking the measurements, it's always a good idea to check the level and plumb of the window installation one more time. That way you can make any adjustments before you install the framing.
After checking the window, measure the distance from the corner of the window to the bottom of the cleat. Then measure the angle of the cut on the window side of the wood using a framing square. You'll repeat the process for the angle at the bottom of the cleat. Next, you must transfer those angles to the 2x6’s and make the required cuts using the circular saw. You should double the studs at both corners of the window and then fill in the framing between them using the same technique to measure each piece every time for the sake of uniformity. Use the cordless drill and 3-inch screws to fasten the framing members to the bay window and cleat.
Install The Wall Sheathing
Once the framing is complete you can now install the ½” plywood sheathing over the framing lumber. You can install the sheathing by cutting the plywood to match the dimensions for both the sides and the front of the bay window framing. If you need to use two pieces to cover the framing for the front then make sure the seam falls on the center of a framing member.
You’ll need to use a rotary drill and 1 ¼ inch screws to fasten the plywood to the framing. Ideally, you should space the fasteners about 8 inches apart and no closer than an inch from the ends. Predrilling slightly smaller holes for the screws will prevent the ends of the lumber from splitting when you drive them in.