Building a Cantilever for a Bay Window

What You'll Need
Circular saw
Framing square
(12) 2 x 6 8 lengths
(3) 4 x 8 plywood
Tape measure
Cordless drill
3 Galvanized coarse thread screws
1 1/4 " Galvanized coarse thread screws

Building a cantilever is an important part in the overall installation of a bay window. Careful attention to this framing is crucial since it will support the majority of the weight of the 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.


Step 1 – Layout the Framing

The framing for your bay window cantilever will consist of 2x6 lumber angled at 45° to the side of 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 fall. 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 hits the exterior of the house. Now, using the 4 foot level, draw a horizontal line that intersects the mark you’ve just made.

Step 2 – Install the Framing

The first step in framing under the bay window is to fasten the cleat to the exterior wall. You’ll have to remove the siding and insulation first so the cleat can lay flush. Next, cut the 2x6 to match the dimensions of the bay window using the circular saw. Make sure you locate the studs when fastening the cleat to the wall. Before taking the measurements, t's a good idea to check the level and plumb of the window installation. 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 the framing square. Then repeat the process for the angle at the bottom of the cleat.  Next, transfer the 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 in between using the same technique to measure each piece. Use the cordless drill and 3 inch screws to fasten the framing members to the bay window and cleat.    

Step 3 – Install the 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 than make sure the seam falls on the center of a framing member. You’ll need the cordless drill and 1 ¼ inch screws to fasten the plywood to the framing. Ideally, you should space the fasteners about 8 inches apart.