How to Install Exterior Casings Around New Windows

Walk to the exterior side of the new window, and set up a ladder under the window if it is not within reach. Measure the window's length and width, standing on the ladder to do so if necessary. Double the measurements to arrive at the total length of casing required for the job.

Lay 1-by-4-inch casing trim on a table, and use a pencil to mark it for each of the four pieces needed for the window. Don safety glasses, and cut out each casing section using a power saw.

Apply exterior-grade primer to all the casing trim's surfaces with a paintbrush if the casing trim is wood. When the primer is dry, apply exterior-grade paint to all the wood casing trim's surfaces. If the casing trim is vinyl, don't apply primer or paint.

Apply a 1/2-inch wide bead of silicone caulk around the entire perimeter of the window opening, placing it at the inner edge where the casing trim will reside.

Pick up the casing trim section for the bottom of the window, and press it into place at the bottom of the window opening, ensuring that it is centered. Hammer 1-inch long, galvanized nails into the trim 1 inch from each of its ends. Insert more of the nails evenly spaced through the middle of the casing section.

Place the casing section for the top of the window against the wall over the window, and ensure it is centered. This section is the header. Insert 1-inch long, galvanized nails through the header to hold it to the wall.

Measure the distance between the bottom of the top casing trim and the top of the bottom casing trim to determine whether or not the side casing trim pieces require trimming. If necessary, cut them to the proper size with the power saw.

Install each side casing trim piece in the same manner you installed the top and bottom casings, pressing them into the silicone caulk and then nailing them into place.

Apply a 1/4-inch wide bead of silicone caulk around the perimeter of all of the casing trims pieces. Fill the nail heads with wood putty.