How to Install a Preassembled Block Window

Take out the existing window if necessary. Insert the pre-assembled block window into the hole, and center it. On the exterior of your home, measure and mark the outside edges of the window. Remove the pre-assembled window, and set it aside.

Pry off the exterior siding located between the window hole and the marks. Remove vinyl siding with a zip tool and wood siding with a pry bar and hammer. Cover the exposed wall sheathing with self-adhesive flashing. Slip flashing underneath the siding on all sides for added protection from moisture.

Add a layer of caulk to the back side of the window flange, and press the pre-assembled window firmly into the hole. Center the window, and partially secure it with a single roofing nail driven halfway through the top edge of the flange. Enter the house, and ensure that the window appears centered from the interior.

Secure the window's exterior by driving more roofing nails through the flange on all sides. Cover the nailed flange with more flashing. Apply caulk to a piece of metal drip edge, and install it behind the siding above the window.

Measure and cut the ends of the brick molding at 45-degree angles, so that you can create miter joints at the four corners of the window. Secure the molding over the edges of the window with galvanized casing nails. Fill the gaps between the molding and siding with caulk.