How to Install a Window Step-by- Step

Measure the roughed-in opening for the new window. This measurement is necessary to ensure you have the proper sized window for the opening. Measure the diagonal of the opening from the lower left corner to the upper right corner. Measure the diagonal from the lower right corner to the upper right corner. The measurements should equal one another if the window opening is square.

Cut strips of 8 inch wide waterproof barrier. Cut four pieces one for each side of the window openings. Each piece should measure about 18 inches wider than the window opening. Tuck the waterproof membrane under the edge of the home’s siding. Wrap the membrane through the window opening and secure to the inside of the window frame with staples. Repeat with the remaining sides.

Set two half-inch spacers into the bottom corners of the window frame, recommends Don Vandervort. Use your level to ensure that the bottom of the window fitting is level. If the piece is not level, add shims on the low side of the frame until it becomes level.

Fold out the nailing fins on the window unit. Carefully slide the bottom edge of the window into the opening. Lean the window forward until the top edge is also installed. Check that the window is level. Secure the window in place by hammering 1 ½ inch roofing nails partway through the nailing fins on all four corners of the window.

Check that the window is level from the outside. Hammer in the nails applied in the previous step. Use your nail gun to fire nails into each corner and then around the perimeter of the window casting. Apply a layer of caulk around the window to further seal and waterproof the area.

Check that the window is level from the inside. Apply shims if necessary to level the window. Add a thin layer of polyurethane foam insulation into the cracks around the window. Add trim around the window to further seal the hole. Most trim can be applied by nailing or gluing it in place.