How to Install a Single Hung Window

Measure the window’s rough opening diagonally, from the upper right corner to the lower left, and from the upper left corner to the lower right, making sure these two dimensions are within ?-inch of matching.

Place a level on the rough openings to verify the sill is level.

Measure the rough opening’s width and cut a length of flexible adhesive window flashing to this dimension plus 16 to 18 inches.

Apply the flashing piece by removing the half-strip of waxed paper from the adhesive backing and positioning the flashing edge flush with the inside edge of the sill and centered in the opening. Spread the flashing across the sill and vertically up each side of the opening. This flashing piece is called the sill pan and prevents water from accessing the wall in the event the window seals fail.

Complete the sill pan flashing by removing the second half strip of waxed paper from the flashing back and stretching the flashing onto the adjacent wall surface. Slowly stretch the flashing material around the outside corner radius and do not cut the flashing as this risks water leaks.

Position sill shims on the sill, flush with the exterior edge, approximately three inches from the sides and two spaced equally across the sill.

Rest the base of the window frame on the sill shims positioned in step 6, and insert the window into place in the opening, keeping the vertical gaps equal on each side.

Nail through the top hole in the nailing flange on both sides of the window with an 8d nail.

Position a framing square at the corners of the frame to verify the frame is square. Adjust the frame as needed to square, then secure with an 8d nail in the lowest nailing flange hole at each side of the window. Make sure to keep the gap between the window frame and the wall stud equal at both sides.

Secure the window to the wall framing by the nailing the remaining holes in the nailing flange with 8d nails.

Measure the height of the window frame cut a piece of flexible adhesive window flashing to the dimension plus 10 inches. Remove both of the paper backing strips from the flashing piece cut in step 11 and apply the flashing vertically to the wall surface on the right side of the window, holding the top end two inches above the top of the window frame and lapping it over the nailing flange and the sill pan flashing installed in steps 4 and 5. Rub firmly over the flashing surface to a insure full contact with the wall surface.

Repeat Step 11 to install the flashing on the left side of the window.

Determine the outside horizontal dimension from the outside vertical edge of the side flashing pieces installed in Steps 11 and 12. Cut a piece of flexible window flashing to this length and apply to the wall surface above the window frame, covering the nailing flange and lapping over both pieces of side flashing. Rub firmly over the flashing surface to ensure full contact with the wall surface.

Apply a continuous bead of silicone caulking to the edge of the flashing where it meets the window frame to provide a final seal against water penetration.