When installing an aluminum window yourself, there are several steps that need to be followed so that the window will operate properly. This is not a novice do-it-yourself job. You should have some understanding of carpentry, such as checking for square and plumb, reading a tape measure, measuring a rough opening and using shims.
Inspect the window. Make sure it is the right size and shape. A quick check with a tape measure will confirm the size marked on the packaging is the size the window actually is. Check for damage such as cracks, gaps or twisted framing. Bent corners can usually be bent back into shape with pliers. Inspect the nailing fin to make sure it is secure to the window unit. This is the main fastening component of the window.
Make sure your window is square. To confirm square, measure the window diagonally both ways, like an X. If these dimensions are not equal, the window is not square and should not be installed.
Remove packaging material from the window unit and clean the frame with a cotton cloth. Check the lock to make sure it functions properly. Leave the unit in the locked position for installation.
Use a weather seal silicone caulk on any gaps in the framework, making sure you are not sealing the working portion of the window.
Check the manual for your window to see if it requires spray adhesive. Apply a thin coating of spray adhesive to the rough opening of the window frame. Make sure not to spray the glass on your window. Once the spray adhesive is on, apply a 3-inch-by-3-inch piece of self-adhesive flashing to each upper frame corner.
Prepare the sill pan. The sill pan diverts water away from the window to the outside and helps avoid water damage. It is three-sided and made out of sheet metal. The window should sit in the sill pan on the rough opening. Follow the manufacturer's instructions to properly cut, bend and size your sill pan. There should be an extra inch for flashing on the outside of the rough opening.
Prepare the rough opening. It's the studded framework that the window sits in, which should be ½- to ¾-inch wider and taller than the window frame. Check the rough opening for square as you did the window. Using a bubble level, check the opening for level and plumb. Level is the horizontal dimension and plumb is the vertical. None of these measurements should exceed 1/16 inch for every two feet of rough opening and this should never exceed 1/8 inch. The rough opening has double studs for reinforcement. If there is building wrap around the rough opening, peel back 3 inches from the rough opening.
Install the sill pan. Apply adhesive spray 3 inches from the edge of the sill pan and 6 inches along the sides of the rough opening adjacent to the sill pan. Seal the corners of the sill pan using self-adhesive flashing. Cut the flashing to the size of your sill pan, attach it to the pan and apply the plastic drain screen if provided.
Check your window and position it with its right side up and right side out. Check the location of the weep holes. If your window is positioned correctly, the weep holes will be facing outward.
Fit the window into the rough opening. Check for square and plumb. If the window is not square or out of plumb, adjust the position using wooden shims to compensate. Attach the window to the rough opening by driving a nail through the nailing fin 3 to 7 inches from the corner. Leave some play so you can adjust the shims again before completely nailing down. Check the shims again for square and plumb. Once you are sure the window is in the proper position, run a bead of sealant around the shims.
Test your window to make sure it will open and close and its lock works before completely securing the frame. Secure nails and apply sealant to all of the seams and the window unit.
Install the drip cap on the header, maintaining a 1/8-inch overhang on both sides. Nail the cap to the header and seal the nail fin completely with spray adhesive. Exercise care with spray adhesive so you do not spray it on the glass. Apply the self-adhesive flashing around the outside of the window frame and pull the building wrap over the flashing. Check the weep holes to ensure they have not become blocked. Take a step back to look over the entire window. If there are no loose ends, your installation is finished.