Sliding screen doors are found on most patio doors. They are attached to a track above and door and along the threshold below. These screens are convenient because they allow the patio door to be open. Air can then come into the house without allowing insects to enter. There are different types of screen doors depending on the manufacturer of the patio door, but for a generic screen, measurements can be taken so the proper size screen can be installed.
Unlock and slide open the patio door. Locate the header at the top of the track on the inside of the patio door. Feel the shape of the track at the top and determine if it is T-shaped or U-shaped.
Measure from the top of the track at one end of the door down to the threshold on the ground with a tape measure. Write down the measurement. Move halfway down the opening and measure again. Write down the measurement.
Measure a final time from the end of the door. Compare the measurements and make sure they all match up. This will verify the door is square and plumb in the opening.
Add 3/8-inch to the measurement for a T-shaped header and 1/8-inch for a U-shaped header.