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A cat flap lets you to allow your pet to roam more freely while saving you the hassle of having to let them in and out. It will also make your pet less anxious and will stop them from coming into the home when they aren't welcome. To set up a cat flap take a measurement of the distance from the bottom of the cat's stomach down to the floor. Draw a line along the door's frame. Make sure the line is level with a spirit level. 1. The kind of door If you are installing a cat flap into an uPVC door panel or double glazing glass sealed