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A cat flap allows you to give your pet more freedom while saving you the headache of having to let them in and out. It will also make your pet less stressed and stop them from entering your home when they are not allowed to do so. To install a cat flap take a measurement of the distance from the bottom of the cat's stomach to the floor. Draw a line across the door panel. Make use of a spirit-level for checking whether the line is straight. 1. The type of door If you're installing a cat flap into an uPVC door panel or double glazing glass sea