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Cat flaps allow your pet to leave and come whenever they want, without you having to let them in or out. They can also help prevent your cat from peeing in the house. It is much easier to put a cat flap on a upvc panel than on a glass one, but you will require some expertise and right tools. Measure the Door The installation of a cat flap inside the door isn't easy however, as long as you choose the right type of cat flap, take your time and follow the directions provided, it shouldn't be too difficult.