Why does my flap rip along the top edge? If you have a flap-type door and the flap repeatedly rips along the top edge, the door is probably too small for your dog – or installed too low. Make sure the top of the pet door opening is a couple of inches higher than the back of your tallest pet. Cut the opening size in a piece of cardboard and see if your pet can go through without problems. You can also read more about measuring your dog in our article, “What size pet door should I get.”