Most dog doors make use of a flap made from heavy plastic, scored around the edges and attached to the top of the entrance with screws that pass right through the frame. The bottom edge of the flap is gripped by a strip of metal cladding, which is drawn to the magnetic field present in the opening to keep the flap closed. Common areas of damage to the flap include splitting at the top where the frame meets the plastic, and bending or warping of the metal strip at the bottom.
Clean the flap in order to check it thoroughly. You may find it difficult to see small tears and splits among the splatters of mud and caked dust that dog flaps generally attract. Wipe it down in position with warm, soapy water and a soft cloth, then dry it to remove any streaks of soap residue that might hamper your view.
Check the flap to see if the material itself is intact, or if any damaged areas exist. If your flap is located in a high wind area, debris blowing against the flap may cause scratches and tiny tears. In cold climates, the metal strip may freeze against the frame and when the dog pushes through the flap, this can cause small tears to expand.
Remove the flap if you find any areas of damage, as you need to be able to work on the flap on a flat surface such as a table to do repairs. Remove the metal strips from the flap by gently easing the two sides apart, or slide the metal off the flap if it is held on by a ridge in the plastic. Flatten and smooth the metal strip if it is bent or warped.
Repair the flap, using industrial plastic curtain material. You can purchase this from most industrial supply companies, and it resembles the plastic used in butcheries to screen off the meat-cutting area from the storefront. Glue a plastic piece cut to the same size as the flap on either side, replace the metal strips and screw the flap back into position.
Replace the plastic flap with a piece of heavy-duty carpeting cut to the same size, or glue a section of carpet to the plastic flap instead of plastic curtain material. This provides a solid flap that closes immediately because of the extra weight and is more likely to block the cold.
Winterize the flap by building a tunnel around it, either indoors or outdoors, with a second flap at the end of it. Do this by removing the two ends from a wooden box, attaching the box to the door around the flap, and hanging a section of heavy carpet over the other end of the box. The dog enters or exits through the plastic flap into the box, which closes by the time the dog pushes open the carpet flap at the end of the tunnel.