How to Build a Cat Flap

Measure your cat at the widest part of the body. Measure the cat's height as well. Add three inches to each dimension: two inches for the cat to have ample room to fit through the opening, and room for the trim that will line the flap opening.

Remove the door the cat flap will be built in. You will need a wooden door to install a cat flap. You can find pre-fabricated pet doors designed to fit metal frames.

Measure up four inches from the bottom of the door. The flap should be positioned at this height for ease of comfortable entry for the cat. Use the level and draw a square the size of the dimensions you measured, centered on the door, with the bottom at the four-inch mark you made from the bottom.

Insert the augur bit at a corner of the square you drew for the door. Use this hole as entry for the jigsaw blade, and cut the square out of the door.

Measure the inside dimension of the opening. Cut four pieces of the 1/2 inch board to fit the inside of the square. Cut the two top and bottom pieces to match the dimension of the inside of the square, and cut the two side pieces one inch smaller, to make room for the top and bottom pieces.

Glue and screw the frame pieces in place inside the square, to finish off the opening.

Measure the inside of the square again. Measure and cut a piece of the rubber mat 1/4 smaller on both length and width for the door. Attach the two hinges to the top of the opening, and screw them to the rubber mat.

Cut eight pieces of 1-by-1-inch trim, to the size of the outside dimension of the opening. Glue and screw them in place on both sides of the door to conceal the seams around the cat flap opening, and cover the hinges where they are hung on the door.

Paint or stain the trim if desired. Return the door to its location.