A cat door, or any pet door, offers your companion animal the ability to move from indoors to outdoors or from room to room within the home. Commonly used in the interior of the home hollow core doors are made of two wood veneers held apart by a frame, creating a door with a hollow centre. Installing a pet door in a hollow core door requires some additional steps to the standard cat door installation process.
Cut the door opening on one side of the hollow core door. Mark the door opening using the template supplied with the cat door. Drill 1/4 inch holes at each corner and cut between them with a jig or sabre saw.
Drill holes through the veneer of the opposite side of the hollow core door from the inside aligning the corners with the corners of the first cut. Cut between the holes with a jig or sabre saw from the outside of the door.
Measure the gap between the veneers of the hollow core door. Rip, cut lengthwise on a table saw, a piece of wood to this exact width.
Cut the ripped board into four pieces with two equal to the height of the pet door opening and two equal to the width of the pet door opening.
Place the pieces between veneers of the hollow core door. Hold in place, that's the tricky part, while running a 1/2 inch screw through the veneer and into the ripped board. The ripped boards serve as a brace to hold the veneers apart and also as anchoring points for the door cat door frame. Run two screws from each side into each of the four pieces around the door.
Install the cat door in the opening according to manufacturer's instructions. This usually involves attaching components of the door to each side of the hollow core door with a flexible segment hanging across the opening.
Train the cat to use the pet door by blocking the covering partially open until the animal is comfortable moving through the door. Cat doors, such as those installed in an interior hollow core door, are often used to give the animal access to rooms where feed or litter boxes are kept away from the family living space.