Installing a cat flap is a job probably best left to a professional. However, if you’re a competent DIY-er and keen to have a go, here are my top tips

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Location. when deciding where to install a cat flap, bear in mind whether a burglar would be able to reach through the flap and access the door latch or key. Never leave the door keys in the lock overnight.
Consider seriously whether the presence of a pet door might compromise your home security. Burglars could climb through dog doors so take special care to secure the internal door
Wooden door. It’s generally quite straightforward to fit a cat flap in a panel of a wooden door. A suitably sized rectangular hole is cut with a jigsaw, reciprocating or circular saw. Special provision must be made where the door panel comprises tongue and groove boards, or where decorative beading interferes with the area occupied by the cat flap.

With wooden doors, it's particularly important to make the installation weathertight - rain penetrating into the door will in time result in the wood rotting. I always form a seal between the cat flap and the outside surface of the the door using a long life weatherproof sealant
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Single glazed window or door. Older doors and windows in particular may contain glass which has not been toughened by a special heat treatment to make it safe. This should be replaced by a new pane of laminated or toughened safety glass in which a circular hole has been pre-cut.
Sometimes the glass is too narrow to accommodate the circular hole required for a cat flap. In this case a pane of solid polycarbonate may be used to advantage as a tighter rectangular hole can be cut in this
Double glazed window or door. Double glazed doors and windows usually contain safety glass which has been toughened by a special heat treatment. Any attempt to cut a hole in this will cause it to fracture into myriad tiny pieces. So if you have a double-glazed window or door you will need to get a replacement unit manufactured with a circular hole pre-cut for the cat flap. A replacement full size double glazed door unit costs from about £100 to £200 or so depending on its size and specification
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uPVC, GRP and composite door or panel. The construction of these varies greatly. Some comprise a very flimsy plastic outer skin bonded to a soft insulating polystyrene core. Great care should be exercised in cutting through these as they are quite fragile - especially in cold weather! Some uPVC door panels incorporate a sheet of ply or mdf for added strength while better quality doors incorporate a sheet of aluminium or steel for security. All these can still be cut with a standard electric jig saw fitted with a fine toothed blade.
Additional complexity occurs where the panel incorporates raised decorative features or beading. Not only must the raised feature be cut away intricately to accommodate the pet flap neatly, but the outside face must be made rainproof where the raised feature has been cut back
Metal faced door.  These doors offer greater security than the ubiquitous uPVC door. The metal may be aluminium or steel, steel being far stronger than aluminium. They are treated like uPVC doors but may be tougher to cut through. Bear in mind that any metal within a door may interfere with the functioning of a microchip cat flap. Sometimes an oversize hole will have to be cut to avoid such interference. In this case it will be necessary to fill in the oversize gap and render it rainproof
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Brick wall. A rectangular hole can be cut through a solid or cavity wall. It can be lined with a special plastic tunnel, or the inside face of the hole rendered with cement mortar. The cat flap is then fixed to the interior surface of the wall. Check that there are no electric cables, gas, water or waste pipes where you are going to cut into the wall. Avoid cutting a hole very close to a door or window as this could weaken the wall. A series of holes drilled through the wall with a masonry drill facilitates chopping out the hole using a cold chisel and club hammer. Cut the tunnel with a slight slope to the garden so that it can shed any rain that might blow into it. On the outside face of the wall, fill the gap between the cat flap tunnel and the surrounding wall to prevent rain getting into the wall
Weatherproofing. Take care to seal between the cat flap or tunnel liner and the outside face of the wall, door or window in order to keep rainwater out -  see the accompanying pictures. Use a suitable flexible sealant such as building silicone or frame sealant, although I prefer to use one of the superior MS polymer sealants
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Accessibility. Sometimes the only place a pet door can be fitted is a window too far above the ground for the cat or dog to reach easily. In such cases a step, stairs, ramp or even a ladder can be provided to help the animal access the pet door

 

The contents of this page have been reproduced with the generous permission of Philip Foulger of Four Paws Doors