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0115 838 9905

0115 838 9905
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Fitting a Cat Flap to Your External Doors

Owning a cat can be one of the most rewarding experiences in a person’s life. They offer companionship without the cloying attachment issues that are often associated with dogs. 

For families, they are also wonderful for teaching younger and older children about responsibility and for bringing families together. They are also, however, wildly individual creatures who value their freedom as highly as they do their domesticity.

For this reason, if you’re going to own a cat, you are probably going to want to invest in a cat flap for your front and rear external doors. The advantages of a cat flap are numerous and should be obvious to anyone who has ever owned a cat:

  1. The days of being constantly pestered (or clawed at) when your furry friend fancies a stroll will come to an instantaneous end.
  2. Whilst the litter box is always an option, many owners prefer their cats to ‘do their business’ outside and with a cat flap, they will be free to come and go (go being the operative word here) as they please.
  3. If you trust your cat enough to leave and enter the house as they please, they are likely to respect and care for you to a far greater degree than if you were to essentially keep them prisoner in their own homes.

cat looking through cat flap


Of course, there are only really three valid reasons to not consider installing a cat flap. If you don’t feel your cat is ready yet, if you are worried about random cats entering your home or if you’re worried about security. 

These are all fears that can be alleviated, however. If you don’t think your cat is ready to venture into the great outdoors on its lonesome, cat flaps can be locked until you and your cat are ready. 

For door security, meanwhile, as long as you are smart about where you place your cat flap, you shouldn’t have any issues. Make sure the cat flap is located far enough away from the latch so that a pair of prying hands couldn’t manoeuvre their way through and never leave your keys in the door overnight. 

If you’re really concerned, you could even install a flap with a magnetic strip, which will only open for your cat, who can wear a corresponding tag on their collar. This also solves the rogue cat problem.


Of course, installing a cat flap in an existing external door can be a difficult task if you’re not a DIY master, but it’s certainly not an impossible task. To save you the cost and hassle of calling in a carpenter, joiner, or handyman, we’ve compiled a short, simple guide that should take even DIY newbies no more than a couple of hours to make it through. 

We will be going through the unique eccentricities you’ll find with each of the main four building materials below, but in all cases, remember to always take precise measurements and make sure you have the right tools for the job.

Fitting a Cat Flap to your External Doors

Tools: The cat flap itself, tape measure, a decent pencil, spirit level, file, drill and jigsaw.

Before going any further, make sure you read the instructions supplied with your cat flap. These instructions should also include a list of any extra tools you might need, aside from those listed above. 

  • Start by measuring your cat against the door in which you’ll be installing the cat flap. This is so you can find a comfortable place to fit the flap, so it will be easy and comfortable for your feline companion. Measure between the bottom of their stomach and the floor right by the door and then draw a horizontal line on the door as close to that height as possible. Remember to use a spirit level to keep everything flat.
  • Now comes the fun/messy part. The cutting. Some flaps will provide a template you can trace against the door, but if yours didn’t, you can always use a sheet of thin paper instead (tracing paper ideally), so don’t panic! If you have a square flap then drill holes at each corner of the shape you have pencilled in on the door and if it’s circular, drill holes at 12 o’clock, 3 o’clock, 6 o’clock and 9 o’clock.
  • Use the holes you drilled as a starting point for your saw. Take your time. If you rush you might make a mistake that will be very difficult to fix. Don’t force it, let the tool do most of the work. Once you are done and the cut is made, smooth out the edges with sandpaper. Don’t use a power sander as you might go too far!
  • Now you have your opening it’s time to fit your flap. Hold it firmly against the door with the flap on the outside for maximum protection. With the flap in place, push it open to make sure it does so smoothly and efficiently. If it’s catching at any points, use a file or sandpaper until the fit is snug and the flap opens smoothly.
  • Finally, pick your pencil up again and place the flap back against the hole, marking where the fixing holes need to be around the opening. Use your drill to make these holes then use the bolts that should have been supplied with the flap to secure it. 

Of course, every door will be a little bit different and the main difference will be the material your external door is built from. Below, we’ll outline the specific eccentricities you might face when fitting your cap flap in doors built from the four most common building materials.

Fitting a Cat Flap to a UPVC Door

Perhaps the most common and affordable building material for external doors in the UK, the quality of these will vary greatly, but in all cases, you’ll want to exercise greater care than you might with a wooden, glass or composite door, as the material is generally more fragile. 

This is particularly true in cold weather. You should be safe following the instructions above, but take into account that some uPVC doors might incorporate a sheet of MDF, plywood or even aluminium/steel for added security, so you might need to use a sturdier blade on your jigsaw. 

In the vast majority of cases, however, a decent fine-toothed blade should do the trick. The primary complication experienced with uPVC doors is seen in cases where the doors feature a raised decorative piece. Avoid these where at-all possible and install your cat flap around them. If that’s not possible, be careful when cutting the raised piece away and remember to rainproof when you’re finished.

Fitting a Cat Flap to a Wooden Door

By its very nature, wood is more natural and easier to cut than uPVC, but it’s not without its challenges! 

Whilst it is a rather straightforward task on the face of it, special provisions should be made where there are decorative protrusions or tongue and groove boards making up the door pane. 

It's also particularly important, with wooden doors, to make sure that the seal on the cat flap is weathertight, as wood can rot if exposed to rainwater over a long period of time. Sealant is certainly recommended in this case.

Fitting a Cat Flap to a Glass Door

cat flap in glass door

When fitting your cat flap into a glass door, the job is obviously a little more delicate. 

All glass doors (such a bifold patio doors, for example) will require the skills of a professional glazer, as cutting glass required specialist tools. Toughened glass also can’t be cut after the toughening process, so if you require a cat flap in a double-glazed glass unit, the glass will have to be replaced with a new double-glazed glass unit as any attempt to cut into the glass will cause it to fracture. 

If the glass is not toughened, you might want to consider ordering a new pane with a hole pre-cut to suit. Once the hole has been cut, the process is largely similar to the process of installing in a wooden or uPVC door. The primary difference is that the flap will be installed into a mounting unit that obviously won't require any drilling. As with wooden door installs, weatherproof sealant is highly recommended. 

If the glass is too narrow to accommodate the opening required for the cat flap, a pane of solid polycarbonate could be used instead. In any case, you will require the assistance of a qualified glazier if you intend on fitting your cat flap into an external glass door.

Fitting a Cat Flap to a Composite Door

Composite doors are generally built to be more weatherproof, secure and energy efficient than standard doors, which means it can be more difficult to make any DIY alterations, but not impossible. 

It's recommended, if possible, to have the doors pre-built with the cat flap already installed, but if this is not possible then you'll need to make your judgement based on the materials in the construction of the door. 

All composite doors are different and built from various mixes of wood, uPVC and metal, so please consult the manufacturers if possible and also, if possible, consult a professional joiner before even considering the job as making any alterations to a composite door can invalidate your warranty or break the weathertight seal. If it can be done, the process is very similar to the process for fitting in a uPVC door.

FAQs About Fitting a Cat Flap

How Do I Stop the Draught from a Cat Flap?

 The simple and obvious answer would be to purchase draught excluding material from your local DIY store. This can be applied as the cat flap is being installed and will make a significant difference. Another idea would be to purchase a cat flap with a built-in, brush draught excluding brush or plastic system, or to fit one into an existing flap. A final option would be to fix a piece of material that's heavy enough to minimise the draught, but not too heavy to stop your cat from easily accessing the flap, above the cat flap. 

How Do I Remove a Cat Flap?

Most of the time you're honestly better off replacing your door completely, as filling in the cat flap could seriously compromise the security. If your door is wooden or simple uPVC, however, you could quite feasibly out the entire panel in which the cat flap is fitted and replace it with an entirely new panel. If the cat flap is installed in a glass panel, however, you should again consult a professional glazier.

Can I Get a Cat Flap That Doesn’t Damage My Door?

If you’re worried about damaging your doors, there are cat flaps available that can be installed in a pane of glass, rather than through the door itself. Some people, meanwhile, choose to put a cat flap on their outdoor shed rather than their house and make the shed into a home for the cat with a bed and litter tray.

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