How do I fit a cat flap in a door?
Choosing the position and the height, what each door material demands, and the cutting sequence that avoids splitting the panel.
- Difficulty
- intermediate
- Time
- 2 hr
- Read
- 5 min
- Safety
- warning
Short answer
Set the bottom of the flap roughly at the cat's belly height, drill the corners, then cut between them with a jigsaw. Timber doors are straightforward; uPVC and composite need checking for reinforcement, and glass must go to a glazier.
Fitting a cat flap is a single irreversible cut in a door you cannot un-cut. Almost all the care belongs in the ten minutes before the blade goes in.
Safety
What you'll need
- Drill with a large bit, and a jigsaw with a fine blade
- Pencil, spirit level and masking tape
- The flap's paper template, if supplied
Step by step
- Establish what the door is made of.Solid timber is easy. Hollow-core internal doors need a frame built inside. uPVC and composite are cuttable but may hide a steel or aluminium reinforcing bar. Glass cannot be cut in place at all.
- Check a uPVC or composite door for reinforcement.A magnet run over the panel finds a steel bar. Some doors have one and some do not, and hitting one with a jigsaw ruins the blade and the door.
- Send glass to a glazier.Double glazed units are toughened and will shatter into fragments if cut. A glazier makes a new unit with the hole formed during manufacture. There is no DIY route.
- Measure your cat, not the packet.The bottom of the opening should sit at roughly the height of the cat's belly when standing. Too high and they will not use it; too low and they have to crawl.
- Check the width against the shoulders.The widest part of a cat is the shoulders and hips. Flaps come in sizes, and a large or long-haired cat needs more than the standard.
- Choose the position across the door carefully.Central is usual and generally strongest. Keep well clear of the lock mechanism and the hinge side, and away from any glazed panel.
- Mark the template with the level.Tape the template on, check it is genuinely level and square to the door edge, then mark through it. A flap fitted slightly out of true is obvious forever.
- Mask the cut line.Masking tape over the pencil line stops the jigsaw's baseplate scratching the door and reduces splintering on painted or veneered surfaces.
- Drill the corners first.A large bit at each corner inside the line. This gives the jigsaw a clean start and, crucially, gives the cut rounded corners, which stops a crack running out from a sharp corner.
- Cut between the holes with a fine blade.A downward-cutting or fine-tooth blade reduces tear-out on the face. Cut slowly, staying just inside the line — you can file out but you cannot put material back.
- Support the offcut.Hold it or have someone hold it as you finish the last edge. A falling offcut tears the veneer on its way out.
- Test fit before fixing.The frame should sit flat without forcing. File the opening rather than forcing the frame, which cracks the plastic.
- Frame a hollow-core door before fitting.Cut the hole, then pack timber battens inside the void around the opening. Without them there is nothing for the fixings to grip and the door face will flex and crack.
- Seal the cut edges.Paint or varnish on timber, silicone on uPVC. Exposed chipboard or MDF inside a door swells with moisture and destroys the panel from within.
- Fit both halves and check the seal.Inner and outer frames bolt through. Tighten evenly rather than one corner at a time, and check the flap swings freely and closes fully against its brush seal.
- Introduce the cat gradually.Prop the flap open at first, then let it close. Most cats take days rather than minutes, and forcing them through once teaches them to avoid it.
Tips
- Drill the corners before cutting. Rounded corners are what stop a crack spreading from the opening.
- Belly height, not eye height. It is the commonest reason a cat refuses to use a new flap.
- Check for a steel reinforcing bar with a magnet before touching a uPVC or composite door.
Common mistakes
- Cutting into a double glazed panel — Toughened glass shatters completely the moment it is cut or drilled. The unit has to be replaced with one made with the hole already in it.
- Fitting a flap in a hollow-core door without framing it — There is nothing behind the thin face to fix to, so the frame works loose and the door face cracks around the opening within months.
If it doesn't work
Flap frame will not sit flat
Cause: Opening slightly too small or out of square — Fix: File the opening a little at a time. Never force the frame — it cracks.
Draught around the flap
Cause: Frame not tightened evenly, or brush seal missing — Fix: Loosen and retighten evenly across the corners, and check the brush seal is intact.
Crack running from a corner of the opening
Cause: Square corners with no relief hole — Fix: Drill a small hole at the end of the crack to stop it spreading, then fill and repaint.
Cat will not use it
Cause: Too high, too small, or a bad first experience — Fix: Check the height against the belly and the width against the shoulders. Prop it open and let them approach it in their own time.
Door swelling around the cut
Cause: Unsealed edges letting moisture in — Fix: Cannot be reversed once swollen. Seal all cut edges as soon as they are made.
Questions people ask
Can I fit a cat flap in a double glazed door?
Not in the glass. A glazier can supply a replacement unit with the hole pre-formed, or the flap can go into the solid panel of a composite door if there is no reinforcement in the way.
How high should the cat flap be?
The bottom of the opening at roughly the cat's belly height when standing normally. Measure your own cat rather than following a standard figure.
Can I fit one in a fire door?
No. Cutting a fire door voids its rating entirely and defeats the purpose of having it. In a flat or an internal fire-rated door this can also breach building regulations.
Is a microchip flap worth it?
If other cats come in, yes — it reads your cat's existing chip and locks out everything else. It costs more and needs batteries, but it solves a problem no ordinary flap can.