Why the glass you have cannot be drilled
Toughened glass is heat treated after it is cut. The surfaces are left in compression and the middle in tension, and that balance is what makes it several times stronger than ordinary glass. Cut it, drill it or even nick the edge hard enough and you release that tension, and the whole pane collapses into granules in one go. There is no way round it and no tool that changes it. Every pane in a door, and most panes with glass low down in a window, is toughened because Building Regulations require safety glass in those positions — so the pane a cat flap is wanted in is almost always the one that cannot be cut.
What the job actually involves
The existing sealed unit is measured, the flap model and its opening are agreed, and a new unit is made to the same overall size with the aperture cut and the edges worked before the glass goes through the toughening furnace. When it arrives the beads come off, the old unit comes out, the new one goes in and the flap is fitted into the opening. The frame, the sash and the window itself all stay where they are, so nothing about the look of the window changes apart from the flap.
Getting the opening the right size
The hole is sized to the flap's tunnel, not to the plastic frame you see on the wall. Every model is different, and the figure that matters is the cutting diameter in the fitting instructions rather than anything measured off the flap with a tape. The safest thing is to have the flap before the glass is ordered. A photograph of the instruction leaflet is enough — the diameter is always printed on it, and it means the unit is made once.
Where the opening can sit in the pane
An aperture cannot go hard up against the edge of the glass. The pane needs a margin between the hole and its edge, and in a sealed unit it also has to clear the spacer bar running round the inside. On a tall door side panel there is normally plenty of room low down. On a small window there may not be, and that is worth establishing before a flap is bought rather than after. Send a photograph of the window with a rough idea of its size and it can usually be answered without a visit.
Height, and what the cat will actually use
The bottom of the opening wants to be about level with the cat's belly when it is standing. Too high and a kitten cannot get through it; too high also stops an older cat using it long before you would expect. It is worth thinking about what is on the other side too — if the flap comes out above a drop, the cat needs something to land on, and a step or a slab there is the difference between a flap that gets used and one that does not.
Doors, panels and single glazing
Not every cat flap needs new glass. A solid uPVC door panel is not glass and can be cut on site, which is a quicker and cheaper job. A single glazed timber window can be cut on site as well, because the glass in it is usually ordinary annealed glass rather than toughened. It is specifically toughened panes and sealed units that need a new unit making, so it is always worth saying what the flap is going into when you ask.
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Certass registered · PWF Trusted Installer · £5m public liability · 40 years' experience
Common questions
Can you not just cut a hole in the glass I already have?
Not if it is toughened, and in a door it will be. Toughened glass is under tension by design, and cutting or drilling it releases that tension and shatters the whole pane into granules. A new unit has to be made with the hole in it before the glass is toughened.
Will it ruin the insulation in that window?
The new unit is a sealed unit made to the same specification as the one it replaces, so the glass performs as it did before. The flap itself is a thin plastic panel in a hole, so it will never insulate like glass — that is true of a cat flap in any door. A flap with a decent brush seal and a magnetic or locking closure makes far more difference than the glass around it.
Can it go in the uPVC panel instead of the glass?
Yes, and it is usually the cheaper option. A solid door panel can be cut on site with the door still hung, so there is no new unit to make and no wait for glass. If your door has a panel at the bottom and glass at the top, that panel is normally the easiest place for it.
How high off the ground should it go?
Roughly level with the cat's belly when it is standing normally. Also worth checking what the cat lands on outside — if there is a drop on the far side, a step or a slab under the flap makes it usable, particularly as the cat gets older.
Are microchip flaps any different to fit?
The principle is the same but the opening is usually larger and the tunnel deeper, so the cutting diameter needs checking against that specific model rather than assumed from an ordinary flap. They also need batteries and a bit of clearance behind the frame, so it is worth having the flap to hand before the glass is ordered.
Can you do it in a leaded or Georgian bar window?
It can be done, but it needs planning rather than just ordering. The lead or the bars in a modern unit sit inside the sealed unit, so the new unit is made with the pattern arranged around the opening. Send a photograph of the window and it can be set out before anything is ordered.
Where we cover
cat flaps fitted into glass across the West Midlands and the towns around it, including: