Cat Flap Fitted Into Glass

What's happening?

This is a job people expect to be small and find is not, for a straightforward reason. The glass in a glazed door or a low window is almost always toughened, and toughened glass cannot be cut, drilled or notched after it has been made: the heat treatment that gives it its strength means any attempt to work it releases the whole pane into granules. So a cat flap in toughened glass is not a hole made in your existing pane. It is a new pane, manufactured to the same size with the aperture formed before toughening, glazed into the frame you already have. In a double glazed door that means a new sealed unit built around the aperture, which is why the glazier measures rather than quotes over the phone.

What usually causes it?

• Existing pane is toughened glass, which cannot be drilled or cut after manufacture
• Existing pane is a sealed double glazed unit, so an aperture means a new unit built around it
• The position is a critical location, so whatever goes in still has to break safely
• Laminated glass in the door, which can be worked but is difficult and rarely worth cutting in situ
• A frame rebate too shallow for the thicker unit some flap models need
• Pet door model requiring a specific aperture diameter and glass thickness combination
• A leaded or patterned pane where matching the existing appearance drives the work

Is it dangerous?

The work itself is not, but two mistakes are. Attempting to drill toughened glass in place shatters the pane, usually while somebody is standing over it. And fitting a plain annealed pane because it can be drilled downgrades the safety glass in a door (a critical location where the glazing is supposed to break safely), which is a worse outcome than not fitting the flap.

Can I fix it myself?

Choosing the flap and confirming the aperture size it needs is the useful part to do yourself, because the glazier needs the model to order the unit. Fitting the flap into a supplied pre-cut pane is often within reach. What is not: cutting or drilling toughened glass, and substituting annealed glass in a door so that it can be drilled.

When should you call a glazier?

Call a glazier before buying anything, so the flap model and the existing glass are matched up front. They will measure the pane and the rebate, confirm what the glass is, and tell you whether a new toughened pane, a new sealed unit or a laminated alternative is the route. Where the door is a fire door or a rented property's entrance door, raise that on the call: cutting an aperture is not appropriate in every door.

What will the glazier actually do?

• Identify the existing glass (toughened, laminated or annealed), and the unit make-up
• Measure the pane, the frame rebate depth and the aperture the chosen flap requires
• Order a new pane or sealed unit with the aperture formed before toughening, in a specification that suits the location
• De-glaze the existing pane and clear the rebate
• Glaze the new pane or unit in on fresh setting blocks and re-bead or re-gasket
• Fit and seal the flap into the aperture and confirm the door still closes and locks correctly

Questions people ask

Sources & References

1. GOV.UK, Protection from falling, collision and impact: Approved Document K
2. Glass and Glazing Federation
3. Glass and Glazing Federation: Technical library