Doors:
Cat/Dog flaps

Cat and dog flaps in double glazing:
Here are the main 'bones' (get it?).
My customers often seem to feel a tad silly or embarrassed when they ask me, ‘Do you do cat flaps in double glazing?’. When I tell them that I do one or two a month, they are often mildly surprised and visibly relieved that I haven’t ridiculed them over the idea.

Dog flaps in double glazing:
The main differences between a dog flap and a cat flap is that a dog flap is bigger, there is less choice and dog flaps are available in several sizes. I generally advise against dog flaps, because of their size of opening, as a small would-be intruder could crawl in through all but the smallest of sizes available. Most of the following text about cat flaps also applies to dog flaps.

Cat flaps in double glazing:
Most cat flaps are fitted in the bottom panel of a kitchen door, as this is often the most practical and convenient place. No problems with leaks at all. Doors with a middle rail (horizontal bar at about waist height) normally have a bottom panel of around 2ft x 2ft (60x60cm), which is ideal. The position of the cat flap is normally in the middle and as near to the bottom as possible. Funny as it sounds, some cat flap fitting instructions suggest you measure up from the ground to the tummy of your cat, and place the bottom of the opening at that point. In practice you will often not be able to get the cat flap quite as low as you would like, but the cats seem to cope if it is just a little higher than ideal. I like to play my customers up by telling them that I need to see their cat to take an inside-leg measurement, and when they do not believe me, I show them the instructions. You have to have a laugh!

Fitting a cat flap in a wooden door:
This is a quite straightforward DIY job, taking out the glass and replacing it with plywood, then cutting a hole in the new panel to the template supplied. 

Fitting a cat flap in a double-glazed aluminium door:
The bottom double-glazed sealed unit could be replaced with plywood and stained up or, if the door is white, then with a PVC-U sandwich panel. Different methods of design and construction of aluminium doors means this may be a simple or a specialist job. The easy doors to do will have a removable interior 'wedge' gasket and four noticeable external aluminium beads. If your door does not have all four external beads, then it may be a job for a specialist.

Fitting a cat flap in a PVC-U double-glazed door:
A white or woodgrain door can almost always have the bottom glass doubleglazed sealed unit replaced fairly easily with a new sandwich panel to match the framework. Be careful to obtain the correct shade of white, as there are commonly seven different shades available in our trade, and you should insist on a virtually perfect match. The supplier should have a colour swatch showing the differences. White is not always 'white', as you will see when you look at two different makes of cars or vans parked next to each other! If doing the job yourself make certain that any corner 'jacking' packers are fitted back correctly or the door could drop, on the lock side. The packers are fitted into opposite corners, so as to act as a brace across the glass or panel.

Important tip:
I suggest that you do not buy the cat flap before you know for certain the exact thickness of the panel it will be fitted into. This is because many cat flaps are not designed to fit the thickness of a double-glazed sealed unit or PVC-U sandwich panel. I myself use the 'Staywell' make of cat flap and model number 17, as I find it ideal to fit. It has four settings: in only, out only, in and out and locked closed. Also, the swinging flap has a clear 'window' for the cat to see through and a magnetic catch so it is not draughty.

Fitted elsewhere:
Although I have heard of cat flaps being fitted into glass and double-glazed sealed units, I would not do it myself or recommend it. Partly this is because any glass in the bottom of a door really should be 'tempered' (four-five times harder to break and if broken will shatter in to hundreds of small relatively harmless pieces). If you really do want the cat flap fitted in either single glass or a doubleglazed sealed unit then you could consider 'laminated' glass, but expect to put your hand in your pocket very deeply.