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I'm going to fit a cat flap in a brick wall. My intention is to drill out a square hole through the wall. Then create a wooden (MDF) tunnel to fit inside this. On one end of the tunnel, I will attach an extra piece of wood with the cat flap fitted.

My question is this How do I secure that wooden tunnel into the brick-work?

Obviously the drilling is going to leave a messy edge, which I intend to tidy up with cement and/or filler, but I can't see that these are going to get much purchase on the MDF. I can imagine, one good kick to the outer face would send the whole structure flying into the house.

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Could someone please tag this "cat flap" –  Greg B Mar 23 '11 at 8:32
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First, I recommend against MDF. Yes, it has a great smooth surface, but it just doesn't have much structural strength. I would recommend plywood, maybe 1/2 or 3/4 inches thick.

If it were plywood, I would say put a plywood "collar" on each end of the tunnel, with screws from inside the edge of the tunnel into the plywood ring around it. If you have one on either end, this locks the tunnel in place, and even if someone gave it a good kick, it should stay in place. However, with MDF, you don't have that much strength, so you may need some other way of attaching the "collar". Hope this helps.

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Agreed on MDF. It's easy to work with, but it sucks up moisture like a sponge. While I've used it for many projects, I wouldn't use it anywhere near an exterior application. –  RQDQ Mar 23 '11 at 15:23
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