Allotments 4 All

Not really, but the question is related. Is a cat flap strong enough to prevent the ingress of a rat?  But not so strong that the cats cannot get in either?

We've had a fox through (when I forgot to lock it one night) but not a rat.  I've also just had to replace the flap part of ours because one of the cats got fed up being locked in at night and headbutted it so hard that the flap cracked and went flying into the garden.  Found a gaping hole in the morning.  Do you mean an unlocked cat flap?  I would think a rat could get through if a cat can if it's unlocked.  But if you have one that is lockable one way, it may do the trick.  Not very helpful, I know! Sorry!

We had one of the magnetic ones, which took the cat a head-butt to get in.  I've never had a problem with rats, but I wouldn't have thought they'd have gotten through.  The magnet prevents much of a flap-swing, and holds it in place well (no swinging, even in very windy conditions).

The cat-flap we had: http://www.staywell.co.uk/Intl/UK/Products/Cats/Magnetic-4-Way-Locking-Classic-Cat-Flap

A micro chip activated one:  only your cat (and anything in it's mouth!) can get in: special collar: http://www.staywell.co.uk/Intl/UK/Products/Cats/400-Series or using their microchip: http://www.staywell.co.uk/Intl/UK/Products/Cats/Microchip-Smart-Flap-Door-Mount-White

All avaliable much cheaper.  Google them!

the only rat we have had in through one was in the mouth of the cat.....

We eventually had to go with a chip sensor cat flap as every living thing within a mile tried and succeeded to get into our kitchen.  Even a really stong wind opened the flap up so I gues a rat could get in easily if they wanted..