I need to install a catflap in my backdoor, i have done this before, except this time my door is metal skinned and im not sure what it looks like inside, i know that there is wood inside and i believe they are wooden struts and im scared if i cut through the struts i will ruin the door. what should i do? thanks in advance!

 

You will have to raise the bottom of the hole for the flap approx 2 inches above the bottom of the door. The reinforcing members in the door are all around the edges. The interior will be a stiff foam. You need to paint the cut edges of the steel or it will rust. Automotive touchup paint (with the brush in the cap), or similar works just fine. Don't use a latex paint. The foam will need to be sealed to prevent damage to the door. I just smeared latex caulk all over the interior surface of exposed foam after painting the steel.

You will also want to caulk where the cat flap frame meets the door. If its a paneled door, make sure the frame is centered so it doesn't overlap the beveled edges of the panels. It shouldn't with the right flap. I've done this on both steel and fiberglass doors and it worked fine.

If the flap is like the ones here in the States...blowing wind and rain can get through and sit on the caulked area of foam, that will need to be dried off. If the door is under a porch roof or protected somehow from the weather, shouldn't be an issue.

You may need to use different hardware (screws, nuts, etc) as some flaps are designed for walls or different doors.

It can be done, just need to protect the interior of the door from water intrusion.

btw...Welcome to the Forums