We appear to have rats in our ceiling cavity. Our pest guy says this year is terrible for them, he's spending most of his time with similar callouts. They apparently get up there through drain pipes, into guttering, then under the eaves. We're in a suburb but backing onto a small amount of bush, which is where they're likely to come from.

We have three drains they might be getting up:

• One that drains a small overhang that comes out beside the house. Rats might be able to get up the pipe onto this small roof then jump the 30 - 40cm from that small root up into the guttering for the main roof (some rats can apparently jump two feet up)
• A small feeder pipe that drops rain water from the main roof to a smaller roof for a part of the house that's been added on. They can get onto the small roof by jumping from a fence
• Our storm water does't go into council waste, it goes down a bank. Rats I guess could Macgyver up that pipe about 10m to a sump, along a 10m pipe beside the house, then up a 5m pipe that goes up to the main roof

I'm looking for a decent solution that's hopefully low maintenance:

• The pest guy said screw up something like chicken wire and stuff it up the drain pipes. I tried this with some plastic mesh I had, it washed out fairly quickly. The downside of this is any leaves or such could block the drain, and it probably requires regularly maintenace
• In the UK you can get products that are basically a spring loaded flap in the drain. Water pushes it open, but rats can't pull it open. This would be ideal if you can get them in NZ.
• If there's a way to put some kind of a cage or barrier over the gutting where the downpipes meet the gutting that could solve it.

The pipes are largely 80mm PVC. Rats can get through very small openings, even 2cm wide.

Any ideas or suggestions for products or techniques?