You need to work out how they are getting into the house and then block that off before killing things – or trying to. When you get to that stage, the Good Nature trap, which is made in New Zealand is – as above – very effective and as humane as something that smacks a CO2-powered bolt into a rodent’s head can be. ie: it’s instant and kills them well dead.
Clasicallly rats get in a few ways. One is to simply gnaw their way in from ouside, but depending on where you live, there’s a good chance they’ve got in from the sewers. We had them in our loft and under the floor space in our front room. It’s easy to think that they’re like The Great Escape and strategically tunnelling in, but it’s more likely an accident.
I’m pretty sure the ones under the floor had burrowed in after finding a cracked join in our ancient Victorian pipework, tunnelled sideways and found a nice dry space to live in. The ones in the loft had come up a drain pipe that came down from the guttering and was open to the drains – I actually saw one emerge from that drain opening and dug in that way.
The answer for us was a rat-flap, a sort of one-way valve that means rats can get out to the sewers, but not back in – lots of versions, the Danes fit them by law and have some pretty neat double-flap ones that are the way to go if you want to be sure. Easy to fit yourself, come in two sizes. I also got a grate for the drain and epoxyed it on, so ratsy can’t lift it – just in case. And for peace of mind stuck some metal mesh over the bottom of the pipe with zip-ties.
Once you’ve blocked access, you need to kill any left over rats. We had two, one died in a snap trap in the eaves of the loft, the other jumped into a record crate, so I took it outside. Next door’s cat got it next day. I think it was weak from lack of food tbh. Bear in mind that food sources are mostly in the sewers / bird feeders / etc. Or, in our case, next door’s dog food cupboard, which they somehow managed to access via our party wall. Bear in mind that they can follow wiring and pipework ducts.
If you’re bemused, the rat flap solution is worth a try, I think. But if you live in a terraced street and have adjoining lofts etc, it may be a little more complicated, but killing them is pointless unless you work out where they got in and stop it off. Oh, they’ll chew through foam, better to use metal mesh and then foam that to hold it in place.
Apparently they’re quite good at not killing themselves nibbling at cables. They have a physiological need to gnaw things as otherwise their teeth just keep growing…
But basically, killing them is the easy bit, keeping them out in the first place is what you need to focus on.