Rats in the cavity wall!

Hi all,

At my wits end with rodents!

We have a three story, stone built farmhouse in the foothills in Italy, the rear of the lower floor is up against earth as the house is partially dug into the hillside, this floor would have been the cellar and cooler than the three floors above but now it's a small self-contained apartment.

When converted a block-work wall to make a cavity was built inside on a suspended floor, this is how they deal with damp cellars in Italy.

Five weeks ago we noticed evidence in the apartment of activity and using our wildlife camera caught site of rats! it took some time to work out where they had entered, on the only exposed (no stucco/render) side of the house they had gnawed through a small area of expanding foam next to the waste pipe, clearly a poor repair job by the previous owners when putting new plumbing in.

I blocked this hole and then started to use humane traps, over the course of the following three weeks, caught one adult and two juveniles but one remains and won't go near any of the traps, I have no idea how it's surviving without apparent food/water sources and am now fearing the worst, that somewhere at the back there is a tunnel system that arrives from outside into the cavity!

We have paying guest arriving in April and the apartment will be largely occupied all summer...HELP....PLEASE!

Thanks for listening.

Can you borrow a cat? Preferably one that is known to kill rats.

I had a similar move-in - cellar foundation; but it was a skunk.  Tried to time closing the entrance with it's night time exit, but with no luck.  So I ended up sealing it in.  It took several weeks for the decomposition odor to disperse; but it was much preferable to the permanent skunk den reek.

There are "one-way-doors" for wildlife exclusion. They are made of wire mesh, and are a tunnel with a spring-loaded flap, and a means of fixing it over the "entrance" the animals are using. The flap allows exit, but not re-entry.
Sealing up an entrance is the first reaction, but one of these doors can be more effective, since the animals aren't trapped inside, possibly leading them to find/create an alternate entrance.

A thermal camera might allow you to see the rat(s) through a wood/plaster wall, or might show you some "leaky" spots where cold/warm air is infiltrating the space, which could also be an entrance.
Dusting the floor inside, or ground outside the building with flour could show you footprints, and maybe lead you to an entrance that you don't know about.
The size of the entrance can be quite small. Surprisingly so. The size of your thumb = big enough for a rat, >6mm = big enough for a mouse.

The barrel of water with a plank that drops the rodent in, seems to be effective.
There are electrocution traps (RatZapper) that are also effective.

Sebastian Köln wrote:Can you borrow a cat? Preferably one that is known to kill rats.

With our dog the cat would dead sooner than the rat!

Ruth Meyers wrote:I had a similar move-in - cellar foundation; but it was a skunk.  Tried to time closing the entrance with it's night time exit, but with no luck.  So I ended up sealing it in.  It took several weeks for the decomposition odor to disperse; but it was much preferable to the permanent skunk den reek.

I think my problem is that it must be getting out somehow, other than how it came in, it's been in there now for five weeks!! Surely if there was no exit it would have died?

Kenneth Elwell wrote:There are "one-way-doors" for wildlife exclusion. They are made of wire mesh, and are a tunnel with a spring-loaded flap, and a means of fixing it over the "entrance" the animals are using. The flap allows exit, but not re-entry.
Sealing up an entrance is the first reaction, but one of these doors can be more effective, since the animals aren't trapped inside, possibly leading them to find/create an alternate entrance.

A thermal camera might allow you to see the rat(s) through a wood/plaster wall, or might show you some "leaky" spots where cold/warm air is infiltrating the space, which could also be an entrance.
Dusting the floor inside, or ground outside the building with flour could show you footprints, and maybe lead you to an entrance that you don't know about.
The size of the entrance can be quite small. Surprisingly so. The size of your thumb = big enough for a rat, >6mm = big enough for a mouse.

The barrel of water with a plank that drops the rodent in, seems to be effective.
There are electrocution traps (RatZapper) that are also effective.

When I closed the hole they came in I made a one way door using 2 inch pipe hoping that it would come back out that way but it hasn't!

It never enters the apartment space itself other than running across that bit of stonework beneath the ceiling in the photo,there is no way for it to do so so the only way I can put traps down is in the ceiling void above the bathroom which has a drywall ceiling, hard to explain but there is only this 4 square metre drywall with a 30cm space between it and the original solid floors and the cavity which encircles the entire apartment.

A thermal camera might be an idea but having never tried one I'm not sure it will work through 10cm clay blocks?

Stuart Smith wrote:I think my problem is that it must be getting out somehow, other than how it came in, it's been in there now for five weeks!! Surely if there was no exit it would have died?

Aren't there smoke machines to detect such things?  I think I recall a pink fog one can insert in spaces to see where it might spill out.

Ah!

https://www.greenbuildingadvisor.com/article/pinpointing-leaks-with-a-fog-machine

Stuart, it sounds as if this "runway" along the top of the stonework is the only spot with an opening between the apartment and the cavity?
Good spot for a trap?
Stakeout with an air rifle?
Maybe plaster this this area off before your guest arrives? not ideal if you haven't eliminated the pest and found the entrance.
Maybe try baiting the outside of your one-way pipe exit with some food? In theory, animals could only get OUT, and you could set up the wildlife camera to monitor that your pest got out.

Not sure if the thermal camera will "see-through" the clay block, but will as I said, show up cold/drafty spots which could be openings, or nearby to openings.

Beware of half-measures... if you need to open a wall or ceiling to fix it, then that's what you've got to do.
Construction/remodeling without an eye towards pest control drives me crazy. I have been struggling with this everywhere I have lived.
Small gaps in framing, oversized holes for pipes and wires, knotholes in wood boards, all either overlooked/done expeditiously/significance underestimated...

Ruth Meyers wrote:

Stuart Smith wrote:I think my problem is that it must be getting out somehow, other than how it came in, it's been in there now for five weeks!! Surely if there was no exit it would have died?

Aren't there smoke machines to detect such things?  I think I recall a pink fog one can insert in spaces to see where it might spill out.

Ah!

https://www.greenbuildingadvisor.com/article/pinpointing-leaks-with-a-fog-machine

Thanks for the reply Ruth, interesting idea, not sure it work in my circumstances as the smoke will come back into the apartment before it finds any other way out, it's hard to explain the construction of the space, somewhat particular. But thank you.

Kenneth Elwell wrote:Stuart, it sounds as if this "runway" along the top of the stonework is the only spot with an opening between the apartment and the cavity?
Good spot for a trap?
Stakeout with an air rifle?
Maybe plaster this this area off before your guest arrives? not ideal if you haven't eliminated the pest and found the entrance.
Maybe try baiting the outside of your one-way pipe exit with some food? In theory, animals could only get OUT, and you could set up the wildlife camera to monitor that your pest got out.

Not sure if the thermal camera will "see-through" the clay block, but will as I said, show up cold/drafty spots which could be openings, or nearby to openings.

Beware of half-measures... if you need to open a wall or ceiling to fix it, then that's what you've got to do.
Construction/remodeling without an eye towards pest control drives me crazy. I have been struggling with this everywhere I have lived.
Small gaps in framing, oversized holes for pipes and wires, knotholes in wood boards, all either overlooked/done expeditiously/significance underestimated...

Thanks again for another reply Kenneth.

I can see I'm running out of options and may have to consider putting a hold in the wall!

I'd also try sending something into that space that would make it uncomfortable for the rat.  Ammonia, perhaps?

I've successfully used hot pepper juice sprayed on wood siding to discourage squirrel gnawing.