Barrier at Top of Wall Cavity?

[bigalxyz]

Long story. Pest control guy believes we've got rats climbing up the inside of a wall cavity and from there getting into a loft space. He advised that we should remove some roof slates so that the top of the wall cavity can be accessed from above, and then that we should block off the top of the cavity (perhaps using some kind of stiff mesh that rats can't chew through) - and then replace the roof slates.

(loft space is sloping and incredibly tight so it's not possible to do this job from inside the loft - we'd need one of The Borrowers for that)

Does this sound right? Is it normal for the top of a wall cavity to be "open" (so that there's a route into the loft space? If so, are there any problems associated with blocking it with mesh etc.?

Pest control guy isn't a builder/roofer, so I wanted to get a second opinion on this.

Thank you.

 

[^woody^]

Cavities would normally be closed when built. However that's a bit extreme to do afterwards. Do you have a party wall? If so, how will that be dealt with?

Surely bait traps will sort them out?

You should look for the entry points at ground level not the exit points.

 

[Ivor Windybottom]

It's normal for cavities to be open at the top in older houses. Closing it to stop rats sounds like a bit of a botch.

As said above, it's much better and easier to find the entry points and stop them at source. But this may be difficult if your house is attached, e.g. semi-detached or terraced. If so you may need a coordinated effort between neighbours.

Rats and mice can walk up walls...

Have a walk around the outside with a rat's eye, see where you think you'd go to get in.

 

[bigalxyz]

Thanks re both replies. To address some of the points made, if it helps:

1. Rat guy doesn't think they can get in by climbing up the outside of the wall (which I expect they could manage tbf) and crawling into the loft from there because everything is thoroughly sealed up. He has himself sealed up a few tiny gaps, and these fixes seem to have held - no sign of them being breached, chewed, etc.

2. He's done camera scope thingy checks of all the drainage underneath and around that part of the building and he hasn't found any breaches

3. The bottom row of slates on the roof can be moved slightly by hand - I'm wondering if I could get one of those cheap "camera on a flexi tube" phone attachments and slide the camera under the first tile & point it downwards to see what I can see.

4. The wall in question would have been built approx in the year 2000 - does that give any indication as to whether or not we'd expect the cavity to be sealed at the top?

5. Not sure how we'd block entry points at the bottom of the wall...? Although yes I can see that that would be preferable if it could be done.

6. No party wall problems here - the wall isn't shared with the neighbouring property etc.

7. Rat guy has put traps all over the place but he says the rats have now learned to ignore them. Instead they're just eating each other he also used some yellow stuff that shows up with a special light (or something like that) in the drains at the bottom, and came back a couple of weeks later to find the yellow stuff all over the loft but primarily at the part, where the bottom of the sloping roof meets the suspect wall.

 

[johnny2007]

It was built in 2000 so there should be cavity closers installed.
Should be...

 

[Ivor Windybottom]

They'd have chewed or pushed through any standard closer anyway. So presumably you're now talking about some kind of armour plated version.

It sounds wrong. Much better to keep them out than let them in just as long as they stay in the cavity.

You can get cheap movement-triggered wildlife cameras. Might be worth buying a few and leaving them outside pointing at the places you think are most likely. There shouldn't normally be an underground route,

 

[^woody^]

Older cavity walls should have always been sealed with a brick course, that was the done thing. Newer cavity walls sealed with insulation.

Poison bait in the loft or eaves, should be more effective than traps.

Rats will tend to get into the cavity walls (or floor void and then the walls) by burrowing from the outside, and these tend to be visible.

 

[sparkymarka]

Disagree …I’ve lived in a 1938 semi and you could rod to the loft …I now live in a 1940s semi and it’s sealed , they vary some are some are not …even up to modern 1990s houses it’s normally 50/50

 

[Ivor Windybottom]

Had a 1930s house, now 1950s, both conventional brick built. There's definitely absolutely nothing at the top of the cavity, it just freely vents into the loft.

Perhaps they were closed in older times, and again more recently. But there were several decades where nothing was the standard.

 

[Ivor Windybottom]

Check your sewage pipes. For this reason...