Advice on installing a concrete "rat slab" in crawlspace

[sbornia]

A couple years ago, we had our 100+ year-old house fairly well protected against rodents by having all external gaps sealed with concrete or chicken wire, and a concrete slab put in the small space under the front stairs where there were signs of tunneling. These measures seemed to work pretty well, but we recently noticed some new activity. The rodent proofer inspected the property and found that rats had tunneled under the foundation and chewed through our insulation encapsulation to get inside the crawlspace. Their proposed solution is to put in a concrete slab in the entire crawlspace to prevent future tunneling. They said they'll also put in a couple of drainage points under the concrete (buckets with gravel) as a preventative measure in case of water intrusion (which we don't have right now, fortunately the space is very dry). And they're going to remove and replace any of our insulation from the crawlspace ceiling that looks impacted by the rats. I haven't received their quote yet but suspect it will not be inexpensive.

Is there anything important I should know about this kind of work, say any potential consequences after the slab is in? Any probing questions I should ask when getting quotes?

Thanks!

[rwsaunders]

See detail 3-15 in the link and there is an animation link as well. Since it’s not load bearing, the concrete slab can be 2” thick and made of lean (low compressive strength) concrete, but I’d use a reinforcing fiber or mesh as the slab will crack. Note the inclusion of a vapor barrier and the need to tightly seal where the slab abuts the wall, as a rat can go through openings as small as 3/8”.

https://foundationhandbook.ornl.gov/...ction3-2.shtml

[Peter Polack]

I don't have a foundation answer but I must ask/suggest-have you tried bait traps?

I have one of the linked traps and looking at the missing bait and lack of rats and mice hanging around, I'd say they're effective.
You might want to consider placing some of those baits in the problem area which I assume is not accessible by pets or children.

There is no single solution to your problem; a multiple approach will yield better results.

[sbornia]

See detail 3-15 in the link and there is an animation link as well. Since it’s not load bearing, the concrete slab can be 2” thick and made of lean (low compressive strength) concrete, but I’d use a reinforcing fiber or mesh as the slab will crack. Note the inclusion of a vapor barrier and the need to tightly seal where the slab abuts the wall, as a rat can go through openings as small as 3/8”.

https://foundationhandbook.ornl.gov/...ction3-2.shtml

Thanks, this is very helpful. I will ask about the type and amount of concrete, the reinforcing mesh, and their plan for sealing the slab against the wall. Fortunately we already have the vapor barrier in place.

[sbornia]

I don't have a foundation answer but I must ask/suggest-have you tried bait traps?

I have one of the linked traps and looking at the missing bait and lack of rats and mice hanging around, I'd say they're effective.
You might want to consider placing some of those baits in the problem area which I assume is not accessible by pets or children.

There is no single solution to your problem; a multiple approach will yield better results.

We've used traps a few times and they eventually catch something....big. The last two rats I saw that took the bait were about 7-8" long. I imagine we could keep setting the traps and catching new ones, but given the tunnel holes, I expect they're just going to keep coming in.

[bigbill]

Our new place in Wyoming has a finished basement with a utility room that contains the water heater and shutoffs. There are sticky traps in the utility room and the closets of the bedrooms. I check them frequently and so far, have found nothing. There are signs of rodents anywhere, but the previous owners placed the traps for some reason. Unfortunately, the prior owner passed and his kids never lived in this house. We have a cat that is a mouser, so I won't use any poisonous baits. Once it warms up, I plan on digging out a 12" wide, 6" deep trench around the perimeter of the outside basement walls and filling it with gravel. Rodents don't like gravel because they can't tunnel through it and it exposes them to predators when they cross it.

[thollandpe]

I’m not sure of your climate zone or local codes, but here in Mass I’d use insulation (min. R5) under that slab along with the vapor barrier.

And if your floor joists are insulated with fiberglass batts, you should consider replacing those, they’re rodent habitat.

[steve garro]

YEah, after a 42K sewer intervention I can say that you wanna have access to any utilities which may be entrapped under that slab.

We have every varmint you can think of but a big battle is burrowing squirrels and they can be stopped with 1/4" steel mesh - could be worth a preemptive layer of mesh as it's more cost effective then concrete as a first salvo, it's held down with cobbles but to get to a pipe is fairly easy, you can also pin it down with cinder blocks.


- Garro

[sbornia]

I’m not sure of your climate zone or local codes, but here in Mass I’d use insulation (min. R5) under that slab along with the vapor barrier.

And if your floor joists are insulated with fiberglass batts, you should consider replacing those, they’re rodent habitat.

We have recycled denim insulation in the floor joists, which we picked in part because we were led to believe they were unappealing to rodents. This turns out to be untrue, based on what we're seeing in the crawl space.

Our climate is pretty temperate and doesn't go below 45 degrees very often. When we did an insulation project a few years ago, PG&E measured a 45% improvement in air-tightness before and after the work was completed. My casual observation has been that it keeps the house about 4-5 degrees warmer in the winter...and also makes it much stuffier in the globally warmed summer.