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jrepp44 03-31-2010 11:47 AM

How to reinforce a "RAT" slab
I am considering installing a thin (1-1/2") concrete slab over an existing gravel crawl space to make access easier/cleaner. Any way to make such a thin slab stronger?

DangerMouse 03-31-2010 11:55 AM

I don't foresee a problem. It's likely not going to be a high traffic area......
As long as you don't try to use it as a support foundation, you should be fine.


jlhaslip 03-31-2010 12:55 PM

specify a stronger mix from the concrete plant. also, get fibremesh added to the mix.

conudrum 03-31-2010 02:48 PM

Pretty sure a "rat" slab is by definition a thin pour of unreinforced concrete. Best way to strengthen a 1 1/2" slab is to pour it thicker.

walkman 03-31-2010 05:04 PM

Should plastic be placed over the gravel before the pour?

conudrum 04-01-2010 10:23 AM

1 Attachment(s)
plastic over gravel is ideal...the gravel provides free drainage and the plastic prevents moisture from seeping into the slab. depending on site specific conditions (is there ground water?) the plastic may be overkill, but it's cheap and easy to install so it wouldn't hurt.

I just poured a slab in our backyard for a new garage. i used gravel but no plastic. this was a shallow excavation with very little ground moisture, and it's a garage so we won't be laying carpet on the slab.

Ron6519 04-01-2010 07:23 PM

If the crawlspace is under a heated area and/or contains water or heating pipes, I would place 2" foamboard under the slab.

jlhaslip 04-01-2010 08:33 PM

Plastic is required under slabs of occupied spaces by code here.
It stops the ground from removing water from the mix so the slab hydrates properly.

Jim F 04-01-2010 08:38 PM

This is a pretty informative article about crawlspaces. . The RAT slab is something I've never heard of before. I wonder what it would cost to get say a 14X24 space done.

conudrum 04-02-2010 01:06 PM


Originally Posted by jlhaslip (Post 422909)
Plastic is required under slabs of occupied spaces by code here.
It stops the ground from removing water from the mix so the slab hydrates properly.

crawlspace is not considered habitable space, so a rat slab would not require plastic beneath it.

the problem isn't "the ground removing water from the slab" but the ground introducing water to the slab. while it's true that heat and sunshine can prematurely dry a pour, causing surface cracks, once the chemical curing process is finshed, there is nothing "proper" about hydrating a slab.

plastic is layed down to prevent moisture in the soil from seeping into the slab, not the other way around.

DangerMouse 04-02-2010 01:19 PM

In the crawl space for the expanded areas of my addition, I laid down between 3" and 4" of concrete as a crawl space "critter barrier". It was not required to be anything more than that as it bore no structural weight. It's a nice, clean area now and I use an old, (but comfortable) mechanic's creeper to get around under there when I need to. This sounds to me pretty much all the OP wants to do, make a bug and rodent barrier and a cleaner area to crawl around if he needs to. If the space is normally dry, not much need for the plastic either. (Depends on location) Just my 2....


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