# Rat Slab Thickness



## nickedemus (Mar 16, 2021)

I've made the rat slab 2" in the past. I just saw an article that said the IRC required it to be 3.5," but the article didn't cite the section in the code.

I looked, and I'm not seeing any code requirement specific to rat slab at all in the NJ IRC 2018. Doesn't even look like it's required. But I don't have the premium version, so searching is not always successful.

There's one section (R317.1) that says the wood framing has to be 12" - 18" above the exposed ground. But again, no mention of rat slab.

Am I wrong to specify a 2" thickness for the rat slab?


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## Teeshot (Mar 16, 2021)

I'm in Ca and don't believe this is covered in the CA Res. Code either. 2" is fine but I recommend closer to 3" in thickness, to help prevent future cracking.


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## mtlogcabin (Mar 16, 2021)

2 inches will not give you the lateral support at the bottom of the foundation wall, 3.5 inches should. Then again backfilling the interior about 12 inches above the bottom of the footing should provide adequate lateral support if your unbalanced backfill is over 4 foot. 

R404.1.1 Design required.
Concrete or masonry foundation walls shall be designed in accordance with accepted engineering practice where either of the following conditions exists:

1.    Walls are subject to hydrostatic pressure from ground water.

2.    Walls supporting more than 48 inches (1219 mm) of unbalanced backfill* that do not have permanent lateral support at the top or bottom.*


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## nickedemus (Mar 16, 2021)

mtlogcabin said:


> 2. Walls supporting more than 48 inches (1219 mm) of unbalanced backfill* that do not have permanent lateral support at the top or bottom.*



I don't think I will have that great of a differential, though I have yet to do a revised building section that shows the updated grading. I'm thinking more like 18" max. It's such a limited amount of space that I have under the floor. 

Here is an outdated section that I did, showing only a 5" space between the ground and the bottom of the truss. Excavating to the top of the interior footing will only give me 17". I think I'll have to lower the interior footing somewhat.... I guess if I match the depth of the exterior footing, I could get the depth for the 12" backfill. Or, I can just increase the thickness of the rat slab.... It's a townhouse, where units step down along a slope, so I'd like to find the simplest solution.



https://imgur.com/MoM7ThL




https://imgur.com/auVNWzl


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## Mark K (Mar 16, 2021)

The purpose of a rat slab has noting to do with providing lateral support for a foundation element.

R404.1.1 is confusing.  What is meant by permanent lateral support?  Does this mean that a 8' or 18' retaining wall could be built without an engineering design if it has what is considered permanent lateral support.  This does not make sense.


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## mark handler (Mar 18, 2021)

A so called "Rat Slab" is not in the code.
The slab needs to be stable enough to resist movement and needs a water proof membrane. the attached was developed by the City of San Diego *under an old* code but may help with the conversation
remember you still need ventilation


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## Joe.B (Mar 18, 2021)

I was told in my younger days that a rat slab was an additional protection used to help keep rodents out of a crawl space. As it's not required, there will be no codes on it. For the same reason we don't find codes for paint. 



(Now somebody's going to throw a code reference at me about paint, right?


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## Teeshot (Mar 18, 2021)

Ditto, Joe.


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## nickedemus (Mar 18, 2021)

Joe.B said:


> a rat slab was an additional protection used to help keep rodents out of a crawl space.



That's what I learned as well. But since codes change all the time, I wanted to make sure.


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## ADAguy (Mar 18, 2021)

As to paint, it depends on where and what it is applied to and for what purpose. Adjacent to urinals?


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## Joe.B (Mar 18, 2021)

ADAguy said:


> As to paint, it depends on where and what it is applied to and for what purpose. Adjacent to urinals?


Haha yup, or for fire rated protection? I knew I'd have to eat that one. Mmmmm, tasty.


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