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Old 11-15-2009, 08:54 AM   #1
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recommended slab thickness?

I'm converting part of dirt crawlspace to a slab so I'll have space for a utility area and storage.

Do I need to shoot for a 4" slab, or would 3" be sufficient?


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Old 11-15-2009, 09:02 AM   #2
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If it's going to be used, I'd put in a traditional 4" slab. If this was just a rat slab, all you would need is 2". In either case, I would put down 2" foam board and a vapor barrier under the slab.


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Old 11-15-2009, 10:02 AM   #3
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Is the extra inch for load, or to help prevent cracking?

I was planning on 2" of gravel and then vapor barrier. I take it the foam board is for insulation? Is it needed in GA in a non-living space?
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Old 11-15-2009, 10:10 AM   #4
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Whether it is "needed" is a matter of asking the local building inspector. A 4 inch slab is usually used because it is less likely to crack under impact loading than a 3 inch slab, and a 2 inch slab is very prone to cracking under impact load. You cannot effectively reinforce a slab that is less than 6 inches thick, due to cover requirements for the steel reinforcing, so the only resistance to impact loading is from the thickness and strength of the concrete.

The foam is a good idea for two reasons. One is insulation, the other is that it provides a cushioned surface for the concrete, similar to the use of gravel. A slab on a soft surface like foam is less likely to crack under impact than an uncushioned slab. The vapor barrier is also a good idea to minimize the amount of moisture wicking upwards into your crawl space. It also reduces (does not eliminate) the amount of radon that will seep into your crawl space. If radon is a problem in your area, you would need a combination of plastic plus underslab ventilation, however if radon is a marginal issue, the plastic alone will help keep the concentration down.
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Old 11-15-2009, 03:30 PM   #5
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If it's literally a crawl space, I would suggest a 3" floor with 6 mil VB min. Not much "loading" present in a crawls space. To be completly honest, I think the 2" foam under a crawlspace in Georgia may be a bit of overkill also. I think the pay-off for the price of foam vs. the energy saved would be too long to justify it.
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Old 11-15-2009, 06:12 PM   #6
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what is the soil type the slab will set on ? if in dirt. i would go with the 4 inch with 3/8 rebar .and some type of crusher fines at least 6 inches thick with moisture added on it and compacted. i say that because of the expanding and shrieking of soil.
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Old 11-16-2009, 06:19 AM   #7
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I would agree with jomama45 that insulation is probably not needed. If the crawl space is well ventilated the heat will be lost anyway. We always use a minimum of 4 inches of concrete which helps stop any vegetation forming as well, although a vapour check has been found to send the moisture up the walls instead.
When a vapour check is used on the solid floor construction is used it is lapped into the DPC and underneath about 2 inches of sand is used to cushion it against the gravel hardcore as this has sometimes pierced it in time.
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