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Slab on grade thickness

Just a quick question on how thick the slab should be on a slab on grade house?...thanks,Bob

Asked by bob holodinsky
Posted Wed, 07/09/2014 - 15:09

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6 Answers

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1.
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Bob,
The answer depends on your soil conditions and whether or not the slab will bear any loads. Talk to an engineer.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 07/09/2014 - 15:30

2.
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Being an excavator by trade I can tell you what I see in our neck of the woods. If you are talking about a thickened edge structural slab, I usually see a 12"x18" perimeter grade beam with the main floor area being 5" or 6" thick and #4 rod 24" o.c both ways and (4) #4 rods continuous around the grade beam. Our soils are clay silt, 2000 psi.

If the building is very large, i see a 18" x 24" grade beam with 6" slab.

Keep in mind that any point loads need additional concrete. Pier or column locations can require large integral footings that are poured with the slab. I've see these as large as 10'x10'. We're beginning to see more grade beam foundations even for large commercial structures.

Answered by Rick Van Handel
Posted Wed, 07/09/2014 - 20:52

3.
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Thanks...the building is on bedrock with the cross corner difference of about 30 inches.The pad is 28 by 44 and the contractor is using ICF as stem walls with a gravel pour...,3 inches of foam...vapour barrier and 4 inches of concrete for the pad ...does that sound right ?

Answered by bob holodinsky
Posted Wed, 07/09/2014 - 21:42

4.
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You really need to do a geotechnical investigation, better known as soils test, to know the soil configuration, NCRS soil regions, and behavior. Some of the issues to study are the expansivity of soils with moisture, which it can varies greatly across a site, and the possible vertical rise and pressures. A geotech report will give you several recommendations for foundation types and their design criteria, and/or remediation recommendations.
Depending on your site, you maybe required to have two or more borings, usually to a 20' depth, unless rock strata is found.
Be careful with using a soil survey, they ARE NOT an geotech report as it does not constitute a site specific investigation for engineering applications.
If you have a foundation failure, one of the first things an insurance company will ask for is the geotech report. Absence of it, you get nothing.

Answered by Armando Cobo
Posted Wed, 07/09/2014 - 22:40

5.
Helpful? 0

I don't have a lot of experience with bedrock but would suspect you will get by with much less concrete and reinforcement if the site is uniform and level. In our area, deep footings can be omitted if on bedrock, since bedrock itself cannot heave with frost.

What is your frost depth and how will you be accommodating your water and sewer services? Blasting?

Answered by Rick Van Handel
Posted Wed, 07/09/2014 - 23:51

6.
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Hi Rick...the site is the high spot on a point surrounded by water .I will be pulling from the lake for water with a filter and UV system...the septic bed will be a couple hundred feet away and slightly uphill so I will have a pumping cistern to a holding tank and then to the septic bed...frost depth here is 4 ft on soil but bedrock doesnt move much(Ihope..lol)...Bob

Answered by bob holodinsky
Posted Thu, 07/10/2014 - 08:13

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