# slab on grade size



## duburban (Apr 10, 2008)

i'm thinking about building a shop this summer. i'm up in vermont and would like as big of a slab as possible. what kind of sizes could i get out of a slab on grade with proper site prep and insulation?


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## Precision10 (Apr 6, 2010)

You could pour a slab as big as the state if you could pay for it. Your question is lacking some details, please clarify.


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## duburban (Apr 10, 2008)

figured as much.

how big would you feel comfortable building a house on?

id like to build as big as 36x40

shallow frost protected foundation techniques could be used. 

shop building, rough cut walls 2x6 or 2x8, storage trusses.


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## tgeb (Feb 9, 2006)

You certainly could build that size and larger.

Monolithic slab with turned down footing to below frost level, cut a few control joints, you're good to go. I'd make the slab minimum of 5" thick for a shop. Throw in some rebar.


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## duburban (Apr 10, 2008)

tgeb said:


> You certainly could build that size and larger.
> 
> Monolithic slab with turned down footing to below frost level, cut a few control joints, you're good to go. I'd make the slab minimum of 5" thick for a shop. Throw in some rebar.



frost depth is 4+ here. what about a shallow frost protected foundation?


when i start digging frost walls i can't help but wonder about a basement...


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## tgeb (Feb 9, 2006)

"Shallow Frost Protected Foundation"?

That's beyond my expertise, frost is 24-30" here depending on the jurisdiction.

You can put a basement under the shop, but it'll cost you.


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## greg24k (May 19, 2007)

What exactly you asking? Do you have any requirement there, codes that you must follow, permits, approvals, inspections... or you just gonna put a 30x40 building without permits or approvals?


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## duburban (Apr 10, 2008)

greg24k said:


> What exactly you asking? Do you have any requirement there, codes that you must follow, permits, approvals, inspections... or you just gonna put a 30x40 building without permits or approvals?


building permit yes, beyond that nothing is required. out in the sticks... its nice. 

i'm just asking what people feel comfortable with in regards to slab foundations without frost walls and the size of them.


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## katoman (Apr 26, 2009)

If frost is 4' then put in a conc. block foundation. No bsmt. One block above grade. Then pour a slab floor inside.


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## Chris Johnson (Apr 19, 2007)

You can put a post tension slab in without frost walls or shallow footing detail but it will move with the frost.

Assuming your shop is heated, pour a thickened edge slab, 12" below finish grade, run 4" styrofoam 4' out all the way around the slab and cover with dirt, asphalt, whatever you want. You know have a shallow footing detail


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## rino1494 (Jan 31, 2006)

Chris Johnson said:


> Assuming your shop is heated, pour a thickened edge slab, 12" below finish grade, run 4" styrofoam 4' out all the way around the slab and cover with dirt, asphalt, whatever you want. You know have a shallow footing detail


If you are looking to save money, I agree with this. 

Our shop is a pole barn. The post are below frost but the slab isn't protected. Our slab is 8" and we have gravel along the edges. Shop is heated and and insulated. Never had a cracking or flaking issue.


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## duburban (Apr 10, 2008)

it looks like the insulation details change depending on if the space is heated or not. if i go with the heated shop insulation detail, will i always have to keep it warm in the winter to avoid cooling the thermal mass/earth below?

its funny after so much research you think you know this stuff. suddenly you might be doing it and your knowledge base is filled with holes. 

i know a guy that just built something similar to what i'm describing, i'll have to pick his brain. i don't think he insulated the wings though.


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## SLSTech (Sep 13, 2008)

Chris has pretty much nailed it for the footings
As for the size of slabs - we did one that was about 250' wide & 400' foot long quite easily in one day - almost anything is possible with enough manpower & tools
Thermal mass of slab - if the money is in the budget, I would recommend 2" or more of rigid insulation under the slab & on the inside of the footing. Along those lines I would also recommend running radiant piping tubes underneath also - granted you may not be planning on heating your shop that way at this time, but you will be glad later if you do go that way. (Just make sure you have it pressurized as you pour the slab)
You might want to checkout out Building Science .com & GBA - both have some good information & details on how to do this


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## Sar-Con (Jun 23, 2010)

Where I'm at I can only build up to 600sf without full frost wall foundations. I think an execption can be made with engineered drawings, but for that expense on a small project you're better off throwing in the frost wall and keep going.


There is a huge cost increase to building a full basement under your concrete slab....form rental, additional concrete (5" vs 8"-10"), rebar, engineered plans, bigger foundations, bulk excavation and oh ya, another slab on grade....perimeter drainage and waterproofing....Don't kid yourself, building a basement under a SOG is not another while I'm down here....:whistling


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## duburban (Apr 10, 2008)

Sar-Con said:


> Where I'm at I can only build up to 600sf without full frost wall foundations. I think an execption can be made with engineered drawings, but for that expense on a small project you're better off throwing in the frost wall and keep going.
> 
> 
> There is a huge cost increase to building a full basement under your concrete slab....form rental, additional concrete (5" vs 8"-10"), rebar, engineered plans, bigger foundations, bulk excavation and oh ya, another slab on grade....perimeter drainage and waterproofing....Don't kid yourself, building a basement under a SOG is not another while I'm down here....:whistling


:thumbsup:


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## slowsol (Aug 27, 2005)

SLSTech said:


> Chris has pretty much nailed it for the footings
> As for the size of slabs - we did one that was about 250' wide & 400' foot long quite easily in one day - almost anything is possible with enough manpower & tools


You must have had one heck of a crew to pour 100,000 sf in a day easily. Was it a big box store? Getting enough trucks running with efficient timing for that big of a pour would be an incredible coordination in itself. I think we poured 700 cu yds in a day as our record.


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## katoman (Apr 26, 2009)

SLSTech said:


> (Just make sure you have it pressurized as you pour the slab)


Qurious, why? We recently poured a driveway with heating tubing. We did not need to pressurize the tubing.


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## duburban (Apr 10, 2008)

katoman said:


> Qurious, why? We recently poured a driveway with heating tubing. We did not need to pressurize the tubing.


how could you know if you ruptured it during the pour?


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## katoman (Apr 26, 2009)

It was pretty tough stuff, mat mesh on top of it, and the pour was only 4-5" thick. Not much chance of puncturing it.

I like the idea though.

In this case the manifold was not run into the house yet, and we needed to get it poured. Worked out ok. But I agree, it would have been good insurance to have it pressurized.

It was a first for me, and I relied on both my heating guy and concrete guy. That's why I asked. Always learning.


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## duburban (Apr 10, 2008)

pex is really rugged for sure. i'd bet the pressure makes it more crush resistant also. 


i'm kind of liking the frost walls and slab on grade idea. 

read a discussion on greenbuildingadviser a while ago where they were preaching how much more effective 4" foam under slab is vs 2". 

i think passive house is more like 10" !!


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