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Old 11-17-2011, 10:06 PM   #1
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Slab on Grade in NY - slab removed...


Hello All,

I am considering buying a small house (approx 1500sf) in Upstate NY (Zone 5/6 border) that was built using slab on grade foundation construction back in the Mid 1940's.

The slab has been removed by the previous owner (apparently it was heaving?) then got foreclosed upon.

I am an engineer by trade with some experience in concrete design, but in my experience it is best to talk to the guys that build the stuff engineers design to get the best understanding and education about practical application of all that book theory!

I have never built or renovated a slab on grade. Can anybody direct me to some good info?

I know I have to tie back into the "stem" with rods and reinforce in (fiber or steel), but I am a bit foggy on joints and subgrade plumbing?

Thanks for any info.

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Old 11-17-2011, 10:49 PM   #2
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Slab on Grade in NY - slab removed...


Is the house sitting on cribbing now?

Andy.

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Old 11-18-2011, 01:35 AM   #3
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Slab on Grade in NY - slab removed...


You do not HAVE to tie the floor to the foundation wall. Floating slabs are quite acceptable up here. Were I in your position, I'd take full advantage of it and dig deeper, replacing at least 6" below the new slab w/ EPS over a good vapor barrier (10 mil black poly, StegoWrap, etc). Isolate the edge of the slab from the foundation wall with foam as well, to kill a thermal bridge' 2 to 4" if aesthetically acceptable.

Last edited by jklingel; 11-18-2011 at 01:39 AM.
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Old 11-18-2011, 07:35 AM   #4
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Slab on Grade in NY - slab removed...


Quote:
Originally Posted by jklingel View Post
You do not HAVE to tie the floor to the foundation wall. Floating slabs are quite acceptable up here. Were I in your position, I'd take full advantage of it and dig deeper, replacing at least 6" below the new slab w/ EPS over a good vapor barrier (10 mil black poly, StegoWrap, etc). Isolate the edge of the slab from the foundation wall with foam as well, to kill a thermal bridge' 2 to 4" if aesthetically acceptable.
If this was a slab on grade there will be no foundation wall by the definition I know of 'slab on grade'.
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Old 11-18-2011, 07:40 AM   #5
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Slab on Grade in NY - slab removed...


Post some photos of the issue so we can see what you're talking about.
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Old 11-18-2011, 01:49 PM   #6
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Slab on Grade in NY - slab removed...


Quote:
Originally Posted by joed View Post
If this was a slab on grade there will be no foundation wall by the definition I know of 'slab on grade'.
Joe: Yes, but the OP called it SOG and mentioned tying the slab to the stem wall. In my mind it gets a bit confusing because some folks build w/ a SOG, no stem wall at all, some with a thickened edge beam (and call it SOG), and others call it SOG when there is a floating slab and a stem wall. I guess technically you should call them SOG, thickened edge beam, or floating slab (and assume there must be a stem wall w/ the floater). Whatever; pictures clarify.
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Old 11-18-2011, 03:02 PM   #7
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Slab on Grade in NY - slab removed...


In Melbourne, the common method is slab on ground, and I would estimate 80% of new houses use it. If designed properly, it's indestructible, and ideal for filled or problem sites. Only on very steep grades would one consider using timber floor framing and flooring. Timber is more expensive, subject to rot and in termite prone areas, can be attacked by them.

The problems associated with concrete slabs are minor, and with careful design can overcome the biggest problem: shrinkage. Concrete shrinks in it's length by about 1/3-1/2 the distance brickwork grows, so if no expansion/contraction joints are provided to the brick wall, it's not uncommon to see a long brick wall with no joints overhang the concrete slab by up to 1'' to 2''. Structural engineers prefer to pour a house slab in one continuous slab to avoid joints that can heave out of alignment if shear rods are not installed properly. Another problem to watch is to use a flexible adhesive between stone or ceramic tiles, as tiles expand and concrete shrinks.

The older method used where perimeter walls were built first (usually of brick) and the slab poured afterwards insde is no longer used, because of shrinkage and heave problems. The inner ground beneath the slab cannot be comapcted as well as the natural stiff clay under the perimeter wall foundations. Result is differential movement between outer walls and inner walls, showing up as corner cracks in plaster, diagonal cracks over corners of openings and in severe cases, cracked glass due to stressed window frames.

Structural engeneers design slabs to stop differential movement and make allowances for shrinkage and expansion of different materials. It is illegal to construct a residential slab without a structural engineers certificate of compliance and computations in Australia.
Cheers! from Oz.

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