Hi all
I’m working on a small addition (digging it tomarrow, Friday).
The addition is 6′ x 12″ tucked into the corner of a house.
The concrete contractor suggested that we pour a slab, and then the walls on top.
This saves him a trip and a third trip from the concrete supplier.
Is this a good or bad idea?
I usually do the usual footings and then wall, with the floor later.
I have done garage slabs with walls on top with no problems.
He says we’ll key it and have rebar.
Any thoughts?
jeff
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Replies
if he's doing a monolithic slab and sizing the footing portion of it correctly, there shouldn't be a problem. but i'm not a CC expert.
whats the additional room going to be? Whats the existing floor?
I personaly dont like mixing crawl space with slab because of the different "feel" you get when you go from one room to the next.
Additional room will be for a water tank, in the basement and a bathroom above.
The basement is a typical 8 ft with a poured concrete floor.
The addition will be about 7 ft deep, and a doorway cut into the basement. Roughly a 1 ft step up into the new basement.
Lots of drainage around and below.
well in that case i would definetly do the slab. as far as before or after the walls are done id be inclined to agree with dovetail and do the slab after the wall were in place.
Im not sure ive ever seen a detail where the basement was a poured slab and then the walls were built on top of said slab.
Keying is not a guarantee that you won't have a continuous leaking seam at the joint. High winter ground water could be a problem.
Are you planning a slab floor for any special reason?
Slab floor is a suggestion, I can have the concrete contractor do it normal.
The budget handles it either way.
Slab will support a water tank and pump.
Personally I would pour the slab after the walls.
They can't get your Goat if you don't tell them where it is hidden.
doable - if he does all the other stuff right
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Jeff, are you putting footing drains in on both sides of the footing?
If so, I don't see how one way is superior to another. No matter what, there will always be a way for water to get to your slab: from directly below! Your waterproofing details will prevent water from going through your basement wall and the joint. The drainage system will prevent hydrostatic water from being forced in.
This is a six of one/half dozen of the other call.
If you really think it matters, he could pour the footing and slab and block out for the poured wall and you could pout the wall later directly onto the footing and get the same look as if you poured the floor later.
Maybe I'm misunderstanding something here.
Bob's next test date: 12/10/07
Quote of the day: "...can't be done, it will take too long, not enough pipeline capacity, yada yada but yet they don't apply the same skepticism to their own "solutions" such as wind and biofuels"
I want to clarify my comment on "personally I wouldn't do it ".
The slab poured inside of the walls acts as a compression member against the lateral force of the soils pressing against the wall. Without the slab acting as the compression member you are relying solely on the key and the rebar to hold the walls in place. This isn't much of a problem with say a 2' foundation wall , but a much greater risk with a 8' wall.
Water intrusion at the keyway can and would lead to deterioration of the re-bar over time so near perfect water proofing would be needed to prevent that from occurring.
While this is doable I am sure with the correct engineering, I simply don't see the point to it.
Just my opinion.
Good points. Bob's next test date: 12/10/07
Quote of the day: "...can't be done, it will take too long, not enough pipeline capacity, yada yada but yet they don't apply the same skepticism to their own "solutions" such as wind and biofuels"
I worked with an engineer for a bit who was an advocate of this approach. He termed them mat slabs, which are supposed to distribute the load across the entire slab. He led me to believe that it was common practice in CA. I wasn't impressed with the method. For one thing, we typically have 12" or so of granular fill under the slab--this would be a problem if your slab was also your footing, although I suppose it could be compacted enough to be under the footing. Also, if you have any underslab plumbing, it's going to be a real treat to locate walls etc. Also, what about insulation? After we pour basement walls, we line the entire floor with foam. Again, this seems problematic with the mat slab.I suppose it comes down to how expensive the extra pour is.
Hi all
No Plumbing under the slab (just the old footing drain.)
There will be new footing drains around the slab.
The wall will be 7 ft, 5ft underground.
Insulated all around with 2" foam board (waterproofed first of course.)
The old water pipe will be re-routed from thru the old wall, to this new wall. And a new pressure tank installed in this new basement addition.
jeff
With a proper rebar detail I would prefer a monolithic pour. And, if I were doing the design, it would be that way, and possibly have an optional joint at the floor to wall line, with a keyway.
The monolithic pour if properly reinforced, with a low water to cement ratio concrete will give better results.
As long as your waterproofing detail is solid, you shouldn't have any issues. We do this is commercial applications quite frequently with a mat slab to act as footing and finish floor slab with either cast-in-place walls on top. Definitely make sure you use a keyway and plenty of rebar. In addition to your exterior waterproofing system, I would also suggest a continuous strip of waterstop. Installed in the keyway, exterior to the rebar.
Shawn
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Checker Contracting - SE Michigan