Concrete over framed floor

weeone · Mar 17, 2015

My first post...woo hoo!

My son needs a concrete slab poured at his main entry. It is being poured over 2 by 12 treated joists with treated plywood over it. The entry is over his root cellar. How thick should this slab be? What strength? How much re-rod? By the way, it is exterior but has a roof above.

A concrete contractor I know suggests a 5-6" slab, 4500psi, with re-rod in a 12"grid. Can anyone weigh in on this?

Many Thanks.

Bondo · Mar 17, 2015

weeone said:
My first post...woo hoo!

My son needs a concrete slab poured at his main entry. It is being poured over 2 by 12 treated joists with treated plywood over it. The entry is over his root cellar. How thick should this slab be? What strength? How much re-rod? By the way, it is exterior but has a roof above.

A concrete contractor I know suggests a 5-6" slab, 4500psi, with re-rod in a 12"grid. Can anyone weigh in on this?

Many Thanks.

Ayuh,.... As in under the porch at the front door of his House,..?? foot traffic only,..?? no cars, Right,..??

I'd think 4" oughta be enough,...

ConCretin · Mar 17, 2015

Just curious. What is the purpose of placing concrete over wood framing?

We regularly place concrete over wood framed floors for radiant heat that are only 1 1/2" thick.

A six inch slab will add a dead load of 75 psf.

RivennHewn · Mar 17, 2015

I just poured a 20' X 20" deck over wood framing.

4" thick with 16" OC grid of #4 bar.

16" OC joist. I upped the plywood to 1-1/8" T&G.

Saw cut relief lines the next day.

4-5 months old, and no cracks yet.

weeone · Mar 18, 2015

Thank you for the replies! I never asked why!!! It does sound like 4" should do the job. Thanks for the advice.

homebuilt burner · Mar 19, 2015

I did a similar thing and I was informed it would be a good idea to put an 1" of styro overtop as a thermal block and rubber roofing membrane to make sure the slab is sealed. I love it the room underneath stays around 45* without any heat in the winter.

weeone · Mar 20, 2015

Thanks burner. I think that is just what he is looking to achieve for a temperature.

thegarageguy · Mar 20, 2015

we regularly apply polymer modified self leveling concrete over 3/4 inch tongue and groove plywood. We pour 1/2 inch of concrete with anti-fracture membrane.

Here is an example over wood and tile;

benwah · Mar 20, 2015

thegarageguy said:
we regularly apply polymer modified self leveling concrete over 3/4 inch tongue and groove plywood. We pour 1/2 inch of concrete with anti-fracture membrane.

Interesting, how do you adhere the 3/4" plywood to the wood and tile substrate?

thegarageguy · Mar 20, 2015

We do not install the wood. We ask that the gc or owner to provide us with 2 sheets of 3/4 inch tongue and groove plywood, screwed and glued to each other. As far as the tile, we just grind it and pour right over it.

yeldogt · Mar 20, 2015

Will self leveling work for a garage? I'm trying to figure out how best to level a slab so I can put a wood floor over it. It's for my studio. I want to level the slab - vapor barrier sleepers/ insulation and plywood (maybe warm-board radiant) then wood floor.

And just so I understand ...you can pour a concrete floor over plywood that is only 1/2 thick without cracking? When you say two sheets -- so it is 1.5 inches thick plywood?

Looks good!

benwah · Mar 20, 2015

Very interesting, thanks for the info! Great looking floor BTW. Dig the dark gray stripe

thegarageguy · Mar 20, 2015

Thank you benwah. Yes, we can trowel apply 1/8" microtopping over plywood without issues. I have found the 2 sheets of plywood will be sturdy enough to minimize vertical movement which could cause hairlines on certain systems.

There are different formulations of self leveling, depending on the substrate and what finish is specified. There are gypsum and portland based, calcium aluminate based, calcium sulphate...the list goes on and on.

@yeldogt, more than likely, you need a low prep, high psi gypsum based product with anti-fracture mesh. Companies to research are CMP Specialties, Drytek, Ardex, USG and Mapei.

Concrete over framed floor

weeone

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Bondo

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ConCretin

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RivennHewn

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weeone

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homebuilt burner

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weeone

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thegarageguy

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benwah

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