I am building a floating slab on grade garage floor.Am I correct in assuming a 3″ slab, 1.5 ” syrafoam over compacted sandy b-grade fillwith a builtup base on the perimeter. Garage one store frame 12’x 20′
How built up should the exterior concrete perimeter be?
Does the plastic go above or below the syrafoam.?
George
Replies
garage slab, three inches thick. It will break
Lik BB said.
Use a 4" pour.
Why foam brd. under the slab? Is it a radiant slab floor on an attached garage or a future shop?
If you are not going to heat the space, leave out the rigid foam.
I put the poly over the foam board, other put it under. I don't pour slabs with a monolithic apron. I pour the apron after the slab and dowel it to the slab edge. 4" is fine for an apron also, if everything is on correctly compacted grade.
Dave
Thank you
The owners are going to use the garage for a cottage bunky or guest room, No heat just summer use, I thought the styrafoam would keep the floor closer to room tempature. I will increase the floor to 4", Any thoughts on size of the perimeter concrete.
George
You can put 2" foam insulation outside of the edge. Use 2x6 sill and cover the edge of the foam. Flash ,paint or parge the foam unless you like the pink look.
4" Slab with a 12" x 12" grade beam (thickened edge) around perimiter. I've done 2 houses and a garge this way and all have held up well in cold climates. Consider fiber cement as well as you will not be heating the building.
Dave.
In reading your post one more time , why do you pour the perimeter/apron separate from the slab?
George
Are you talking perimeter as in edge of slab, or only the portion outside of the overhead door?
I pour aprons (outside the ovrhd dr) separately and dowel them to the slab so they can move somewhat independent of the slab. Even in an unheated detached garage the inside slab is subject to milder winter stresses than the outside apron. The perimeter frost wall or foundation walls reduce the amount of freezing the interior slab will experience, and the sub grade acts as a thermal stabilizer to some extent. That difference is enough to cause shrink or expansion cracking in the interior slab. By pouring the apron separate from the slab, I reduce some of the stress on the interior slab. I use smooth dowels poured into the interior slab, and then grease the exterior part of the dowels before I pour the apron. This allows the apron to move into or away from the slab as conditions change, but keeps it from moving up and down.
Now if you are talking about how thick to pour the interior slab perimeter, I would ask why? It is unlikely that the perimeter of your floating slab is likely to experience as much load as the area that vehicles will be parked on. Unless you are building a monolithic slab/footing with framing on top of it there is no reason to make the perimeter of the slab thicker.
If you are doing a slab on grade your under slab insulation isn't going to help much unless you do perimeter insulation also.
I guess we need to know more about your plans to give you specific suggestions. BrownBag is our resident concrete expert. He'll jump right in to give you some good advice.
Dave
Thank you again Dave
Yes I am building a monolithic slab with the exterior walls on the perimeter of the slab. No footings.
George
I'm no concrete guy, but as I recall from when my garage was built (floating slab, no footings) the floor is 4" thick in the center and then it is increased to 12" thick around the perimeter.
I have built many a garage and IMO your 4" slab will be fine for "living" space. However, you should have turned down footings along the perimiter. More so if you plan on 2x6 walls.
Good compaction of the base material cannot be stressed enough!!!
Regardless of what you may hear, 5" is min. if you plan on parking your car/s inside. A turned down footing with rebaer is also suggested. Rebar, Rebar and more Rebar is also recommended.
Your interest in putting insulative foam is IMO a great idea, since you don't know what your future wants will be-- we call that planning and CYA . Heating your garage is not out of the ordinary anymore.
If you are doing the work yourself, do not forget to put in control joints. If for example you garage is 22x24, cross hair control joints will be fine(the floor will have 4 squares). I quite frankly don't recommend that you pour it yourself, hire this end of the job out per your requirements. The concrete guy will bitch a little but it's your garage so stick to your guns. A six bag mix is good. As an extra, if you can foot it, order fibergass re-enforced concrete.
And lastly, 90% of garages that I can see will develop a crack parallel to the overhead door within 3' to 6'. Your turned down footing should follow this space also.
This may all sound over the top but pouring a simple slab will always be followed with regret after the first cracks are found.