Concrete Slab Overlay

[DE LLC]

I have a home owner that would like to raise his living room which is 11" lower than the rest of the main living area of the house. This would require pouring 11" of concrete on top of the existing 4" monolithic slab. I would generally be concerned about shear along the perimeter of the overlay area but there is a 10"x20" footing that runs about 90% along the perimeter of the effective area. Anyone have experience with concrete overlays of existing slabs?

 

[MotorCity]

Assuming this is a slab on grade, my approach would be simply to raise the floor with wood framing. If you are adding an 11" thick concrete overlay to the living area of a house, I would not be concerned about shear, flexure, or any failure mechanism that did not involve nuclear warfare or a zombie apocalypse.

 

[GC_Hopi]

Is the existing a slab on grade? How about using sand as a filler... maybe 7" then vapor barrier and 4" concrete with WWR. Tie the the new slab in with a few dowels. You could price that against wood but my guess is wood is cheaper.

Edit: The main item to consider is the floor finish (tile, vinyl, hardwood) and the transition. You dont want a crack forming there.

 

[phamENG]

MotorCity - I'd be worried about settlement. 11" of concrete isn't weightless. You're adding 133psf of dead load surcharge the supporting soils. Some soils can take it, others can't. Where I am, it could cause soil problem.

 

[MotorCity]

phamENG - fair point on the settlement.

As long as were talking details of the overlay, I would throw in some reinforcing near the top of the slab to restrain any shrinkage cracks and add a few sawcut control joints. Forming the overlay could also present issues. If this is being poured within a traditionally framed room with drywall, the drywall cannot serve as formwork. May need to consider some sacrificial strips of plywood around the room or more robust formwork. Also what happens to any electrical outlets....are they being raised too or will they be 2" above the floor? Any interior or exterior doors? Any floor air supply or return vents? Lots of things to be considered.

 

[gte447f]

I would go with treated wood sleepers instead of 11" of concrete.

 

[DE LLC]

Thanks all. The existing slab is on grade (sandy soil) and the finish will be tile. Good observation on the details. I intended to specify removal of drywall and form with 5/8" plywood (studs are at 16" o.c.) and the electrical outlets raised to NEC code height.

 

[Tomfh]

Raised timber floor is the standard approach.

If done in concrete we sometimes use waffle pod void formers to cut the volume of concrete.