Concrete Slab with no walls

[Anonymous]

i have a 12' x 20' slab were a garage used to be.. i dont know the right names of all the stuff "like what you would call the walls or anything like that..i like to do stuff myself if you know what i mean.... when i build my walls .....do i need to add concrete walls to nail/bolt the treated bottom plates too or could i just drill some holes and put lags in the ground and slip a bolt into it??? the slab is old maybe build in the early 1900s and its about 3inch thick ...and i was going to build the garage 12'x20' and help would helpful

[Rich]

The problem with having a non-thickened edge, or no stemwall, is the windload/snowload moves the structure around. When this happens typically the thickened edge, or stemwall, will take the brunt of the force. When you put that all on to a 3" slab it will most likely crack the edge off. The easiest thing to do would be to dig out a couple feet down on the outside of the slab and form a small stemwall I would also suggest tying this to the slab by doweling into the edge of the slab with #4 rebar and turning them down into the stemwall.
I'm not sure where you are located but you should think of frost heaving also when digging the stemwall. That may be the deciding factor, or I should say is the deciding factor, on how deep the stemwall should be.

[Anonymous]

ok thats sounds great and kinda easy......how thick should i make it??? and what is the rebar for? im new in doing anything with concrete
and also im in seattle so we hardly ever get snow manly RAIN:(

[Anonymous]

opps for got to ask if i should poor some of the concret onto the slab? and can i make a form using 3/4 plywood? or do i have to go out and buy some just for making forms??? thanks again for your FAST REPLYS :roll: :shock:

[Rich]

The rebar is for tying the stemwall to the slab. By doweling epoxying into the edge of the slab approximately 4" and then turning them down into the stemwall they will act more like a solid unit and give the stemwall more solidity. My suggestion for thickness of wall would be 8" for a typical cast in place stemwall. I'm not up on the depth of frost in Seattle but wouldn't imagine it would be more than 36"-48" deep. This will dictate the depth of your stemwall.
Forming is fine for one time use with 3/4" plywood.. use CDX grade. It may be hard to take off but it's a bit cheaper. I don't think you need to pour any concrete onto the slab - pouring it flush should be alright. One thing to consider when pouring flush to the slab is what the grade of the ground is in case of larger rains and possible water problems. It's always nice, but not always achievable, to have 6" from ground to the top of concrete.
Another point to consider is when backfilling against the new stemwall - compact the dirt back in in 6-8" lifts. This will help out with water runoff later.