Monolithic Slab with Two Slab Levels?
gerrha
| Posted in Construction Techniques on
What I would like to do, is have a monolithic foundation and slab, poured at one time, but with the garage area 4″ lower and sloped 1% to where the doors will eventually be.
Myself, I have a hard time visualizing how this might be done, but I also thought surely it is possible. However, I cannot find any source of information for how, which must mean it cannot be done – in a single pour anyway.
Also, in my dream, at the same time the above was done, I would put in a 40″ wide wheelchair ramp from the 4″ lower garage floor back up to the main floor level.
Must this be two separate pours of concrete with a joint between the levels?
Replies
To answer your main question - Yes, it can be done as a monolithic placement (i.e. one pour).
However, it all depends on the skill and accuracy of your concrete contractor. It will require a 4"+ high form, suspended above the top elevation of your lower garage floor slab. Not knowing where the slab step is in relationship to the sloping garage floor complicates matters a bit. In one direction the 4" step would be constant, but 90º to that the 4" step can increase because of the 1% slope.
The challenge with a suspended form like this is holding its accuracy in location, as typically there is very little to brace to within a reasonable distance. Further, any bracing used poses a tripping/stumbling hazard to the people placing the concrete.
Can it be done? Yes. Will it be accurately located once the concrete has hardened? Depends. You might get lucky.
IMHO, a 1% garage floor slope is too shallow. I would recommend a minimum of 2%.
Residential concrete floor slabs are rarely perfectly flat and perfectly true to level (or slope).
A 2-placement approach will yield better results with respect to the location accuracy of the step in the concrete.
Your desired ramp should be located when to lower slab is placed, leaving the top of the slab rough and about an inch lower where the ramp meets the garage floor. It is not possible to add concrete to the top of an existing slab and feather it to 0" thick where it meets the lower slab.
Ok, it looks like two pours it must be.
I guess you would do the entire turned down edge foundation along with the higher-level slab floor at one time. Then the lower-level slab with the wheelchair ramp in a second pour understanding that this second slab will be isolated from the previous concrete.
Thanks for the suggestions for the wheelchair ramp and garage slope.