Topping Slab LWC or NWC?

[davab]

Fellow structural engineers!

I have a residential custom home that requires a topping slab to accommodate the drainage system. Between the interior and exterior space, I am dropping the composite steel decking by 4" and allowing them to add a topping slab to slope to drain.
I am faced with a challenge in terms of assuming the right amount of dead load for the topping slab.

Since the topping slab will be sloping, I am not assuming 4" thick additional topping slab. Rather, I am finding the average between the minimum thickness and the maximum thickness of 4".

The question are:

What would be the minimum thickness required for a topping slab?
Is NWC preferred for a topping slab that is exposed to water?

Any thoughts or recommendations would be helpful.
Thank you.

 

[jayrod12]

if you're willing to reduce the maximum aggregate size, you could do down to a fairly thing topping if desired. I would be designing around NWC unless I was really strapped for depth or checking an existing condition. If new design and no drastic headroom issues, NWC all the way.

 

[KootK]

I just wrapped up a project where, to my chagrin, it turned out to be pretty much impossible to get light weight concrete. That may well be a oddity of where I'm at though (Western Canada). Best we could do for aggregate was apparently Utah. And I tried hard to make it happen as weight was a problem for my structure. Moral of the story for me: I'm never specifying light weight aggregate again unless somebody local has confirmed it's availability for me. I actually used light weight on a project here back in 2011 and don't recall any fuss. Now I wonder if I actually got it or not. On this latest job, they just subbed in normal weight concrete and didn't bother to bring it to anybody's attention in anyway. Only picked it up in submittal review.

 

[r13]

Other than cost concern, I think rigid forum insulation is good way for creating sloping surface on the flat roof.