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Re: STEEL ROOF DECK: Question About Using Lightweight Concrete Fill

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Bill Polhemus wrote:

> Every now and then, just for fun, I actually bother to look at some of the
> Archy's details, and often I find the most amazing things. For example, I
> see that he has called for "lightweight concrete" as a fill on the roof deck
> on which the built-up roofing is to be applied. I'm used to seeing something
> like rigid board, or maybe poured gypsum. This is the first time I've seen
> "lightweight concrete." It just isn't that usual here on the Texas Gulf
> Coast.
> I usually use something like Vulcraft 1.5 B for the roof deck. But I can't
> really see where that would be appropriate if "lightweight concrete" is
> going over it.


Several architect clients I had in the past used the term "lightweight concrete"
for the product "insulating concrete" (Zonolite, Perlite, Elasticell, etc). What
they wanted on the roof was EXTRA-lightweight concrete.  Years ago, a Post
Office under construction in a town northeast of Houston collapsed when the
contractor placed what was called lightweight concrete on the plans for the
roof. The design intent was for insulating concrete fill to be used on the roof.

If they are trying to achieve a UL Class 90 uplift rating on the roof, then one
of the cementitious fill would be appropriate if the use of mechanical fasteners
to hold the roof insulation is ruled out.

If the architect truly wants lightweight concrete, then I think you need to use
a "C" or "VL" deck. I think this would be overkill for the roof structure.
Davis G. Parsons II, PE RA AEI
a practical structural engineer
in Fort Worth, Texas

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