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Re: Roof Slope for Drainage

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Jeff,

The UBC has always been a little nebulous about roof drainage.  IIRC, the 
first place that a 1/4"/ft roof slope requirement appeared was in the 
glue-laminated requirements.  If the roof doesn't drain, and it isn't 
designed to support the weight of the accumulated water, then I would say 
that Section 2305(f) of the 1973 UBC had not been met.

Haussler published a paper in Civil Engineering Magazine, October, 1962, on 
roof ponding.  (Haussler suggested limiting deflection under a 5 psf load 
to 1/2".  Since water weighs 5 psf/ft of depth, the limitation means that 
water would always run off over the edge of even a flat roof.  However, if 
the roof had an initial dish, then all bets are off.)  Marino expanded on 
Haussler's work in AISC's Engineering Journal, July, 1966, covering ponding 
of two-way roof systems.

I developed a very simple equation relating deflection to slope necessary for 
drainage:

   slope = 0.0104 + 4*deflection/L

where:   0.0104 is 1/8"/ft (0.125/12) as being a minimum slope at the low end 
               for drainage (can be changed to whatever slope you feel will
               be adequate for drainage);

and,

      deflection is all the deflection that occurs under loading, shrinkage 
               and ponding, less camber, and

      L is the span in the direction of drainage.

Hope this helps.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Jeff Coronado wrote:

> We are currently evaluating a structure built in accordance with the
> 1973 Uniform Building Code.  The roof does not have a 2% slope for
> drainage.  I do not have a copy of the 73 UBC.
>
> If someone is familiar with 73 UBC, or has a copy of it:
>
>         1. Is the 2% slope a requirement in the code.  If so can you
> please tell me the section so that I can reference it.
>
>         2. If 2% is not in the code, is it in the standards?  Were
> there            no guidelines for roof slope in 1973?
>
> I would like to go on record that the slope of the roof is not in
> conformace with the UBC but I need something to stand on.
>
> Thank you,
>
> Jeff Coronado, S.E.
> West Covina, CA

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