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Re: Roof Slope for Drainage

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Roger,

Thanks for the info.  I'm not sure I am using the equation correctly.

For a typical delta/L = 1/240, s = 0.027 > 0.021 (code stipulated).  Is
this the intent?

For a 20 foot long beam, with 1/2" deflection, s = 0.019.  Therefore
elevation difference from end to end should be 4.56".  After deflection,
the elevation difference at midspan is 1.78".  The slope from end to
midspan is now 0.015.  I would have thought this would have been 0.0104.

I will struggle with it a little more to see if I figure it out.

Again, thanks for the info.

Jeff Corondao, S.E.
West Covina, CA

Roger Turk wrote:
> 
> Please correct my previous post.
> 
> Thanks!
> 
> Roger Turk
> 
> ----Forwarded Message(s)----
> 
>  #: 35460 (C) S0/CompuServe Mail  [MAIL]
>      26-Sep-00  16:43:30
>  Sb: Re: Roof Slope for Drainage
>  Fm: Roger Turk 73527,1356
>  To: INTERNET:seaint(--nospam--at)seaint.org
> 
> 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/in*** 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
> 
> [TAPCIS: Message was sent to 1 address]
> 
> 
> ----End Forwarded Message(s)----
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