# Re: Roof Slope for Drainage

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Roof Slope for Drainage
• From: Scott E Maxwell <smaxwell(--nospam--at)engin.umich.edu>
• Date: Fri, 29 Sep 2000 23:08:24 -0400 (EDT)
```Jeff,

You might also want to look at the most recent Structural Engineer
magazine.  It contains an article on roof drainage/roof slope.  It may or
may not help.

Scott

On Thu, 28 Sep 2000, Jeff Coronado wrote:

> 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
> >
> >
> > > 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]
> >
> >
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