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Re: Roof Slope for Drainage

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Roger,

It helps very much.  Thanks once again.

Jeff Coronado, S.E.
West Covina, CA

Roger Turk wrote:
> 
>    Jeff,
> 
> For a deflection = L/240, the minimum slope to avoid ponding would be,
> 
>       s = 0.0104 + [4 * L/240]/L
> 
>          = 0.0104 + 4/240
> 
>          = 0.0271
> 
>    or,
> 
>       0.0271 * 12"/ft = 0.325"/ft
> 
>    and for a 20 ft span, the required difference in elevation would be 6.5"
> for positive drainage off the low end.  (This also works with camber, i.e.,
> P/S beams to prevent ponding at the *high* end.)
> 
> The code minimum of 1/4" per ft will only avoid ponding if deflection is
> restricted to L/360.
> 
> In their adoption of the 1994 UBC, the City of Tucson and Pima County have
> modified the minimum slope to 3/8" per ft.
> 
> Hope this helps.
> 
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona
> 
> 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<<
> 
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