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Re: Roof Slope for Drainage

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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


      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:


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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