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Re: CORROSION OF JOIST HANGERS AND ROOF

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Ben Yousefi wrote:

>>I'll put in my 2 cents in regard to a possible root cause:

"Ponding"

Based on my personal experience of reviewing plans, and the observations in 
the field, I believe one of the most overlooked areas of building design is 
roof drainage. Architects are normally responsible for specifying the slope 
and showing the drain and overflow details and their locations. However, they 
are not apparently very keen on this issue. Many times the slope is not even 
defined on the drawings. Also, until the 88 edition, the UBC did not even 
specify a minimum roof slope. Technically you could have designed the roof 
completely flat and still complied with the code. (the exact language was: 
"Roof systems not designed to support accumulated water shall be sloped for 
drainage" ).

I have made it a habit of paying particular attention to this during the 
review process. My suggestion to all structural engineers is to get proactive 
on this issue and make sure the architect has addressed roof the drainage 
properly. Keep in mind that the even the minimum slope of 1/4 inch per foot 
may not still not be adequate, 1/2 inch per foot is much more functional.<<


Ben,

Several years ago I developed the following equation to approximate the 
minimum roof slope necessary to provide positive drainage:

   roof slope = (min. end slope) + 4 * d/L

   where:

      (min. end slope) is the slope desired/required at the low end of the
         span/bay so that water will not pond.

      d = deflection of structural member, or sum of deflections of *all*
         structural members in a bay under the loads being considered.

      L = span between supports in the direction of slope. (units 
         consistent with "d").

For a (min. end slope) of 1/8"/ft (.125"/12" = .0104)

the equation becomes:

   roof slope = 0.0104 + 4 * d/L

If the structural members span in only one direction, the direction of the 
slope, and the deflection is limited by the common criteria of L/360, L/240, 
and L/180, the minimum roof slopes become:

      1/4"/ft for L/360
      3/8"/ft for L/240
      7/16"/ft for L/180

Local amendment to the UBC has changed the minimum roof slope in Tucson and 
Pima County to 3/8"/ft, unless otherwise justified by engineering 
calculations.  I have seen apartment complexes with (supposedly) dead flat 
roofs where maintenance personnel put submersible pumps on the roofs after 
each rain.

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