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RE: ASCE 7-05 Section (#4) clarification needed

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I interpret this as if the flat roof snow load (Psubf) exceeds 30 psf, then you take 20% of the FLAT roof snow load (Psubf) and add that to the seismic mass.  You do NOT take 20% of the SLOPED roof snow load (Psubs)...that is the "regardless of actual roof slope" part.  And you do NOT take 20% of the portion that exceeds 30 psf.
Thus, if you have a 40 psf flat roof snow load and a 12:12 roof pitch on a non-slippery roof such that the sloped roof snow load is 24 psf, then you would add 8 psf (20% of 40 psf) to the seismic mass...NOT 4.8 psf (20% of 24 psf).
Adrian, MI
-----Original Message-----
From: Dennis Wish [mailto:dennis.wish(--nospam--at)]
Sent: Saturday, July 19, 2008 9:56 PM
To: seaint(--nospam--at)
Subject: ASCE 7-05 Section (#4) clarification needed

I need an interpretation to note number 4 of ASCE 7-05 Section which states:

“Where the flat roof snow load, Pf, exceeds 30 psf (1.44 kN/m^2), 20 percent of the uniform design snow load, regardless of actual roof slope.”


This seems to conflict with itself. My first interpretation is that if the snow load for a flat roof (say ¼” in 1-foot) slope or a shed roof exceeds 30 psf then you would add 20% or 6 psf to the roof dead load. However, the last section of the statement reads “regardless of actual roof slope” in which case I would interpret the 20% of the Snow Load exceeding 30-psf to be added to any sloped roof.


How do others interpret this statement. If the roof is flat should 20% of any Snow Load be considered whereas only 20% of the Snow load exceeding 30-psf be used if the roof is pitched?


Dennis S. Wish, PE


Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant

La Quinta, CA 92253