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Re: Sloped Roof snow load

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For small roofs (most simple residences) with high pitches there was a major change (I say omission) slipped into the ASCE 7 a few years ago, and most 12:12 or higher roofs in use would not pass todays code provisions.  In 7-02, the unbalanced load for rafters 20' or less was 1.5ps/Ce.  Since ps = pf * Cs, and pf = 0.7 x pg (Ce cancels out, Ct and I ==1 for most structures with this rule (claimed attic R values notwithstanding)),  then the drift load is simply 1.5 x 0.7 x pg = 1.0pg (1.1pg if you believe the insulator did his job)

That's what the 7-05 code is - pg.  The rub is that if you have either a steep sloped roof (steeper than 12 in 12 for well insulated roofs or 9 in 12 for anything built before 1980) or practically any slippery roof, your Cs is likely to be in the 0.4 to 0.6 range, which means 1/2 the load.

A nominal example: 12 in 12 metal roof, 16' rafter span, 30psf ground load, R38 insulation with vented rafters/attic

ASCE 7-02:    pf = 0.7 x 1.0(Ce) x 1.1(Ct) x 30psf = 23psf
                       ps = 0.4 x 23(pf) = 9psf
         unbalanced = 1.5 x 9 / 1.0(Ce) = 14psf

ASCE 7-05:   unbalanced = 30psf (pg)
 
ASCE 7-05 is certainly easier,  but quite a bit more conservative. It also tends to fail the sniff test, as it is somewhat hard to believe that a roof which will hold 9psf on both sides in a balanced condition will hold 3-1/3 times as much on one side due to drifting. Where did the snow come from? The intent is that it drifted over from the windward side, but clearly that isn't possible.

Even if you take a non-slippery roof you get Cs=0.75, making the load 26psf - a 13% variance. Of course for older homes, the actual thermal resistance is likely less than R25 net, resulting in a Cs=0.62 and a net 21psf, a significant difference. It may seem trivial, but a good deal of my practice is in renovations of older homes, and many have steep pitches and have seen a 100 year snow event in their lifetimes without visible damage. Such roofs are rare enough today, apparently, that they do not receive allowances in the code.

Sorry...it seems I've acquired a soapbox under my feet.

Jordan


Pinyon Engineering wrote:
In ASCE7-05 it seems the sloped roof snow load is the same as the flat roof snow load except for a few extreme roof slopes no reduction for the typical 6:12.  Is this what others are finding?
 
Tim Rudolph
Pinyon Engineering
Bishop CA
 

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