Sliding Snow Loads

[smokiibear]

ASCE 07-05 section 7.9 discusses sliding snow requirements for any roof above another roof.

I'm curious if this load should apply for dormers, where the roof above is not very high above the roof below?

Secondly, does sliding snow or drift loads apply to roofs at a pitch change?

Any comments? I've attached a picture for your review.

 

[DaveAtkins]

Question 1--the answer is yes, but there won't be much sliding snow, because the dormer roof is so small.

Question 2--the answer is no.

DaveAtkins

 

[smokiibear]

Thanks, Dave. I would have thought that the sliding snow would be small for that dormer. However, it ends up going from ps=25.2psf to sliding of 100psf over 15 feet. That ends up being pretty significant.

 

[DaveAtkins]

Your numbers don't make sense. How wide is that dormer--10 feet? Then each side of the dormer receives 5 feet of snow from the upper roof, spread over 15 feet--something like 8 psf, not 100 psf.

DaveAtkins

 

[smokiibear]

one correction. the flat snow load = 25.2psf. the sloped snow load = 20psf. as I see it, the sliding snow load + superimposed sloped load = 20psf + 0.4 x (pf =25.2psf) * (W=8ft) (ridge to eave) = 100psf.

I suppose one area of discussion could be in ASCE 07-05, section 7.9, is the defintion for W as "eave to ridge." I'd say that this defintions would more clearly read "overhange to ridge." If it truly is eave to ridge, this would reduce my total load a bit.

Am I missing something?

 

[smokiibear]

Upon re-reading the section, I think I missed a very important part. The code reads, "...The total sliding load per unit length of eave shall be 0.4p fW, where W is the horizontal distance from the eave to ridge for the sloped upper roof."

I missed the part about per unit length "OF EAVE." I thought the 0.4*pf*W was per unit lenght over the 15 feet, so I interpreted the total load as a uniform load over 15 feet, not the total load / 15ft.

I think what you were getting at was this:
0.4 * 25.2 * 8 = 80psf / 15ft = 5.3psf
Ps = 20psf + sliding snow = 5.3psf = 25.3psf over 15 feet.

Is this what you were trying to say?

 

[DaveAtkins]

Yes. I was just throwing around rough numbers, but I knew the load is to be spread out across the 15 feet.

DaveAtkins