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Home > Forums > Engineering Codes, Standards & Certifications > Engineering Codes, Standards & Certifications > ASCE (civil) Code Issues Forum

Snow loads on a trough roof (butterfly truss)Helpful Member!(4) 

thread176-312956
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UpNorthStruct (Structural)
22 Dec 11 11:20
I am reviewing the design for a structure with a slight inverted pitch (2.385 degrees) to the center.  The width of the building is 294 feet.  I am concerned about snow accumulation in the valley (6'-4" height difference).  Since the slope is inverted, I am not sure the flat roof provision (less than 5 degrees) of ASCE 7-05 Sect. 7.3 pertains.  Should this be evaluated as a sawtooth roof in Sect. 7.6.3 or is there another provision that applies?   
MiketheEngineer (Structural)
22 Dec 11 15:39
Not sure - but I sure know that this roof will cause GREAT problems in the future even if it doesn't collapse.

I would check fOR drifting and saw tooth might be one way to go.

REMEMBER THIS - IT WILL LEAK - IT WILL ALWAYS LEAK!!!!!!
Helpful Member!  ajh1 (Structural)
2 Jan 12 8:32
Yes, this falls under Section 7.6.3.  I would consider it a folded plate structure (or valley structure) more than a sawtooth, but the same rules apply.  Section 7.6.3 applies for greater than 3/8:12 slope (1.79 degrees) so your building falls into the requirement.  In effect you have a drift similar to Figure C7-1 in ASCE 7-05.
Helpful Member!  dik (Structural)
2 Jan 12 10:39
Mike... have done this a few times using an EPDM membrane with crickets between the roof drains... and none leaking yet... some have been in service for 15 or so years... just have to do the roof right, and in the case of the long term one, had a proper roofing inspector around.

Dik
Helpful Member!  MiketheEngineer (Structural)
3 Jan 12 15:00
The big word here is "YET"
dik (Structural)
3 Jan 12 19:31
Mike... I agree, but, I've encountered regular roofs with greater problems...

Dik
Helpful Member!  msquared48 (Structural)
5 Jan 12 12:57
The true function of any roof is merely to delay the inevitible.   

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
 
UpNorthStruct (Structural)
5 Jan 12 14:49
Thank you for the responses.  It is good to have confirmation that I was applying the code correctly in regards to unbalanced snow load in my review.  We are planning on using an EPDM membrane on the roof and will use crickets where necessary.   
dik (Structural)
8 Jan 12 21:42
Just a caution, the two Mikes shouldn't be taken too lightly... The valley creates an ideal location for tractions from freezing water to cause problems.

Dik
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