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Snow loads on a trough roof (butterfly truss)

Snow loads on a trough roof (butterfly truss)

Snow loads on a trough roof (butterfly truss)

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?   

RE: Snow loads on a trough roof (butterfly truss)

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.


RE: Snow loads on a trough roof (butterfly truss)

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.

RE: Snow loads on a trough roof (butterfly truss)

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.


RE: Snow loads on a trough roof (butterfly truss)

The big word here is "YET"

RE: Snow loads on a trough roof (butterfly truss)

Mike... I agree, but, I've encountered regular roofs with greater problems...


RE: Snow loads on a trough roof (butterfly truss)

The true function of any roof is merely to delay the inevitible.   

Mike McCann
MMC Engineering

RE: Snow loads on a trough roof (butterfly truss)

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.   

RE: Snow loads on a trough roof (butterfly truss)

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.


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