Drifts and sliding snow on lower roofs - Considerations for impact

[DST148]

Last week I sent an email to Dr. Michael O'Rourke requesting him to suggest some guidelines for accounting for impact on the lower roofs due to drifting and sliding of snow from the higher roofs. Dr. O'Rourke is chair of the ASCE 7 Task Committee on Snow & Rain Loads. I am posting his reply here with his kind permission.

Drift load provisions are fairly detailed since there are a lot of case histories of structural failure - lots of information upon which to base code provisions. Unfortunately there are relatively few case histories of structural failure due to sliding loads. ASCE currently envisions a static load due to sliding snow - i.e. a load condition without impact. I recently was provided information on a sliding load failure from the Dakotas. I plan to analyze that case history and possibly suggest changes to ASCE 7.

 

[JAE]

Thanks for the info.

 

[swall]

The impact load from the sliding snow will depend on how long the snow had been in place before sliding. If it had been in place for months, it could have compacted/melted/refroze to where it became an ice chunk. I saw this happen in my off campus housing in Upper Michigan. The snow sat on the porch roof most of the winter and when it finally came down on a warm day, the ice mass caved in the roof of a car parked close by.

 

[SteveGregory]

I looked at 3 roof failures from sliding snow impact 2 years ago in Virginia. They were all schools. In each case, a pitched metal roof for a gymnasium dropped snow/ice 8' to 10' onto a lower roof.

 

[cvg]

I have direct experience with this. Standing on a redwood deck at a mountain cabin. Poked at a drift / ice dam on the edge of the roof. A large chunk broke loose probably 8 feet x 8 feet. Landed on the deck and broke through several 2 x 12 deck boards. If I'm not mistaken they were supported by joists at approximately 24" OC.