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Snow drift

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One approach with new Roof Top Units (RTU's) is to elevate them so that the wind can blow underneath, thereby eliminating the creation of a snow drift on existing structure. If your new structure is truly "open" so that you will have limited wind stagnation at the leeward edge (towards the existing structure), you could argue that a leeward drift will not form on the existing roof. The next question is how much elevation difference you have. In "Snow Loads, A Guide to the Use and Understanding of the Snow Load Provisions of the ASCE7-02", the authors recommend the underside of the RTU be elevated the roof snow depth plus 2 feet above the roof to prevent drifting. If you are going this route, I think adding a parapet to the new structure would defeat the purpose of trying to prevent wind stagnation.

Dmitri Wright, PE
Cascade Engineering, Inc.
2459 SE TV Hwy, PMB #202
Hillsboro, OR  97123-7919

Subject: Snow drift
From: "Wesley Werner" <wwerner(--nospam--at)>
To: <seaint(--nospam--at)>

   I am doing a preliminary design for an open structure next to an
existing building. The new structure will be higher than part of the
existing building. I don't want to surcharge the building with snow
drift from the new roof. If I put a parapet around the new roof that is
higher than the calculated windward drift height, do I have to consider
snow drift on the low roof? There won't be a windward drift from the
length of the low roof because the new structure doesn't have any sides
for the snow to pile up against. My only concern is with snow blowing
off the high roof.
Wesley C. Werner, P.E.=20
Phone (717) 632-7722=20
Fax (717) 632-8665=20

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