snow drift at penthouse
 

snow drift at penthouse
does anyone have any references for determining the height that roof top equipment or a penthouse has to be above the roof( up on posts with a space below) to prevent any additional load from snow drift.
our best guess here in the boston area is one foot, but i would like to confirm this.
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no such rule exists. the structure above the roof will block/slow the wind and cause the snow to fall at that location. a two meter horizontal separation for low rise buildings is used for no drift impact from adjoining buildings.
there is no code or technical reference that i am aware of. it is our obsevation that as long as you leave enough clearance under the equipment, the snow will blow under the equipment instead of creating a drift. we, generally, use the base snow load depth plus am little extra (6" to 12").