RTU screens with gap below - Height / clearance where i dont need to consider snow drift 1

[efFeb]

Hi!
I am designing a roof with an RTU screen at a mechanical area.
I am wondering if there is a clause in the NBCC (or ASCE) that tells me how much clearance i need between the bottom of my screen and my roof to ensure that snow drift will not occur at the screen? Is it as simple as calculating the depth of the drift that would occur if this clearance werent provided?
Thanks!

 

[JAE]

I like to use 1.5 x flat roof snow load. If the typical snow can't "pile up" on the screen to begin with, drifting theoretically won't occur.

 

[andrews2]

ASCE 7-16 Section 7.8 has this exception:

Note the bottom of the opening has to be 2 ft above the balanced snow load, not the roof itself, so the higher your flat roof snow load, the higher clearance that is required to ignore drift.