Snow Drift - spacing of mechanical units

[WIEngr]

ASCE 7 states that drifting does not need to be considered if the projection is less than 15 feet. How close can these 15ft projections be to each other? 15 ft, 2 ft, 1/2 inch? If the projection is 13'-4" would 5ft bee OK?

Also if you lift a projection off of the roof will drit loading be negated? how high? height of snow, heigth of drift, or even higher?

 

[cap4000]

Interesting thread. The Old BOCA Code I have uses the same 15 feet but the separation used in BOCA is in the direction of the drifting and NOT along the projection. Try contacting ASCE via email. Good Luck.

 

[dik]

The Canadian code has provisions for roof obstructions... see attached spreadsheet... verify before using.

Dik

 

[B16A2]

Depends on your judgement and how forgiving your roof system is.


Lifting the projection does little to limit drift. I was just out to inspect a failed roof on a PEMB due to drift loads. Come to find they added parapets around the perimeter of the roof. Designer left a 12" tall continuous opening at the base of the parapets to "prevent drifting".

Needless to say, it doesn't prevent drifting and it brought the building down. I think any interruption in the wind deposits snow, so unless you're lifting it 6' high any lift isn't going to do squat.