Roof snow load residential projects

[DoubleStud]

Every building department here uses different guidelines on how to determine roof snow load. Some give you the roof snow load, some give you the ground snow load and you go ahead and add the appropriate factors. Some counties specifically tell you to use ground snow load with no reduction. Some counties give you an equation based on elevation to get the roof snow load (not ground snow load). You will end up getting roof snow load that is very different sometimes even though it is only a few miles a way and at same elevation.

There is one county in particular that requires us to use ASCE Hazard Tool. My question is regarding the Roof Slope Factor Cs. Often they install snow guard or snow fence on the top of the roof to prevent snow sliding down. Do you think I should basically ignore the Cs factor (follow the the solid line on the graph)? Just wondering why some building departments tell us to use flat roof snow load no matter what.

https://www.rockymountainsnowguards.com/snow-guards

 

[Eng16080]

Is the graph that you refer to Figure 7.4-1 of ASCE 7? If so, the dashed line is for a slippery, unobstructed roof surface, and the solid line is for "All other surfaces." In your case, if you are using a Cs factor, then you should be using the solid line because even if your roof surface is slippery the snow guards will obstruct the snow from falling off.

But what you're asking is a little confusing because if you were to ignore the Cs factor, then I think you would just use Cs = 1.0. You would not use Figure 7.4-1 at all. Ignoring the Cs factor would in my mind be the same as saying that the roof is flat.

 

[lexpatrie]

What state, by the way, Colorado? I take it you're not under the Pikes Peak Regional Building Code? Montana?

ASCE Cs (sloped roof correction factor) is at most 1.0 so ignoring it will produce higher loads on sloped roofs than might normally "occur". I.e. it's conservative.