Snow drift displacement

[Catacus]

Any concerns with building up a triangular obstuction on a roof adjacent to a higher roof or parapet to displace the code required snow drift load?

 

[ToadJones]

I'm not too sure it would eliminate the drift snow load.

 

[Catacus]

Any reasoning for it Toad? See attached for a bit more info.

 

[ToadJones]

I just don't think the drift load caused by the wind blowing opposite of the way you are showing will be eliminated.
In situations like this, I have superimposed the drift load on the slope.
Might be a more of a matter of interpretation of the code than it is a fact.

 

[ToadJones]

in other words, I think you still have a "windward step" as shown in figure 7-7 of ASCE 7-05 (if this is your governing code). I suspect the parapet and the shape of the roof will still cause snow to gather in this area.
Keep in mind, snow can easily build up on the leeward side of a low-pitched gable roof as in unbalanced snow loads.

 

[paddingtongreen]

I have used this myself on an existing building where we were increasing the snow load because we were putting up an adjacent, long, higher building where the drift would come from the wind, scouring the snow off the high building and dropping it on the lower. It was easier to put up a false roof than strengthen the existing steel roofing and framing.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Catacus]

Yes, ASCE 7-05, and yes a windward step, but the point is that the snow that composes the drift is replaced by a "light weight" dummy drift. The wind does not "know" whether it is snow under the drift or a cavity filled with air, at least that is the idea.

 

[ToadJones]

I guess i can see it both ways to some degree.
I myself, have a had a few occasions where I had one building with a shed roof abutting a higher building on a gable end. The lower shed had a slope but I guess I felt it was not effective in eliminating the drift in my case.
I suppose in your case, you have essentially eliminated "Pd" in figure 7-8.
Sorry to waste your time.

 

[Galambos]

I believe the asce guuide to snow loads makes a comment about this situation.

 

[Catacus]

You are correct Galambos, thank you. It appears to be an acceptable way to go (if done correctly of course). Will feel more comfortable with it now. Catacus

 

[BAretired]

I agree with your suggestion of using a triangular void and have used the idea myself upon occasion.

BA

 

[JAE]

Just be sure to verify that the weight of the new "false" drift structure is OK when added to standard Pf snow roof loading.