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Snow Load on RTU?

Snow Load on RTU?

Snow Load on RTU?

Should I be adding snow load to the top of a roof top unit?  If so, do I add the minimum roof live load (30psf) or the flat roof snow load (~20psf)?  My RTU is covering 460 sf of the roof, at 30psf, that's an extra 14 kips I need to account for.  

Engineers I know seem to have different views on this, what do you think?

RE: Snow Load on RTU?

If it's actually covering the roof, meaning keeping the snow off the roof itself, then add snow load to the RTU and not the roof, to ensure you account for point loads from the RTU.  No need to double dip.  Make sure though that snow can't get under the RTU, and that the RTU is a permanent thing.

RE: Snow Load on RTU?

I do not know what the minimum roof live load is for, but it definitely should be designed for if required by your local code.

Additionally, if the RTU has any parapets, which I doubt, don't forget to consider drifting of the snow which could locally exceed the 30 psf live load value.  

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Snow Load on RTU?

You should consider snow accumulation on top of the unit and the unit's self weight (I usually treat the unit weight as LL, but it could be considered as DL because it's weight is generally known.  If an existing condition and there is no reserve for treating it as LL, I generally use DL).  In addition, there can be an accumulation of drfiting snow adjacent to it depending on the size of the obstruction.  This latter load often exceeds the weight of the unit and causes loading to be applied to a much greater area.


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