Snow load on adjacent canopies

[vmirat]

I am designing a new 30'x30' hip roof canopy to be built next to an existing 20'x20' hip roof canopy. Both canopies have a 12 foot eave height. This is in Colorado, so snow load is a big factor. The new canopy eave will be 1.5 feet away from the existing canopy eave. The new canopy will be standing seam metal with a 6 in 12 pitch and snow clips on the eaves to prevent sliding. The existing canopy is canvas fabric with the same slope.

Is 1.5 feet enough of a separation between these canopies or should I worry about bridging which would then create drifting? The snow clips on the new roof will prevent sliding snow. I thought of using the balanced snow load depth as separation distance, which would be 14 inches. But then I thought I may have to use the "Unbalanced Other" in ASCE 7, Figure 7-5, which would be 2.3 feet.

Since the canopy eaves are at the same elevation, can I ignore drifting onto an adjacent building per ASCE 7? The value of 6h would be zero, so "s" would be greater than that.

 

[RHTPE]

While your upwind fetch distance for the existing has not changed (wind blowing toward the existing with the new canopy on the leeward side), turn the wind in the opposite direction and the new canopy will give a longer fetch distance for the "pocket" formed between the 2 canopies. That pocket may tend to collect more drifting snow than would the leeward side of either canopy alone, and could add to the load on the existing canoppy. Bridging of drifts may be possible unless the separation distance is large enough to insure it never happens. I live in New England so I have not seen the variations of snow events in Colorado, but I have seen some freaky things occur here. Some of our blizzards have formed drifts on the leeward side of gable roofs that defy explanation (i.e. 2' cantilever of drifted snow beyond the edge of the eave). While not commonplace, it can occur.

It's all in the probabilities of what might occur in any given snow event. I think I would opt for as much separation as possible (say 3-4') and would recheck the unbalanced condition on the existing assuming a larger drift depth than ASCE 7 would suggest. While ASCE 7 may judge drifting onto adjacent building by eave height, your overall ridge (peak) height on the new canopy will be higher than the existing. My gut says there will be some contribution to drifts on the existing.


Ralph
Structures Consulting
Northeast USA

 

[ajk1]

See the National Building Code of Canada User's Guide. Need 15 foot separation for the case of low roof adjacent to higher roof, to avoid snow drifting on the lower roof. Not sure what it would be for you condition though. Not sure what is ion the latest ASCE.