Snow Guards on slippery metal roofs and Decks

[Said the Sky]

Hello all,(and specifically Canadian engineers!)

say we have a gabled roof house and it is sloped approximately 1:3 pitch with a metal roof. The slope direction corresponds to a deck on one side and on the other side just open field. So my question is, is the deck considered a lower roof and needs to be designed for snow drift and or sliding? if so is that a safety hazard and thus require snow guards? With snow guards installed I will analyze it as a projection in the roof and snow drifting would occur similar to if it was a parapet at the eave.

however I am looking at commentary G for NBCC, and it notes "where a parapet or other effective means are incorporated into the design of the upper roof to retain the snow and not allow it to slide to the lower roof, the additional sliding load on the lower roof need not be considered. Retention effectiveness will depend on design approach. Eg. Snow guards, parapet walls"

I guess my question is: what determines where a snow guard or parapet is required. And can a deck be considered a lower "roof". Logically speaking if it was me I would install a snow guard or parapet, however I am looking for something in the code that strictly tells me when I would need it or not.

thanks!

 

[phamENG]

In US residential codes, deck framing span tables are for Ground Snow Load/Live Load...and given that the snow doesn't care what it's falling onto...I'd say designing it as a lower roof for falling snow or snow drifting would be prudent. But I can't point you to a specific code reference...just good engineering judgement.

KootK went into snow loading on decks a while back, though his was complicated with a canopy. He can probably provide some insights. thread507-484307

 

[ryaneng]

Snow loads apply to a roof or any other building surface subject to snow accumulation (4.1.6.1). However, this sounds like a residential building so I believe you would be justified in using the simplified Part 9 snow loading requirements which does not include drifting and sliding.

In terms of whether an upper roof needs snow guards, I've only ever seen this specified by the designer or architect, and I cannot find a code requirement for them.

 

[jayrod12]

I've always still accounted for drift loads in residential where I feel that build-up could be a concern. However, in doing so, I usually use a Cb=0.55 which is essentially what the simplified version in part 9 is doing anyway so the drifts are manageably less.

Regarding the sliding snow, I would 100% be putting up snow fences. We put a metal roof on the cabin at the lake, the one side slipes towards the deck, it most definitely was depositing sliding snow onto the deck. So I'd recommend installing them. I don't feel as though they would drastically change the snow load on the roof when compared to the design loads. At least I haven't seen evidence of that so far.

 

[TLHS]

That commentary clause isn't requiring a guard, it's giving an option. Basically, you're allowed to neglect the snow drift onto the lower area if you stop it from going there, but then you have to account for the additional snow at the upper level.

If you're in a situation where the client doesn't want drift or falling snow from the roof you can put up a variety of things ranging from a tall parapet to a minimal ice rake. I personally haven't seen a code requirement, but canopies over doors along with ice rakes are pretty typical on institutional and industrial structures in high snow areas to avoid the dump you're talking about.

 

[dik]

I've often use Zaleski snow guards on sloped metal roofs to reduce the chances of snow/ice sliding off... I often used them at entrances and exits only, where people might be walking. There are other brands.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Said the Sky]

thanks folks, I just need to convince the owner to get them, because it is not "code" its harder to make them get it... trying to explain to them the potential liability and damages.

 

[dik]

It's his nickel... make sure that he is the owner of the decision.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[jayrod12]

He can always pay to have them installed later once he spends a winter with a giant ice pile where that roof dumps everything.

That's why they got installed at the lake, figured they weren't required, spent a winter with a giant block of ice on the deck. Took forever to melt. Snow fences went up next summer.

 

[Brad805]

If you have a high quality roof package that does not ice, the roof guards are fine. If you have a low end roof that ices the roof rakes typically fall off.

 

[dik]

Even snow can slide off, it gets more dense over time. Sloped metal roofs are slippery. Best to have him own the decision, but make sure that it's his nickel. Also, let him know that ice guards are not a panacea... you don't want to guarantee a result.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik