Snow Drift with Parapet at High Roof. 1

[tmlim]

There's some debate going on here at our offices.

High Roof and Low Roof. Height difference approx. 16ft. There is a parapet at the high roof. At what height of parapet can you ignore the snow drift??

Thoughts??

BTW, I'm going by Canada Building Codes (Alberta to be specific)

 

[msquared48]

I would think that a parapet at the high roof would only affect drifting at the high roof, not the lower one with the 16 foot difference in height.



Mike McCann
MMC Engineering

 

[tmlim]

msquared48,

Agreed, but what if the parapet is only 12", there would be drift onto the lower roof, but not at say 72". What is the limit??

I was under the impression it had to do with the depth of the snow that can accumulate on the roof.

 

[SKIAK]

I don't see how the height of the parapet would prevent a drift from occuring.

My understanding of snow drifts is the lower velocity behind the parapet/wall allows the snow from the higher roof to drop out. I don't see how a taller parapet would prevent this situation. There will also be a windward drift but I would think that the majority of snow would still go up and over (assuming it has nowhere else to go) and onto the lower roof.

I'm going by ASCE, BTW.

 

[tmlim]

SKIAK,

You say "up and over" but say the parapet is, to be extreme, 20ft. high. How is the snow to go up and over that??

Perhaps I'm misunderstanding what you are saying.

The Canadian code says "The Magnitude of the drift on the lower roof depends primarily on the amount of snow that can drift off the upper roof and be trapped in the step." Now if the snow is already existing behind the parapet, how can it lift up and over a really high parapet??

 

[SKIAK]

Well... I don't have a good answer for that.

If you have 20'-0" parapet around the entire perimeter of the upper roof I think you will still have the potential for significant winds inside that area. Enough to pick up snow and blow it downwind? It depends... snow is light.

Odviously if your roof is 2'-0"x2'-0" for 4 sf with 20'-0" parapet (looking to the extremes) you would have very little chance for snow getting out. The more open the upper roof becomes, the more likely you are to have higher winds.

I've never had parapets very high so I ususally just use the upper roof area and the height from the lower roof to the top of the parapet and call it good.

The ability of snow to get over the parapet will probably depend on the dimensions of the roof. Small roof with high parapet should not have as high of a drift.

 

[strguy11]

Wouldn't you have to look at a windward drift on the lower roof, blowing up against the sidewalk going up to the high roof??? I don't think you can ignore this direction, but I can see an argument for ignoring a leeward drift coming across the high roof and onto the lower one

 

[ajh1]

Commentary G paragraph 38 of the User's Guide - NBC 2005 Structural Commentaries provides a formula to use to determine the effect of the parapet. The criteria appears to be a function of both the parapet height and the basic snow load.

 

[tmlim]

ajh1,

Yes Paragraph 38 does have a parameter for parapet height under equation (4). But it states that F is a min value of 2. Therefore the parapet height no longer has any effect. In my case by doing the calcs, a parapet with a height of 5.7ft or higher will have the same effect. So essentially it's saying that a parapet at 5.7ft will do the same as a parapet at 20ft??

 

[BAretired]

The area of the upper roof is another important variable. If the upper roof area is very large, the volume of snow available to drift and hence the parapet height required to reduce F to a value 2.0 is considerably more.

BA

 

[tmlim]

BAretired,

Agreed, and that's what the characteristic length does. But even if you have a small characteristic length, there has to be a point where the parapet doesn't allow any drift. But with the calcs and F being a min. of 2, it doesn't allow that.

 

[BAretired]

tmlim,

Perhaps the parapet becomes a roof obstruction as illustrated in Fig. G-8. You would still have a triangular distribution of snow on the leeward side of the parapet.



BA

 

[JAE]

You have both leeward and windward snow drifting conditions and an upper parapet does little to affect that. Even with a 20 ft parapet snow will drift onto the lower roof.

For leeward snow - the snow doesn't blow sideways across the high roof and drop vertically down to the low roof. Rather, there are upward wind flows on the overall building that keep high roof snow from settling onto the high roof. The snow turbulence eventually results in the upper roof snow settling around the building. It's really not a linear effect the way we would normally envision it.

Windward snow blows across the low roof and piles up against the wall - sometimes both occur during the same event.

(good job by the way - using ridiculous extremes - a 20 ft parapet- is a way we many times help ourselves to visualize difficult problems).

 

[tmlim]

Thanks for everyone's input. It all makes more sense to me now!

 

[geoffdale]

I believe you can max out the snow load at 3X ground snow.