Roof snow removal 16

[mijowe]

I have been ask by an owner for the number of inches of snow fallen on a roof that should trigger its removal. This is for a building that I designed and was recently completed.

The simple answer is to provide the snow load I designed for, in psf not inches. I feel like I have not exposed myself to more liability if I define what I designed for. Without getting compensated I do nto want to make a recommendation for their snow removal plan, and I also do nto want to provide a conservative number that would have them cleaning off the roof more that necessary (it is a fairly large roof).

I have never been asked for this and am curious how others address the question.

 

[Ron]

mijowe....you are correct to limit your statements to just your design parameters. Yes, you can back-calculate into a depth of snow; however, when you do that you are taking the liability of determining the density of the snow, which can vary quite a bit. Stay with your initial gut feel!

 

[DaveAtkins]

A properly designed roof should not require snow removal--am I missing something?

DaveAtkins

 

[msquared48]

I would tell the owner, as stated above, that the roof has been designed for the locally required snow load of XXX psf. As the density of the snow that accumulates will vary, so will the depth to achieve that maximum design load. The local weather is something that you cannot accurately predict, as it involves the intermixing of the effects of rain, snow, ice and wind. As the homeowner, learn to read the weather conditions and use your judgment.

Mike McCann, PE, SE (WA)

 

[Brad805]

It is up to the owner to determine the snow/water equivalent. You can tell them the inches of water, but that is about all you can do. They can use some sort of ABS tube to extract cores and use the microwave if they are so inclined to figure out the actual snow load.

 

[msquared48]

Ideally, he would take the sample (after obtaining a lot o9f Medical insurance) away from eaves, valleys and parapets where the depth would be greater.

However, giving the owner too much information would increase your liability here too. Stupid people do happen.

Mike McCann, PE, SE (WA)

 

[KootK]

I'm going rogue on this one. Your design PSF is based on a code specified snow density. I think that you can specify a snow depth, in conjunction with the assumed density, without taking on any meaningful, additional liability. If I'd just paid ___ million for a new facility and the designer couldn't tell me when to shovel the damn roof, I'd be some annoyed.

Owner <> meteorologist. Poor guy's just trying to do the right thing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[allgoodnamestaken]

Discuss the issue with the owner and explain exactly as has been done here and ask him if you'd like him to get a more or less conservative answer.

I agree with KootK that if you just ignore his request for snow depth outright, he'll be really annoyed. Explain the difficulty in determining snow density and how it varies with temperature, and perhaps give him a range depending on light snow vs dense snow.

 

[mijowe]

Fair enough, but is what I designed the roof for the same thing as when he should shovel the roof? As David pointed out, if the roof is designed correctly, he does not need to shovel the roof when the snow is as deep as the assumed design load. If the design load is reached he can wait for it to melt, it may only make sense to shovel the roof if an additional storm is anticipated to gain back the capacity.

 

[allgoodnamestaken]

Shoveling snow off roofs is not uncommon in cold climates that get lots of snow. My school did this.

 

[KootK]

Snow loads get factored for ULS. Surely that factor includes variation in both snow depth and density. As such, if you specified a depth based on the unfactored snow load, I would think that you'd be conservative but not excessively so. You'd be incorporating variation in both depth and density when, technically, there would no longer be a need to account for depth variation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I prefer not to see properly designed roofs shovelled. Lay people seem to think it acceptable to shovel interior bay snow out into giant exterior bay heaps prior to shovelling it off of the roof altogether.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

Sorry, but I cannot agree here.

To me the request is no different than asking under what wind speed or magnitude of earthquake the structure will fail. Realistically, who knows? To answer with any value is to take additional risk you are not being paid for, and do not have to assume.

Mike McCann, PE, SE (WA)

 

[ergus]

This is an increasingly worrying problem here in B.C. with global warming. In each succeeding year the density of snow is getting higher. The building code is based on 3 kN/M3 (20 pcf) but we are measuring 4.5 kN/M3 (30 pcf) when we are investigating roof collapses. Snow used to blow off the roofs around here, with the design snow load for residential being 55% of the ground snow load. Now I find with all the Freeze/Thaw conditions we have the snow dose not blow off and actual loads on the roofs that collapse are very close to the ground snow load. 20 Years ago when people asked me when they shoud start shoveling I would say when it reaches 3 feet, now I say when it reaches 2 feet

 

[Ron]

Well said, Mike.

 

[wannabeSE]

Mike,
The section 1603 of the IBC requires the design wind speed and seismic accelerations to be on the construction documents. (earthquake magnitude does not correlate well to seismic loads on the structure).

 

[Triangled]

I was asked this some time ago......
code prescribed snow ground loads are based upon statistics based upon lots of data, as I understand it, and it seems reasonable that it should be so. I think there is something like a 2% probability of code snow loads being exceeded in a 50 year period.....I don't have my books before me, just a nice West Coast IPA...
Considering that a Winter season..3 months, 4 months, 5 months.. may include heavy snows, rain, freeze, sleet, freeze, partial melt and re freeze, snow, light and heavy ..... an 18" deep appearance of snow on a roof one season may equal the weight of a 36" appearance of snow on the same roof in another season.
Prescribing a particular allowable depth of snow in linear dimensions to the client is.... not accurate.

 

[JAE]

an 18" deep appearance of snow on a roof one season may equal the weight of a 36" appearance of snow on the same roof in another season.
Prescribing a particular allowable depth of snow in linear dimensions to the client is.... not accurate.

I agree 100% with this.

The depth of snow isn't a good way to determine if there is a problem.

We had a major series of snowfalls several years ago - a series - where each layer of snow began to partially melt, get dense, then get covered with another layer...then melt a bit more, etc.
The snow density, as measured on several roofs, was very high, much higher than that assumed by the code.

So the depth was below that assumed by the code defined snow load but heavier than the code load.

The only thing I'd suggest, for the owner here, is to tell them the design psf, then perhaps offer a range of approximate depths where the code safety factor is reached.
Then educate the owner further about how these densities can vary and if there's any question about too much snow, a quick study should be done on the roof to see what is truly up there.

In other words, the owner should hire an engineer to assess the situation, not depend on a fantasy snow depth.

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[LittleInch]

Why not give him the snow load you've designed for and then tell him to go buy something like this.

http://www.hydrologicalusa.com/product/meteorology/snow-measurement/ssr

Just like the inuit have (apparently) many different words for snow, so the depth of snow may be very different for the same weight. Tell him your roof is designed to handle a certain amount of weight, not snow depth.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[CANPRO]

We saw a lot of over loaded structures in our area last winter and snow/ice removal was common – in many cases the layer of ice exceeded the design loads. We were asked many times to evaluate a roof that had deflected to the point that it damaged interior finishes. We provided our opinions on the state of the structure (taking into consideration the weight of snow/ice on the roof, age, condition, location etc…), which ranged from “well within expected deflections” to “this building is no longer safe for occupancy”. We provided recommendations for snow removal – do not create unbalanced conditions, do not stock pile the snow on the roof etc… and we also provided recommended depths for removal. I always provided the local design load, the load at which we recommend snow is removed (again this value took into consideration age, condition, forecast etc…) and I also provided a corresponding thickness of ice and typical snow depth – this was always followed by a very clear note that the density of snow can vary greatly and MUST be confirmed. I always assumed a wet and heavy snow when determining the depth, but made it clear that the actual weight of snow must be monitored.

I don’t think there is anything wrong with providing them a depth of snow/ice as long as you make it clear that there is more to the story than just the depth of snow/ice.