Snow loads on roofs

[rmayo]

Neal

Dry snow will weigh as little as 3 lbs/cu ft and wet saturated snow can weigh about 21 lbs/cu ft. Depending where you live building codes prescribe the minimum design snow load. In New Jersey the design snow load is about 20 lbs/cu ft so a new house built to code will be at its upper design load with 12" of saturated snow. If its an older house built before codes, without codes or just poor construction roof collapse is highly likely. Atlanta is in a 5 lb/cu ft snow load zone so if they get that freak 12" wet snow they will be 4 times over the design load. Typically roof failure due to snow loads starts at the weakest link. That may be a bad rafter with a knot, or a bad joint connection, over spanned rafter,etc. Once one rafter or truss fails then its load is transfered to the adjacent rafters or trusses and then a chain reaction of failure begins.

[gtblum]

I'm wondering if years of smaller ice dams have slowly deteriorated rafter tails and top plates to where they finally let go.

Which brings up the question...What's the dominant failure mode with excessive snow on the roof? Rafter spread? Rafter failure?

Marc

I'm going to do a little digging to try to find that out. Numbers are a great guideline. Not a perfect answer though. I'd be real surprised to find out the older homes here were built any different than in NJ.

We've only had 170" so far this year. Syracuse has only had 140".

If this was as simple as roofs being underbuilt, this area would've been leveled long before we were born or, every place would be an A frame.

[jfmann]

Density of snow can vary greatly. The governing code for determination of building loads (ASCE 7) includes a basic equation (7-3) for density of snow (in pounds per cubic foot)....[ 0.13 times "ground snow load" + 14 (not to exceed 30 pcf) ]......so that, if ground snow load (per standard snow map) is 20 psf, snow density is 16.6 pcf ........and 12 inches (one foot) of snow weighs 16.6 psf.

In New Jersey......ground snow load varies from 20 psf to 50 psf per standard map in the NJ UCC (Bulletin 94-.

Failure from snow is more likely due to inadequate connections at low end of rafters........than by failure of the rafters in bending.

For standard rafter construction.........outward thrust force at low end of rafters can be quite substantial........especially for low-slope roof (4 on 12 or less). Typical nailed connections between low end of rafters and attic floor joists can and do fail .........especially if there has been decay of wood due to roof leaks.

New article (with diagrams) about gable roof framing will soon be on structural101.

[Marc]

Density of snow can vary greatly. The governing code for determination of building loads (ASCE 7) includes a basic equation (7-3) for density of snow (in pounds per cubic foot)....[ 0.13 times "ground snow load" + 14 (not to exceed 30 pcf) ]......so that, if ground snow load (per standard snow map) is 20 psf, snow density is 16.6 pcf ........and 12 inches (one foot) of snow weighs 16.6 psf.

But snow density varies along the vertical height of the snow doesn't it? At what point would the 16.6 pcf figure apply?

Marc

[gtblum]

Typical nailed connections between low end of rafters and attic floor joists can and do fail .........especially if there has been decay of wood due to roof leaks.

And the fasteners?

The one thing that seems to be different around here from years past, has been the size of the ice dams. They were huge.

I say were because we just had a few days where it was warm enough to either get rid of them all together, or at least bring them way down from where they were.

That seems to be a pretty good argument for using Ice and Water for at least six foot around here.

[jfmann]

Increase of snow density with depth of snow is taken care of (in ASCE equation).......indirectly.........by using ground snow load as a proxy for snow depth on a roof.

More important is the fact that "wet" snow is much more dense / heavy than dry snow....which is not entirely reflected in the ASCE equation.

The following quote from a July 2000 research paper (Density of Freshly Fallen Snow in Rocky Mountains is interesting;

"From studies that have been published, the density

of freshly fallen snow in the United States varies from

10 to about 350 kg m#8722;3 (Diamond and Lowry 1954;

Wilson 1955; LaChapelle 1962; Judson 1965; Grant

and Rhea 1974; McGurk et al. 1988; Doesken and

Judson 1997). Other pertinent contributions to the literature

came from Oda and Kudo (1941), Bossolasco

(1954), Power et al. (1964), Gunn (1965), Stashko

(1976), and Meister (1986). Most of these publications

are not in mainstream meteorological literature, and

the results have not been widely distributed among

meteorologists. For this reason, and because of the

general dependence of operational meteorologists on

gauge data, snow density has not been available to

most operational forecasters."

[jfmann]

Another reason for roof failures due to recent snow may be the occurrence of snow drifts on the roof.............which may not have occurred before, at least to the current extent.

A roof should be designed for "unbalanced" snow ........in accordance with standard code provisions (ASCE 7).......to take account of drifting potential.

[allseason]

Just a side note, DeGreco is a member of the Home Inspection Advisory Board in New Jersey, the people who watch over all of the licensed inspectors in NJ.

[Neal Lewis]

Just becuase he's on the HIACommittee, doesn't mean he knows what he's talking about. I've seen that first hand.

[allseason]

I'm not refering to his abilities,I just thought anyone not familiar with who he is might be interested.

[Denray]

This photo was taken 3-25. I lived here the last 2.5 years of H.S. About a week after I moved there a big barn like building had its roof collapse about 8 hrs before a school sponcered dance/party was going to be there.

Click to Enlarge

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[hausdok]

Hi,

Yeah, I think we are probably mostly all pretty familiar with DelGreco. He's constantly releasing press releases or inspection articles via eZine Articles or PR Web. These are grab bags for internet content where folks can pick them up for free if they are a subscriber to the grab bag, or pay a modest user fee and anyone googling "home inspection" sees them. Each article drops the name of his company onto sites hungry for content all over the country. I've seen his articles picked up by publications in the southwest where half of what he talked about only applied to snow country or New Jersey but it apparently provided them with content that they needed - either that or they never bothered to read the articles.

All that writing has managed to get him categorized as an inspection "expert" by these sites. I'm sure it is good for business and helps to get him expert witness work. Those of you who are looking for a way to get things going a little better could take a page from his book, start knocking stuff out and submitting it to these grab bags. It doesn't cost you anything and it might help you out. Just realize, though, that it probably won't earn you props in here, especially if what you are putting out is the same caliber as some of the stuff that DelGreco's been generating.

Whatever works though; it's not like home inspectors have a whole lot of options when it comes to affordable advertising media.

ONE TEAM - ONE FIGHT!!!

Mike