Can snow cause damage to a flat roof?

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Can snow cause damage to a flat roof?

Postby paulsiu » Tue Feb 09, 2010 9:07 am

With all of the snow that the East has been having, this is probably one of the worse I have seen in terms of amount of snow at one time. I was wondering if the roof can be damage or even collapse from all of this weight?

Thank goodness the roof was replaced last year and all of the gutter cleaned.

Paul
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Postby SpringMan » Tue Feb 09, 2010 9:26 am

Yes, indeed, all that weight can cause a collapsed roof. In the north you see steep pitched roofs because they shed the snow better. Some folks actually shovel their roofs when the snow gets real deep. Good luck.
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Postby jamacq » Tue Feb 09, 2010 9:26 am

Yes. Here's some news from DC area from the storm we are calling "Snowmageddon".

http://firelink.monster.com/news/articl ... w-to-blame

Jeff
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Postby DaveS » Tue Feb 09, 2010 9:35 am

Most building codes say a roof has to hold with up to three feet of snow. Beyond that get a ladder and shovel and take some weight off. Or if we are talking about a parking garage, park on the street. Dave
PS Most insurance policies exclude collapse of structures from coverage.
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Postby 3CT_Paddler » Tue Feb 09, 2010 1:10 pm

Depending on where you are in the US, engineers design building for a certain design snow load. So yes in places that typically don't see much snow, it can be a building hazard.
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Postby FrugalInvestor » Tue Feb 09, 2010 1:53 pm

DaveS wrote:Most building codes say a roof has to hold with up to three feet of snow. Beyond that get a ladder and shovel and take some weight off. Or if we are talking about a parking garage, park on the street. Dave
PS Most insurance policies exclude collapse of structures from coverage.


This may or may not be true depending upon where you live and the building code in the area. Codes typically specify a particular minimum lbs./s.f. rating which would roughly translate to a snow depth depending upon water content and other factors.

However, flat roofs are notorious for not draining well and water is heavy. If you have a pool of water on the roof it can add a lot of weight in one area. This together with the snow can overload a roof and cause it to sag or collapse. This is more likely in a commercial structure than in a house because of the long, unsupported roof spans in commercial buildings (i.e. few interior walls) but can still be in issue in a home.

If you have a LOT of snow on a flat roof...much more than a normal winter would bring...and especially if it is wet snow, it's a good idea to shovel the roof off or have it shoveled.
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Postby chaz » Tue Feb 09, 2010 2:00 pm

Paul, anything can smash a roof if it weighs enough, e.g. a meteor.
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Postby SpringMan » Tue Feb 09, 2010 2:17 pm

chaz wrote:Paul, anything can smash a roof if it weighs enough, e.g. a meteor.

Chaz,
You are correct with a bad example. Meteors burn up before landing, I think you mean meteorite. Sorry for getting technical.
http://www.mentalfloss.com/difference/m ... meteoroid/
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Postby chaz » Tue Feb 09, 2010 3:10 pm

SpringMan wrote:
chaz wrote:Paul, anything can smash a roof if it weighs enough, e.g. a meteor.

Chaz,
You are correct with a bad example. Meteors burn up before landing, I think you mean meteorite. Sorry for getting technical.
http://www.mentalfloss.com/difference/m ... meteoroid/


Thanks for getting technical.
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Postby VictoriaF » Tue Feb 09, 2010 3:22 pm

Washington Post wrote:WASHINGTON, DC -- Washington DC Fire Chief Dennis Rubin is warning residents from trying to clear heavy snow off rooftops in wake of several collapses caused by overwhelming amounts of snow.

"We think that would be a design for disaster," said Rubin when asked if residents should take action to prevent more collapses.

Rubin said the risk of life threatening falls and other serious mishaps are too high.

Instead, Mayor Adrian Fenty says residents who are concerned about the possibility of structural failure should stay with relatives or check into one of the city's "warming centers" until the danger has passed.


My take: Clearing one's roof is the matter of probabilities and consequences. The probabilities of the roof collapsing and of one falling off the slippery roof are unknown and are roughly equal. The consequences of falling from the roof are more dire than those of the roof collapsing and are exaggerated by the difficulties of getting an ambulance through snow-piled streets, understaffed hospitals, and "competing" for hospital resources with other emergency victims.

Thus, it is better to leave roof clearing for later.

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Postby btenny » Tue Feb 09, 2010 3:59 pm

The school building around here (Sierra Nevada moutains) have huge signs posted on them telling everyone how much load each building can hold. Each is labeled 56-65 pounds per sq ft. as I remember. This means that if we get a big snow fall on top of standing snow they have to shovel the building roofs before school can be started. This happens every few years. Similarly we get houses and garages that collapse after these big storms. The last time I remember this happening was 2002 I think. The big K Mart store roof collapsed in Reno. The store closed and never reopened. The same thing happens in places like Salt Lake and Denver.

So yes you need to shovel your roof if you get too much snow.

Also watch out for snow cornices and big ice cycles that can fall on people or cars. Also be especially watchful of metal roofed buildings. These things shed snow in BIG amounts all at once and crush cars parked next to them.

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Postby 3CT_Paddler » Tue Feb 09, 2010 4:19 pm

How about Option C for your roof covered in snow... pay the neighbor kid down the street to clean your roof :) . I think those with the most to worry about are commercial building owners with flat roofs. In those cases I would be clearing off the roof or paying someone to do it if there was a significant snow load and the building was very old.
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Postby grabiner » Tue Feb 09, 2010 5:21 pm

DaveS wrote:Most building codes say a roof has to hold with up to three feet of snow. Beyond that get a ladder and shovel and take some weight off. Or if we are talking about a parking garage, park on the street.


We may have some falling roofs in Washington, then. Most of the area got 30 inches of snow over the weekend, very little melt, and another 10-20 forecast for today and tomorrow.
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Postby TJAJ9 » Tue Feb 09, 2010 5:26 pm

We got a lot of snow in Philly. There has been a lot of melting, though -- probably at least half of it. We're supposed to get another 1-2 feet tonight and tomorrow.
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Postby VictoriaF » Tue Feb 09, 2010 5:40 pm

TJAJ9 wrote:We got a lot of snow in Philly. There has been a lot of melting, though -- probably at least half of it. We're supposed to get another 1-2 feet tonight and tomorrow.

The problem with melting snow is that not all of it is drained and a good part of it is refrozen over night. The same 2-feet pile is heavier if it is a mix of snow and ice.

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Postby alec » Tue Feb 09, 2010 6:13 pm

For all those people in the mid-atlantic, the roof to our local Safeway food store here in Gambrills, MD collapsed yesterday. I shoveled off our lower roof just in case.

btw - it's snowing here again. :x Schools are closed until next Tuesday.
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Snow Weight

Postby TT » Tue Feb 09, 2010 7:25 pm

The average cubic foot of snow weighs in at about 15lbs. if it is not drifting and packed or rained on or very wet snow which in all those cases it can weigh considerably more.
So if you have a 1500 square ft. ranch with one foot of snow on the entire roof it would weigh approximately 22,500 lbs.
That is why in New England we have steep roofs and snow rakes.
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Postby SpringMan » Tue Feb 09, 2010 7:55 pm

Roof rakes work well for preventing ice damming. They also remove some of the snow's weight. More appropriate for pitched roofs than flat roofs though. Link has a short video of how they work.
http://www.minnsnowta.com/
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Re: Snow Weight

Postby 3CT_Paddler » Tue Feb 09, 2010 8:14 pm

TT wrote:The average cubic foot of snow weighs in at about 15lbs. if it is not drifting and packed or rained on or very wet snow which in all those cases it can weigh considerably more.
So if you have a 1500 square ft. ranch with one foot of snow on the entire roof it would weigh approximately 22,500 lbs.
That is why in New England we have steep roofs and snow rakes.


Like you said roof pitch matters. I doubt many residential homes (if they were built in the last thirty years) will have issues with caving in. Even a moderate pitch makes a big difference. I think some people may be overly fearful when the flat topped big commercial building caves in that this means their home is the next to go. Many of those are experiencing much greater stresses than a residential home due to lack of pitch.
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By all means!

Postby jwf3 » Tue Feb 09, 2010 8:17 pm

Even a pitched roof with insufficient trussing or collar ties is susceptible to damage from heavy snow load.

We rake our roof (here in New Hampshire) after every snowstorm. I would not let heavy snow accumulate on a flat roof.
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Postby schwarm » Tue Feb 09, 2010 10:13 pm

The house I grew up in had a section of roof with a shallow angle... it was tarred instead of shingled. That part of the roof was also not well insulated.
An issue we had one very cold and snowy December was that that snow started to melt from the bottom up, and we had water leakage problems until I climbed on the roof and shoveled it off.
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Postby Charleville » Wed Feb 10, 2010 12:08 am

I have got new you already know--every flat roof leaks period, snow just speeds up the process with ice dams etc.
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Postby stratton » Wed Feb 10, 2010 1:06 am

VictoriaF wrote:
TJAJ9 wrote:We got a lot of snow in Philly. There has been a lot of melting, though -- probably at least half of it. We're supposed to get another 1-2 feet tonight and tomorrow.

The problem with melting snow is that not all of it is drained and a good part of it is refrozen over night. The same 2-feet pile is heavier if it is a mix of snow and ice.

Which is when the chance of roof collapse goes up.

The typical snow pattern in Western WA (Seattle area) is it snows and then it gets warmer causing the snow to get wetter and new snow is very heavy. That's when roof collapses happen or in 1998 a lot of apartment complexes had metal car port posts bend and collapse onto cars underneath.

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Postby retcaveman » Wed Feb 10, 2010 1:28 am

I heard on the news tonight that an airport hanger roof collapsed, damaging two planes.

Here in the "frozen tundra" (Wisconsin), a lot of people have problems with ice clogging the gutters and getting under the shingles. In extreme cases, it can actually get into to wall.

You want to make sure your attic space and eaves are well ventilated and that the insulation doesn't touch the roof. Newer homes have roof and soffit vents with little styrafoam panels that keep an air space between the inside of the roof and the insulation.

Some people install electrical wires to heat the edge of the roof. In the UP (of Michigan) many homes have a foot or so of metal along the lower edge of the roof to stop ice from getting under the shingles and help the snow melt. We have "ice shield" (a rubbery sticky tar-like membrane) in all of our roof valleys under the shingles. And as previously mentioned, many people have "roof rakes" that are used (from the ground) to pull snow off of the lower edge of the roof to keep snow away from the roof edge and stop it from freezing and creeping under the shingles. Some people even take an old sock or a nylon stocking, fill it with rock salt and lay it in their gutters to keep them from freezing. I have never needed to do this and would worry it might eventually eat through the gutter.

None of that will help a flat roof however.
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Postby stratton » Wed Feb 10, 2010 1:50 am

3CT_Paddler wrote:How about Option C for your roof covered in snow... pay the neighbor kid down the street to clean your roof :) . I think those with the most to worry about are commercial building owners with flat roofs. In those cases I would be clearing off the roof or paying someone to do it if there was a significant snow load and the building was very old.

That works too. The problem is the kids can get a little too energetic and damage the shingles if they are using snow shovels. When my mom had that done it did about 8 or 10 years off the life of the roof.

Paul
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Postby stratton » Wed Feb 10, 2010 2:08 am

FrugalInvestor wrote:
DaveS wrote:Most building codes say a roof has to hold with up to three feet of snow. Beyond that get a ladder and shovel and take some weight off. Or if we are talking about a parking garage, park on the street. Dave
PS Most insurance policies exclude collapse of structures from coverage.


This may or may not be true depending upon where you live and the building code in the area. Codes typically specify a particular minimum lbs./s.f. rating which would roughly translate to a snow depth depending upon water content and other factors.

Even if building codes allow it may not be worth it to go cheap. For some reason Seattle allows 3/8" plywood roof decking. Whereas other places with heavier snow require 1/2". The problem is 3/8" decking is just asking for problems because it's so easy to damage it when someone walks on it. 1/2" decking won't have that problem.

I've even met a couple of roofers at home shows which refuse to use the 3/8" plywood because it does break so easily and it's just not worth the hassle to come back and fix it. Especially since the price difference is usually $50 or $100 difference for a roof on a new building. The later repair costs are way more...

Paul
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Postby TJAJ9 » Wed Feb 10, 2010 4:15 am

VictoriaF wrote:
TJAJ9 wrote:We got a lot of snow in Philly. There has been a lot of melting, though -- probably at least half of it. We're supposed to get another 1-2 feet tonight and tomorrow.

The problem with melting snow is that not all of it is drained and a good part of it is refrozen over night. The same 2-feet pile is heavier if it is a mix of snow and ice.

Victoria


Thankfully, we just got a new roof with a 15 year warranty. No worries here.
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Postby metalman » Wed Feb 10, 2010 12:26 pm

So far in DC area only some flat roofs on commercial buildings and a firehouse have collapsed, nobody hurt. But if it gets bad enough, anything can happen even to pitched roofs. Usually there are warning signs well in advance, like noises and bowing of walls. The problem with codes is that even if they are followed during building, roofs deteriorate with age. Walking on the roof to clean off the snow is just dumb. Guess when it is most likely to collapse?
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Postby Bob B » Wed Feb 10, 2010 3:23 pm

TJAJ9 wrote:Thankfully, we just got a new roof with a 15 year warranty. No worries here.


If we are talking roof collapse here, your new roof doesn't mean anything. It's the support structure that fails, not the "skin". In fact, it could increase the risk if the new roof was placed over the old, thereby increasing the weight load. (Unless your new roof included reinforcing the structure, in which case, never mind.)
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Postby 3CT_Paddler » Wed Feb 10, 2010 3:44 pm

metalman wrote:So far in DC area only some flat roofs on commercial buildings and a firehouse have collapsed, nobody hurt. But if it gets bad enough, anything can happen even to pitched roofs.


I agree that even pitched roofs can collapse if it gets bad enough. The one thing about large commercial flat roofs is that costs become a big factor when it comes to how much load you design those roofs to handle. So your factor of safety is only going to be so high.
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