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View Full Version : Snow on flat roof - when to worry?



Dean Karavite
02-10-2010, 10:20 AM
Hi All,

Here in Philly me and most of our neighbors have either total or partial flat roofs. Now that we are looking at up to an additional 2 feet of snow on the 28" we got last Saturday, I have been thinking about these roofs. The houses are old and all have beefy old 2x10 fir joists, but I'm no engineer or roof expert. What do you think? I actually can access our roof via a hatch that makes things easy, but my wife isn't thrilled about me going up there.

Thanks in advance!

Rob Russell
02-10-2010, 10:42 AM
3+ feet of snow would be enough to be me up there, especially if it's a flat roof.

Dave Johnson29
02-10-2010, 10:52 AM
What do you think?


If your house was built to Code, check with the local County Zoning department for the Snow Load. That will give you a depth figure that your roof should have been built to support.

Not much help I know, but at least a guide to what should be safe and acceptable. Go up there and measure to see where you are in the scheme of things. You wife will be happier at you getting some College kids in for 10-bucks and hour to shovel snow off.

Bill Rogers
02-10-2010, 12:04 PM
Keep in mind that 1/2" of standing water over a 10' x 10' area weighs 500 pounds, or so I have been told.

Also from wikianswers.com: Ken Hellevang from the University of North Dakota says, "The weight of snow varies greatly. Light fluffy snow may only weigh about seven pounds per cubic foot. More average snow may weigh 15 pounds per cubic foot and drifted compacted snow may weigh 20 pounds or more..."
Source: http://www.ag.ndsu.nodak.edu/coping/snowload.htm

Based on the figures given above your snow accumulation of 52" could be adding anywhere from 30 pounds to 87 pounds per square foot of roof area.

David G Baker
02-10-2010, 12:21 PM
Dean,
If you have a way of getting the snow off of your roof by all means do so. It is not worth taking the risk. The higher the moisture content of the snow the more dangerous it becomes.
I have a snow roof rake that sort of works but the better type is the one that has a long plastic tail that slides under the snow and helps it slide off of the roof. Check out snow rakes on Ebay or roof rakes.
Good luck.

Lee Schierer
02-10-2010, 12:26 PM
You can usually tell by interior doors not fitting right in their openings when you open or close them. If you start seeing door binding problems or they are forecasting rain before the snow melts then you want to get some or all of it off. It can be dangerous work so be careful. Also shoveling snow is heavy work so if you're not used to vigorous exercise, take frequent breaks for your heart.

Dave Johnson29
02-10-2010, 12:46 PM
Based on the figures given above your snow accumulation of 52" could be adding anywhere from 30 pounds to 87 pounds per square foot of roof area.

Bill, while that may be true, if the Code for the roof 130lbs/sq ft and the building was professionally built to Code then there is no real risk.

http://www.id.nrcs.usda.gov/snow/data/geninfo/snowload.html

Dean Karavite
02-10-2010, 1:21 PM
Thanks everyone. This was nagging me in a big way and I knew I would not be able to sleep unless I did it. So, I just got down from shoveling the roof. It was easily 25 pounds per shovel full, maybe more. I couldn't find the code for Philly, but a few sources said the area has about 20" snow per year and we will double that in the past 5 days! If code is for 25 lb per square foot, or even 30 lb - we reached that easy. I'm betting our old house is built beyond that, but I am not taking any chances. Now it is all clean and waiting for the 12-18" we expect today. Quite the cardio work out let me tell you (very heavy snow), but one I would rather have skipped today! :-) Now I'm too tired to play in the shop!

Only half of our roof is flat and I pity anyone with a larger flat roof or some of the 3 story monsters around here. We lucked out by having access via a hatch. Thanks again for all your advice and I'm glad I did this.

Ted Calver
02-10-2010, 1:45 PM
I think you did the right thing. No brainer to me. Flat roof with easy access = shovel it and rest easy. I had a small electric snow blower I used to use on my upstate NY roof. To me the danger is the possibility of a cold rain being absorbed by the snow to make it all that much heavier.

Bob Lloyd
02-10-2010, 1:45 PM
Dean

I would have shoveled it off too. Also, beats going to the gym!! According to this link the design ground snow load is 25 lbs/ sq ft but who knows what your house can take.
http://webapps.phila.gov/li/UpdDocs/2007struct_criteria_.pdf

Bob

Steve Schlumpf
02-10-2010, 2:09 PM
Dean - as someone who has shoveled off roofs quite a few times - there is a trick to it that will save your back. Start at the edge of the roof and just push the snow over the edge. Back up a little, cut a chunk with your shovel and slide it over the edge. With luck, you should be able to clear your roof without lifting anything heavy! You'll still get tired but not as much as lifting each shovel full!

Dean Karavite
02-10-2010, 2:41 PM
Thanks Steve, I sort of figured that out as I went along! Next time (which might be tomorrow!) I will try it.

Brian Loucks
02-10-2010, 2:48 PM
+1 for Steve. Here in upstate NY it's rare to see more than 2 foot of snow on anyone's roof before you start shoveling. Less if it's flat. Remember not only the weight of the snow, you have to watch out for ice build up along edges as it melts and refreeezes. That can cause some serious problems.
Even if you can just get the bottom with a roof rake that will help. Just be safe. Don't be a macho man and go climbing when you shouldn't. Plenty of young guys out of work that can lend a hand.
Looks like our son will get to shovel 6 to 8 inches later today. (I'll miss the little guy (6'2") when he moves into his new home next week :-)
Be safe, Brian

David Bridgeman
02-10-2010, 3:55 PM
I have a family room addition that is not exactly flat, but too shallow for shingles. Always worries me. Luckily, I can climb out from the upper bedrooms.

Myk Rian
02-10-2010, 4:32 PM
I use a roof rake to get the first 3-4'. Keeps the gutters from falling off, and the sun keeps the melt running down.

Joe Pelonio
02-10-2010, 10:32 PM
If they predict rain you'd better get up there soon as you can. The weight from the water that will be absorbed into the snow before it melts can definitely cause problems. We rarely get that much snow but had a lot of caved in commercial buildings and boat sheds back about 1996 after 24"
of snow followed by light but steady rain at just above freezing. It even caved in a huge skylight onto a couple of brand new cars in the Mercedes showroom.

Dean Karavite
02-10-2010, 10:56 PM
Thanks Joe, already taken care of. While neighbors who let it go seem okay, I'm going to be sleeping much better tonight.

So, did you get a deal on any of the Mercedes after the cave in? :)

Joe Pelonio
02-11-2010, 12:18 AM
Thanks Joe, already taken care of. While neighbors who let it go seem okay, I'm going to be sleeping much better tonight.

So, did you get a deal on any of the Mercedes after the cave in? :)
No, I didn't even ask. They were a mess. That glass was thick and huge panels.