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Should We Be Worried About Our Roofs?

by Prince Of Petworth February 8, 2010 at 9:06 pm 44 Comments


A reader asked about this in one of yesterday’s comments but I thought it was worth a separate post. At one point should we start to freak out about our roofs? If we get another 20″ are we in trouble? If the roof is slightly pitched will it be ok? Anyone know roughly how many inches of snow our roofs can support? I feel like I hear 18″ somewhere. It is not possible for many of us to shovel our roofs, if that is the case do we just drink and hope for the best?

I guess lots of folks are thinking about this because I just received the following email:

“I live in one of those two-story mini-houses discussed here with some frequency. It’s 13-feet wide, 30-some feet deep and two stories tall without a basement.

My question is this: Should I shovel my roof? My adjoining neighbor just did so to explore a leak that developed on Sunday and he must have pushed tons of snow off the edge. I wonder if all that weight could breach the roof or cause it to fail? Now there’s more snow coming — more than this house has seen in years — and although my roof has never given me any trouble, it is more than a decade old. If anybody is wondering, my roof has a slight pitch back from the street, probably less than 10%.”

  • deezn00tz

    i live in the attic of my house, its pretty old, i wonder if i should be worried?

  • j

    they did a spot about this on the news. firefighters advise that residents do not attempt shoveling their roofs because it is extremely dangerous and the addt’l weight could be all it needs to collapse. if you hear or see that your roof is in trouble, leave and call 911. if you suspect there may be a problem, call your insurance and get an inspector out.

    i have no access to my roof so there is nothing i can do. i am just hoping for the best — though the worry is on my mind with this new storm approaching.

    be careful out there!

  • Victoria

    What would Dickens do?

    • Eric B

      Eat a spoonful of sugar? Oh, wait. That’s what Poppins would do.

  • EPF

    I did it on Saturday morning during the height of blizzard #1. Probably not wise but, on the other hand, now I’m good for another 20 inches. In case you want to brave it…all the snow I shoveled off the roof was wet and incredibly heavy so that’s a load off my mind and my roof. On the other hand, now I have a 9 foot tower of snow/ice in my backyard near the door.

  • I assume homeowner’s insurance covers the destruction of gutters, etc. due to the snow? Has anybody had their insurance pay for this? I will certainly call me insurance agent, but just curious to learn if anyone has advice? Any good gutter people out there?

  • Just did some googling and found that DC’s current design requirement for roofs is either 25 pounds per square foot (psf) or 30 psf, depending if a recent amendment proposal was passed. According to the following web site addressing the issue for residents of North Dakota, The snow we’re getting is probably about 15 pounds per cubic foot, or 30 psf with 2 feet of depth.


    So, technically, our roofs are probably sitting near the limit. That said, here are some things to consider before stressing over it:
    – Almost every roof in DC can support the weight of a roofer performing maintenance (~175 psf when standing with feet together), so it’s not like your roof will immediately collapse once it reaches 26 psf.
    – If a roof IS close to its limit, standing on it to clear snow is a bad idea
    – Performing physical tasks on any sloped surface covered in snow and ice high above the ground with no guard rail is a very bad idea.

    For what it’s worth, we’re getting ready to build a green roof for a client in Northeast, and the city requires a load capacity of 75 psf. If the homeowner intends to use the roof for entertaining, the load capacity must be 100 psf. Clearly, the city does not expect that all guests will be children, so 100 psf can support the load of adults who could weigh twice that much. Similarly, a well-maintained 25 psf-rated roof can support the load of 2, maybe 3, feet of average snow on occasion even if 25 psf is exceeded. One difference, however, is that the snow covers every square foot of the roof, unlike people (unless you throw a fantastic party). I don’t think a roof could handle a full doubling (or ~4′) of the expect snow load.

    Conclusion, don’t rush out to clear your roof, but if you see any bowing, new ceiling cracks, new water damage, or hear atypical creaking and groaning, your roof might be one of the unlucky few that will give out this winter. Upon observing such warning signs, get down to the first floor or basement immediately. It’s a good idea to know where your water shut off is, as well.

    • ah

      The roofer example isn’t a very good one, because the weight although located in a small area is spread out over more than 1 square foot through the joists. If you had a roof-full of roofers all packed together, you’d have a problem.

      For the OP–if the slope means water drains off from melting snow that should help a lot

      • I acknowledged continuous loads closer to the end. My point with the roofer was that 25 psf isn’t the absolute max before catastrophic failure. For most structures, there is wiggle room. Otherwise, we would have no AC condensers, TV antennas, herb boxes, and deck chairs dotting our skyline.

    • Victoria

      god I adore real facts! thank you!

    • La Niña

      great analysis BUT I have started seeing a lot of porch roofs sag – not a good sign, I am not sure what the load capacity for those roofs are and since they do not have as much support I went ahead and cleared my porch yesterday and the back addition of my house this morning. hard work but a “weight” off my shoulders.

  • Petworth Res

    We live in a Wardman style home and cleared about two feet of snow off the roof on Saturday. I wouldn’t advise anyone doing the same, but having that much snow off the roof is certainly a relief!

  • KStreetQB

    I shoveled my roof off during our first blizzard and I did it twice during this one.

    1000 sq foot roof. 2 feet of snow at 15 lbs/cuft = 30,000lbs of snow on my roof. Another 20 inches? I’d be looking at 45k+ lbs. My house has parapets on the roof, so it drifts even higher than the normal snowfall.

    Forget about the roof failing, I’m worried about z-fractures along my masonry and windows blowing out at that point!

  • EPF

    Irvinggreen – Thanks for that deailed analysis. Much appreciated. My house CH house is of 1916 vintage and I’m sure was not built with much modern load bearing thinking…or the materials are beginning to show their age. There are some sagging of beams going on. On the flip side, the roof has been up in it’s original state since 1916 right!

    • np. I should have said that new bowing is a concern. Nearly all DC row houses have sagging beams to some degree. Gravity takes its toll over time.

      You’d be surprised, though, how little some building codes have changed in 100 years. The basic principles of keeping a house upright haven’t changed all that much since colonial settlement even. Furthermore, much of the wood used for rafters and joists 100 years ago was much stronger than today’s new growth pine. Assuming no rot or termite damage, those old beams notched into the brick parapet walls will probably be fine until you’re ready to build your pop-up or green roof!

  • pinto

    holy cow, now I am scared! I can hear it groaning ;)
    Nevertheless, I would think that shoveling it could possibly damage the roof and cause a leak.

    • ah

      pinto – if you do shovel, you should leave an inch or two to prevent this. Shovel off the top layer, but no need to get to bare roof.

  • In short, I’d say it’s a case by case basis depending on multiple factors.

    The structural integrity of a DC row home depends on multiple variables including the materials used, whether or not the structural materials have sustained previous damage from leaks, renovations, fires, termites, etc.

    Also, it depends on whether or not the most recent roof resurfacing involved stripping off the preexisting roofing materials. A lot of roofs I see have several old layers of roofing, which create a massive continuous load on the structural members.

    You also have to take into consideration the evaporation of snow and snow melt. Lots of attic spaces in DC are not insulated and therefore cause snow melt and draining.

    If anyone is renovating a row home and can jack up the roof and replace the double headers across the party walls that carry the roof deck, it should be done. I’ve replaced multiple beams with LVLs, which should be fine in this weather.

    • hg

      I had my roof done 7 years ago. I made sure the roofer took out the old roof before putting the new one. He thought it would be 3 layers of roofing, but no, to our surprise there were close to 5 layers of old roofing, two of which were gravel. It is amazing how the roof held up for that long. I had them take everything out until they reached the wood part before installing the new roof.

  • PoP fan

    I grew up in a DC house with a partial flat roof. We always shoveled it, which obviously can be dangerous. Main issue was ice dams and then leaks because the extra stress can open up cracks, gaps etc that wouldn’t normally let water in. In some of the big storms with a lot of snow or multiple snows the leaks got a lot worse. But be careful!

  • Finstock

    well, I’m from Colorado, and we don’t worry about snow on our roofs like you pansies in D.C. you all sicken me.

    • John Denver

      And we wish you were still in Colorado.

      • L

        I think Finstock was joking (in reference to an earlier thread)

  • DB

    i grew up in a row home in Pennsylvania (where these kinds of snows happen every year). We shoveled the flat roofs whenever there was a big snow.

    Yes it’s dangerous if icy. But you’re not stupid. You know what ice looks and feels like and not to f with it. You know never to even approach the edge of your house except, at best, from the far end of a shovel. You know not to move fast or turn suddenly. you might even tie in to your chimney since an ice pick is out.

    Be careful not to scrape your roof with the shovel like you would a sidewalk.

  • Lots of snow melted off my Sunday/Monday. I could hear a constant trickle coming down the gutter drain. I’m not going to worry about 10-15 inches over whatever is left up there… Just doesn’t add up.

  • Nofiction

    I shoveled my roof for the first time in the six years I’ve lived in this houe and learned a few things:

    My roof must be well-insulated: My neighbors all have a lot less snow on their roofs, apparently from melting and reduction from heat loss. I don’t see much runoff happening from their roofs, so I wonder if it’s all just compacting/becoming icy?

    The snow up there is really heavy, and varied in depth from 6 inches to over 2′, basically leveling along the roof’s slightly slope.

    After that experience, I’d advise using a plastic shovel – and, as one of the commenters above noted – for the same reason i would avoid scraping the roof clear in order to preserve its seal.

  • Something I noticed on buildings next to ours…the wind tends to sweep a lot of the snow off onto the surrounding ground. So the likelihood that there is a full depth of snow on any roof is pretty small. That, plus the melting effect from heat leaking up from the house–and the sun from the past two days–probably means there is a lot less snow and weight than you might initially assume.

  • georgetowner

    We shoveled two feet of snow off the roof last night. I stood in the hatch opening and pushed snow off with a big plastic dustpan duct-taped to a broom handle–a technique I highly recommend. (I also cleared the gutters with a metal soup ladle duct-taped to another broom handle.) Once I’d cleared the area, my husband crawled out on his knees and shoveled the rest. It helps that we have a foot-high edge along the long sides of the house.

    • “metal soup ladle duct-taped to another broom handle”

      hahaha! thats determination!

  • hg

    just got back home to take a break after shovleing almost half the snow from my roof. there is more than a foot of snow up there. scary.

  • Bella Bot

    I’m from upstate NY. More rooftops are pitched there, but the ones that are flat…people would be trying to shovel them after this much accumulation. I don’t think you should risk your life to do it, but it’s a good idea if you can.

    Decks too!

    I saw someone’s gutter fall off on Sunday in Petworth.

  • crin

    This old snow + new snow for most won’t be a structural problem. But if we get a third storm with warm temps and rain on top of all the snow, you’re talking 3 storms worth of water on your property at one time. Make sure your gutters, downspouts, drains and all other parts of your drainage system are ready when the defrost comes.

    Shorter version: you’re more likely to get a flooded basement than a broken roof.

  • cookietime420

    The good news is that there haven’t been a lot of collapsed roofs so far – just a handful by my count. From what I’ve read, the most vulnerable roofs span much larger areas than the typical roof on a house in DC, such as those on top of commercial spaces. For instance a roof collapsed over an ice rink in PG county.

    We can all take comfort in the fact that our roofs have withstood major snows in the past. I also take comfort after seeing what my own roof can take. I own a condo in a row house that had an insanely dangerous deck. The supports sat on top of the roof, and not even the joists. We reconfigured it so that the supports are now on top of the brick wall. That deck supported people for years before it was fixed.

  • Newman

    …just a few comments on roof snow loads, from a civil engineer visiting DC during the snowfall.

    Mr. Green’s discussion are really on point, and should put at ease building owners, and discourage dangerous roof visits. Added reasons to not worry are that occupied structures are built with an extra margin of safety beyond the code-required 25 lbs/sq ft design snow load for the DC area. That margin occurs whether calculated by professionals or by the intuition and tradition of builders going back centuries. Many older structures are stronger than newer calculated-designs.

    Data on snow depth is usually the fresh-fallen depth, and fresh-fallen snow is usually under 10 lbs per cubic ft; it usually compacts with time, up to 15 and even 30 lbs per cu ft. (but then it has less depth). So, don’t be so sure that 30 inches of snowfall equals 30 inches on your roof.

    Yes, be watchful for new sagging, cracks, groaning. Most winter roof collapses I’ve seen result from 1] ice buildup (like on flat roofs with plugged drains), 2] weak construction (like on porch roofs, DIY garages and sheds, or low-cost buildings using wrong standards e.g. a pre-fab bldg designed for more southern climates), and 3]decrepit old buildings. My advice– maintain your building well before a storm, inspecting drains and poking for wood structure rot or metal structure corrosion. If you have a flat roof, look for ponding on a rainy day. Best candidates for winter roof collapse– an old flat-roof garage with rotted framing and leaves from a big shade tree nearby.

    And, yes, a flat roof is often at more risk for several reasons. One is that lots of ice or water can accumulate if a drain is plugged. On a sloped roof, the gutters simply overflow. Secondly, a pitched roof will have more triangular framing (strong) as compared to the trusses or beams for a flat roof.

  • lou

    There’s an excellent article in the Post about roofs in DC and snow.

  • It is a worry. However, if my 100+ year old town house survived the Knickerbocker storm and its 28 inches of snow, I’m hoping it can make it this week.


    captcha = Poland sunbelt

  • hg

    Just finished shoveling my flat roof. Before this task, I looked across in the back to see my neighbour’s roof and from far it appears there was about 7″ or so snow on their roof. I expected the same as I went up the roof, but to my surprise, our side of the row houses all have about 20″ of snow on them. I took three breaks in between, but finally finished. Now I have to go outside and clear the same snow off the path.

  • GoldCoast-16th St Heights

    I just shoveled a portion of my house that has a flat roof because I was more concerned about damage to the roofing membrane and leaks rather than a full collapse. I was very surprised by the sheer weight of the snow/ice mixture. Additionally, I quickly learned that the flat roof became the repository for all of the snow sliding off of the pitched part of my roof. I had about four feet of accumulating snow and ice. In short, I recommend evaluating your flat roof, and don’t assume that the depth of snow is equivalent to the snowfall. It could be considerably deeper, particularly if the flat roof is adjacent to a pitched roof.

  • Fresh off the wire:

    Man injured as roof collapses while clearing snow
    February 9, 2010 – 2:46pm

    UPPER MARLBORO, Md. (AP) – Fire officials say a man attempting to clear off snow from his garage roof in Upper Marlboro was thrown from a ladder when the roof collapsed.

    Prince George’s fire department officials say the man, who was in his 60s, fell about 20 feet Tuesday and sustained potentially serious injuries. The man was taken to a regional trauma center for treatment.

    Officials are urging residents not to venture onto their roofs to clear off snow. They say a person’s weight can be enough to cause a collapse.

    Instead, officials are urging residents to call roofing contractors to remove the snow. Fire department officials say residents should check for sagging roofs or the sound of breaking wood. If the roof structure seems weak, they say residents should evacuate and call 911.

  • carol

    we have 3 new cracks in our ceiling – middle of the house. we braced them with an old door and lots of lumber. we’re clearing off that damn snow tomorrow morning!!! i wish we did this before this latest snow!!!

  • To say that building codes haven’t changed much in 100 years is the equivalent of saying building cars haven’t changed much in 100 years. The building code is drastically different when compared to the code from 30 years ago. I could go through a systematic list of changes over 30 years that have saved lives.

  • victoria

    Anyone know about clearing outside snow-buried HVAC units? How much surface do they really need? Top? Sides? Thanks!


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