We had a snowstorm this week. A couple foot of wet snow. Usually the wind scrubs all the snow off pitched roofs around here, but this time the snow drifted and stuck on the roofs and then the wind kicked up.
Some ag buildings with long clear spans and the cheapo mail order corrugated storage buildings had roof failures.
Word got around and now we have people in town clambering around on their house roofs scooping snow. They didn’t rig a rake, but are actually on the roof!
I saw a whole family on the roof, grade school kids and all. A business with a steep shake roof put the newbies up top with shovels.
This is obviously unnecessary and ridiculous.
The older houses in town are underspanned 2×4 rafters, but steep enough pitches to shed snow. The newer houses are typical ranch style with 4/12 pitches.
Just out of curiousity, what would kind of snow load would it take before your typical ranch style roof would fail?
Replies
CraigF
It's not just the snow, it's the snow and wind combination..
However your question is much harder to answer than you'd suppose.. You see a lot of factors enter into things.. span and pitch (that's the angle of the the roof) plus internal bracing and is it prefrabricated engineered trusses or hand framed?
Douglas fir is stronger that the more common SPF (spruce pine fir) or western woods.
Roofs can have a 12 inch on center, 16 inch all the way to 30 inch on center. decking could be OSB or plywood and the thickness too enters into it..
In general most well built homes should carry such a snow load. Poorly built homes that may have slipped past a builting inspector or never inspected at all could fail..
sorry it's not the answer you were looking for..
Yeah, there is quite a variation of how houses are built around here and an infinite variation in the weather. Hard telling about the combination.
I don't think there's an easy answer to how much an "average" roof can carry, as it varies a great deal.
Where you're at I'm guessing most roofs are designed for about 20 PSF live load. In other places the roofs are designed for MUCH higher loading, so a couple of feet would be no big deal.
It makes a lot of difference how the roof is framed. Some stick framed roofs are poorly done, IMHO. So they may fail more easily. Trusses that are built and installed right have a lot of safety factor in them, so residential failures are all but unheard of.
Since the average guy off the street isn't skilled in determining how much his roof can hold, I don't blame them for wanting to clear the snow off their roofs. Nobody wants the roof of their house to collapse.
There is quite a bit of difference in the stick framed houses. Some are braced to an interior wall, some not at all.I don't blame anyone for worrying about their roof. I just think it's a poor decision to skate around on your roof.I'm just asking in case someone would ask me. Maybe I could talk someone out of an ER visit and some orthopedic surgery.Thanks for the reply, it'll let me answer somewhat intelligently.
When I used to live in Anchorage Alaska I saw a LOT more roofs damaged by snow-shovelers than I ever did by a snow load. A properly designed roof for your area should hold a heck of a lot of snow. Of course there are exceptions , but they are few and far between.
Walking on 10 degree shingles couldn't have been good. There was a layer of ice underneath. I still don't know how they made it up on the roof.
In my part of MN. we design (by code) for a 40# live load. In many applications engineering is factored by 4, so theoretically new roofs probably wouldn't fail until they had around 160# of snow per square foot (assuming no wind load).
There are a whole lot of older roofs that are over-spanned that have held up for 100 years and still going. I did have a friend who had a roof fail quite a few years ago. The renter had built a room in the previously unused attic. The renter took out the knee wall under the rafters. It was wind that did the roof in, in the middle of the summer.
This season, we would be fine building roofs to the standard they use in Phoenix. I think we've had a grand total of 3" in Minneapolis. Winter here this year has been like a Seattle winter.
I wondered how much slack was in lbs per sq ft. deal.There was a fire house in a nearby town that had a roof collapse. I heard the roof was modified to fit a bigger truck in. The same city is having a debate about building codes. Should be interesting.There alot of very old buildings with parapets and low slope roofs around. Haven't heard of any problems with them. Its all construction newer than thirty years old that is having problems.
BAck in eastern Ontario about 20 years ago the code used to require 80#/sf snow load rating. I once built an addition on my brothers house in Baltimore and built it like I used to back in Ontario. The inspector thought I was nuts. The next week they got the biggest storm in decades dumping 24" of snow. There were scores of roof collapses all over town. At the next inspection, the inspector was quite humble with me (without me saying 'I told you so').
Never Never, and Never get on top of a snow loaded roof ( unless you are on skis or a snowboard for some kind of stunt). The loaded roof is always more than 5 years old, it's pitched, so you get pitched when you fall, you are going to shock load it with harmonic footsteps and step on weak spots. The weak spots can crack and cause leaks later. If there is moisture as well as snow you are going to have additional weight that you cannot gauge. Deep snow from several snowfalls will layer with weak boundary layers that will slide on top of each other, it's typically called an avalanche.
WW
WW, On a side note, we might try to make it up to Whistler this year, any suggestions or recommendations on accommodations. Also considering going to the Olympics in 2010.
TGNY;
This year is an outstanding year, you should have until late May to enjoy the snow.
My top 3 picks that I suggest are
#1) The Tantalus Lodge, you can't hurt it, 80's ambience, lots of repeat customers and a superb location. Someday soon they will tear this beast down, but it's paid for and brings in lots of revenue for the owners, It's #16 on the official accomodations map at Whistler-blackcomb.com. Full Kitchens available.
#2 for the High rollers or whenever someone else is paying, the Fairmont Chateau, #14 on the map. smallish rooms, no kitchens, but fabulous service and great location at the base of Blackcomb.
#3 Club Intrawest. Located next to the Chateau, but kid friendly, kids pool and hot tub, outdoor barbecues, full kitchens in the 1 bedroom units, Adult Hot tub and pool area, Eucalyptus steam room and massage service avail. #8 on the map, the upper village location keeps you free from the noise from the party animals exiting the watering holes in the main village.
I said three but one more is the 4 seasons resort Whistler, next to the Club Intrawest, #4 on the map, again away from the party animals but has it's own restaurant and bar areas, superb service, new rooms and decor, steps away from the Chateau, and upper village amenities.
All four of these can be hard to get into at peak times, but late season always has near giveaway bargains at every location. Check everything out at Whistler-blackcomb.com. Do your leg presses and buy a helmet.
WW
Thanks for the tips, any thoughts on 2010. How soon do you have to book. Would it be better to stay in the city, or up on the hill. I imagine the hockey games will be played at the Garage, but I would hope to see downhill venues too.
Book now if you can, almost all venues in Whistler Village are sold out, however many events will take place in Vancouver and suburbs, Skating, Hockey, Moguls, and snowboarding events will be within a bus ride of downtown Vancouver. Opening and Closing Ceremonies will be in Vancouver.
Practice events at local rinks will offer the most bang for the buck.
Sliding and Bobsleigh events will be in Whistler.
WW
WW, what's the current design snow load for roofs in Whistler?
Snow load tables have been discontinued by the province, generally the average ratio of maximum average roof load to 30 year return ground load for a specific location is .25 for sheltered, and .14 for exposed roofs.
Decks are treated as roofs, Whistler has had 22 feet fall since November this year, I am seeing three to four feet of packed snow on rooftops today in sheltered areas. I don't do roofs, but often the actual snow load includes the maximum amount of water from the occasional heavy rainstorm. Normal snowfall for January is 96 cm.
Good question, I don't have a specific answer, but know I will never get on top of a snow loaded roof.
WW
I was trying to figure out what I would add in #/sq ft by being on a roof, but after the holiday wt. gain thing, it was too depressing to think about. :)I was always taught not to drop a bundle of shingles on the roof. Never thought about how much force someone bouncing down a roof would generate.
What does it take to frame a 80#/sf roof?
"What does it take to frame a 80#/sf roof?"
With trusses, it means larger chord sizes and bigger plates. It also may mean that some sloped ceilings that would have worked at lower loading will not work.
Girder situations also become much more difficult. Sometimes it only means adding a girder ply. Other times it means they simply don't work and you have to change something.
The only tie my prayers are never answered is on the golf coure [Billy Graham]
Obviously it depends on span and structure type (truss or stick built).
I've often used 2x10 or 2x12 rafters @ 16" o.c. with 5/8" or 3/4" ply sheathing.
Nowadays I would also use I&W shield over the whole roof or a synthetic underlayment such as Roofshield or Roofguard . The last roof I built was a 4:12 pitch hip roof and the subfascia was designed as a tension ring. This was accomplished by using the longest lengths of 2x8 possible and strapping the splices and corners with at double rows of metal banding 48" long (typically used for hurricane zones for strapping studs to rim joists and /or sill plates with tico nails).
TGNY and Boss- It's interesting to find out about the world outside my little world.
Hey, the world is your oyster ;-)
That's one of the reasons I like this place. It's amazing how much building varies across the country.
In the Lord's Prayer the first petition is for daily bread. No one can worship God or love his neighbor no an empty stomach. [Woodrow Wilson]