Code Allowance for Reduced Snow Load
maine_tyler
| Posted in General Questions on
Any of you snow country folks know if there are any code allowances to reduce the snow load based on roof pitch and/or material? As in snow sliding off. In this case, it is a 10/12 pitch, metal roof. GSL where I am in Maine is listed as 80 psf.
I find it odd that many span tables don’t even go up to 80 psf snow. And where I am is not actually ‘snow country’ by local standards.
I’m not looking for an engineered solution as that is out of the scope of this project. Surprised to see how large the headers get with 80psf snow.
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i did find this:
https://jonochshorn.com/scholarship/calculators-st/example2.3/index.html
I notice that changing roof surface from 'slippery' to 'non-slippery' makes a massive difference. For my parameter, is goes from 40psf to 74psf (80psf input). I would assume screw down metal roofing is 'slippery' but I'm not sure I would want to derate to that degree...
The only thing along those lines I'm aware of is in the 2018 and newer IRC, R802.4.4: https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction#IRC2018_Pt03_Ch08_SecR802.4.4. It's not in the 2015 IRC we use in Maine.
Despite popular opinion, I have seen metal roofs hold a lot of snow, especially ones with exposed fasteners. Maybe worse in Maine than other places because our snow is often wet and sticky or icy. If your official snow load is 80 psf I would design to that: https://www.maine.gov/decd/sites/maine.gov.decd/files/inline-files/Maine_GSL_Listing_by_Town_2015.pdf. You won't get that load every year but it's for those once-in-ten-year storms that have many feet of heavy, wet snow.
For 80 psf snow load, I use the 70 psf table and 20 psf dead load. Most roofs are well below 20 psf dead load. Or I do my own engineering, or work with a licensed engineer.
I've always wondered the same thing. For what it's worth, my 12/12 pitch roof in Northern Vermont never has held more than about 6" when it was nail down, rusty, repainted, corrugated steel with ice dam issues. Now with a new ventilated standing seam cold roof it maybe 2" max.
tjanson,
A lot of metal roofs have snow-guards to intentionally retain the snow.
Appreciate the thoughts! Point taken Michael about the wonderful ice and slush we can get here.
I did notice on my truss design paperwork a note:
5) Roof design snow load has been reduced to account for roof slope.
Truss companies have their own engineers who can design whatever they want, though they are supposed to follow "established engineering practice." The structural engineers I work with use this document: https://awc.org/publications/2018-wfcm/. I have a copy, and a copy of the accompanying commentary, and searched for "snow" in both. In over a hundred instances of the word "snow," none mentioned reducing the load for slope. They did mention that the design does not allow for sliding/falling snow from a higher roof.
I might be missing something but I believe the truss engineer is making their own judgement call to reduce the load based on slope. The only copies I have of truss designs are for shingled roofs. I will be getting designs soon for a 10:12 standing seam roof in Pownal and will see what they say about the loads.
IBC 1607.14 cover snow loads and has some allowances for reductions. ASCE 7 section 7.5 covers snow loading, including acceptable reductions and unbalanced loading. It is not a straightforward equation just based on slope and so it is not included in prescriptive code. If you are an engineer you follow ASCE equations, hence the truss reductions.
Great info, thank you.
Climate change has me reluctant to bet against Mother Nature these days. We might see less snow every winter but we're also more likely to see one big freak "1000 year" storm that has a perfect combo of historic amounts of sticky snow, rain, and wind storm that could push the limits of that 80psf rating.
This is so anecdotal that it's maybe not helpful, but my 1880 house has an exposed fastener 12:12 roof (not particularly well done), 2x4 (actual) rafters spanning about 14' horizontally (so, 14' rise, 14' run). And I'm pretty sure the only reason it wasn't crushed by the weight of Bozeman snow loads over the last 143 years is that the roof is steep as hell and the snow avalanches off terrifyingly quickly after every storm.
ASCE 7 may help you. An older copy should still have what you need. We're in snow country and I agree that snow often slides right off our 7/12 slope metal roof, except sometimes it doesn't.
I've debated snow guards on a metal roof but my response from installers is that snow guards are mainly there for liability. If the snow decides to slide off in on instant swoop off a metal roof, it's to prevent it from falling and injuring/killing someone below.
Anyone with a metal roof on a steep pitch knows how fast that snow/ice sheds. Gutters can be instantly pulled off with shedding snow.
Your second paragraph gives another reason for snow guards.
DC,
When I was young I worked at a neurological hospital that had a metal roof sevens stories up which shed large chunks of ice onto the sidewalk out front in the spring. Luckily help was close by.
Ground snow load is used as a parameter to determine snow load. If you talk to an engineer (a licensed one in Maine), they will use that parameter along with your roof pitch, roofing material, criticality of the structure, etc. to determine snow load. Don't be afraid to talk to an engineer, they're not that scary..... and the cost isn't that bad compared to not talking to an engineer....
I thought of this thread this week, waiting for the snow to slide from the 8:12 galvanized steel roofing on my shed, three full, sunny days after it fell. It's only about 6" but was relatively heavy and wet.
Michael,
My house two years ago on temperate Southern Vancouver Island. A mixture of shingle and metal roofs.