Snow Loads Simplified

I'd like to address a common misconception I see in calculating the dead weight of a solar system. Ground Snow load does not equal load on the Solar Modules!

ASCE 7-05 addresses this pretty clearly, although I don't have my book in front of me, I'll summarize, then bring some math home tomorrow.

Snow load on flat roofs =.7*ground snow*importance factor*exposure factor*thermal factor

Where:
ground snow= ground snow load at your location

Importance factor=most often 1 unless its an area where many people often congregate, then it can be more like 1.1 or 1.2, .8 for Cow barns, unoccupied buildings.

Thermal factor= I mostly use 1.2 as the Solar modules aren't exactly heated but the buildings usually are.
1.2 when the structure is totally unheated
1.1 when the roof is cold ventilated, and with R value between heated and unheated space exceeds 25
.85 for heated greenhouses with r value less than 2



Exposure factor= Depends on whether your installation is fully exposed, partially exposed, or sheltered, also what exposure category you have, usually b, c or d

Fully Exposed: Roofs exposed on all sides with no shelter. Shelter is defined as trees, buildings or obstacles within 10 times the difference between building height and obstruction height, trees that drop their leaves in winter do not count.

Sheltered: Roofs located tight among Evergreen trees or other shelter
Partially Exposed: All other roofs.



Category--Fully Exposed --Partially Exposed --Sheltered
b
.9
1.0
1.2
c
.9
1.0
1.1
d
.8
.9
1.0

Note: Monoslope roofs of less than 15 deg, Hip/gable roofs with the slope less than either 2.38 deg or (70/width of roof from ridge to gutter+.5) may not be reduced below 20 psf.

Also note:Rain on snow surcharge load applies to locations where ground snow is 20 psf or less, and the roof slope is less than (gutter to ridge length in ft/50) shall have a 5 psf added load.

Next, if you have a sloped roof, multiply the roof snow load by the slope factor to get your adjusted snow load on the modules.


slope factor=Most conservatively (thermal factor 1.2)on a slippery surface this is a range between 1.0 @ 15 deg and 0 at 70 degrees with linear interpolation between these 2 values. Note: slope factor only applies to installations where the modules are at the lower edge of the roof, or over a slippery roof surface.

So for slippery surfaces approximately
4 on 12=.93
6 on 12=.8
8 on 12=.68
12 on 12= .45

shingles, composite roof surface ect:
12 on 12 =1
45 degrees=.8
60 degrees=.4
70 degrees=0

Wow, I thought I could make this quite simple, but after rereading, it might be more difficult for someone who hasn't done this before to get it.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Snow Loads Simplified

    A nice seasonal topic. ;)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Snow Loads Simplified

    I'm lucky, living in Nova Scotia, the Winter sun is low in the sky, so I get EXCELLENT results by repositioning my panels to the vertical for Winter. The result - - I have ZERO snow load, never have to clean them off, and get extra output because of the sun's reflection off the snow covered ground :p
    The only down side is that on cloudy days, I don't get as much of the sky light. But the advantages far, far outweigh the negatives.
  • topper
    topper Solar Expert Posts: 113 ✭✭
    Re: Snow Loads Simplified

    Well according to a local engineer, my roof should be built for 25lb static load plus a 50 lb snow load.
    Doing the calculations on unirack, 3-400 lbs per mounting point seemed almost unreasonable. Or maybe I am out of context.
  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,730 ✭✭✭✭✭✭
    Re: Snow Loads Simplified
    I'm lucky, living in Nova Scotia, the Winter sun is low in the sky, so I get EXCELLENT results by repositioning my panels to the vertical for Winter. The result - - I have ZERO snow load, never have to clean them off, and get extra output because of the sun's reflection off the snow covered ground :p
    The only down side is that on cloudy days, I don't get as much of the sky light. But the advantages far, far outweigh the negatives.

    A dual axis tracker does this every day, and you get the sky light on bad days.
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

  • decabob
    decabob Registered Users Posts: 1
    Re: Snow Loads Simplified

    My snow problem is how to prevent the accumulated snow from sliding off the panels in one large sheet destroying anything near the foundation. The most practical suggestion I have had is to insert some material between panels acting as a restriction to the snow pack from sliding till it melts off. The "break" would also serve to break up the snow pack resulting in smaller pieces sliding down, less potential for damage. This is not mentioned as a problem in the average installation of solar panels because an avalanche from the panels is a rare occurrence. Nevertheless we want to prevent the snow pack from dumping onto the plantings and greenhouse roof located at the foundation. Question: Would inserting a piece of sheet aluminum or copper interfere with the operation of a panel and would it affect the electrical wiring or connections? Question:Has anyone had experience with what are basically metal roof brackets regarding the impeding or breaking up of the snow pack on the roof?
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Snow Loads Simplified

    They make (and sell) "snow breaks/brakes" for steel roofing. You could simply adapt.

    http://www.idealroofing.com/english_canada/html/snow_guards.php


    http://www.snoblox.com/

    A couple (of many) ideas.

    Tony
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Snow Loads Simplified
    decabob wrote: »
    Question:Has anyone had experience with what are basically metal roof brackets regarding the impeding or breaking up of the snow pack on the roof?

    We have the snow guards like #211 in the link from Icarus on our house and garage. They work very well. No PV panels on those roofs though. I want the snow on my PV arrays (one on a pole mount/tracker and one a shed roof) to slide off ASAP!
  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Snow Loads Simplified
    mtdoc wrote: »
    I want the snow on my PV arrays to slide off ASAP!

    So true. The longer they have snow on them, the longer their output is far, far from where it should be. This is extremely significant during the short days of late Fall and Winter.