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Re: Photovoltaic panel weights

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Drew,

     Being outside the building envelope, the solar panels would probably 
use the unheated slope factor chart regardless. At 4:12 the slope factor is 
 almost negligible. However note that the slope factor reduces snow load, 
the chart overstates the slopes required for snow to slide in order to be 
conservative (i.e. snow will almost always slide off a 12:12 pitch before 
reaching what would otherwise be the full design load in the absence of 
sliding, so it is OK to reduce). Referring to Sec 7.9 (ASCE 7-05), load 
caused by sliding snow is a consideration beyond 1/4:12 slope if slippery 
or 2:12 slope if not slippery. This is almost certainly conservative going 
the other way (i.e. snow will most likely never slide off roofs below this 
pitch) My own practical observations and experience would suggest that in 
reality snow will really begin to slide off a surface somewhere in between 
these two extremes. Solar panels in particular seem to warm up once the sun 
 hits any exposed area of the panel, which appears to precipitate melting 
and sliding under the right conditions. Point I'm getting at is that while 
you may not be able to get the reduction there is still a good chance that 
snow could slide off the panels which requires at least a passing consideration 
 of where such snow would end up.

- Mike

Drew Morris <dmorris(--nospam--at)bbfm.com> wrote:

> Thanks for the idea on treating the panel as a slippery slope.  The
> project is located in interior Alaska and coming up with a design snow
> load has been problematic.  The roof slope is 4:12, so the roof slope
> factor looks like it will be equal to 1.0, the array is located over an
> unheated portion of the structure.
> 
> On 3/17/2014 4:48 PM, Michael James wrote:
>> Drew,
>> 
>> Solar panels tend to shed snow (slippery surface). Depending on your
>> roof material and slope (e.g. asphalt shingles), you might realize a snow 
>> load reduction in the area of the panels compared to the bare roof if the 
>> snow is free to slide off and go elsewhere (panels close to eaves
>> perhaps) any snow shed from the panels has to go somewhere however, so
>> you could have a drift wherever any such snow might end up (lower roof,
>> break in slope etc). Then again snow downslope on a non-slippery roof
>> might hold it all on the panels if the snow is deep enough and panels
>> close enough to the roof. Conversely you could have your panels
>> effectively acting as snow bars and holding snow upslope on an otherwise
>> slippery roof (eg standing seam). These types of things come up from

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