[RE-wrenches] Snow loads

Fri Feb 22 04:50:00 PST 2013

Jeffrey,

I agree. We do structural analysis on every roof design and factor in the point loading at the attachment points. I had floated this idea because it came from an SE that I work with that also has a PV system. His theory is the definition snow loads are accumulation of snow over time. And most of the time snow will slide off the panels in 3-4 days. The panels would have to be placed so the snow would completely slide off the roof. I wanted to see what others thought of this approach who have been doing structural analysis. I appreciate your responses.

Gary

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of JRQ
Sent: Thursday, February 21, 2013 8:28 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Snow loads

Gary,

The snow loads that structural engineers design to are a worst-case scenario. It is true that snow tends to slough off of PV arrays quicker than your normal roof. However, if there is a major snow event, especially if it starts at night (this recent blizzard on the East Coast would be a good example), snow will build up on the array the same as it does everywhere else.

In many cases, a PV array actually makes the engineering assumptions worst. Any snow that lands on the modules transfers down to the attachments, and point-load the structural members. The weight of the snow is less distributed when it sits on a PV array, placing additional stress on individual members. I work in Massachusetts currently, and we often have to perform rafter upgrades to roofs that are fine otherwise for exactly this reason.

If you have any snow load requirements in your area, I would highly recommend you have a structural engineer review the structure, even if a structural stamp is not required.

Jeffrey Quackenbush

________________________________
From: Bob-O Schultze <bob-o at electronconnection.com<mailto:bob-o at electronconnection.com>>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org<mailto:re-wrenches at lists.re-wrenches.org>>
Sent: Tuesday, February 19, 2013 3:00 PM
Subject: Re: [RE-wrenches] Snow loads

I would certainly agree with Will here. You can't make that argument as stated, it's wrong. Please don't try to make it to your AHJ, you take the chance of f-ing it up for everyone around there. I think the best argument you can try is:  aside from the usual 3-4lbs/sq/ft that the array itself ads to the roof load, snow will be very unlikely to build up on the modules to the degree that it builds up anywhere else on the roof. The freezing-rain-before-a-snow phenomena is very real and it WILL happen sooner or later. Not often perhaps and under most conditions the snow will leave the modules before the roof. But...Gotta be real about that.
Bob-O Schultze

On Feb 19, 2013, at 11:06 AM, Will White wrote:

Most structural engineers I’ve seen take a reduction for slippery surface but I think you’d be stretching things to say adding PV panels reduces the load to zero.  I have a 45 degree pitch roof and I get snow build up some times especially if we get freezing rain before the snow.

Also if you have an asphalt shingle roof you can create an unbalanced load with snow sliding off the south side and sticking on the north side.  Most engineers take this into consideration too.

Thanks,
Will

__________________________________________________________________________________________
Will White
Regional Field Operations Manager – New England

Real Goods Solar
64 Main St.
Montpelier, VT 05602
Tel: (802) 223-7804
Cell: (802) 234-3167
www.realgoodssolar.com<http://www.realgoodssolar.com/>

From: re-wrenches-bounces at lists.re-wrenches.org<mailto:re-wrenches-bounces at lists.re-wrenches.org> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Gary Bassett
Sent: Tuesday, February 19, 2013 1:48 PM
To: re-wrenches at lists.re-wrenches.org<mailto:re-wrenches at lists.re-wrenches.org>
Subject: [RE-wrenches] Snow loads

Has anyone in the snow belt areas have any luck with convincing their local building inspectors that the snow melts or slides from panels, much like a metal roof only better. Thus your snow load would be reduced to zero where the panels have been installed.

Gary

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