Wes you have a great point.
Using PSF for a solar array can be a little misleading. With your
example with 3.3 psf and a worse case down force wind load of 29 psf we exert a
total of 32.3 psf on the 300 ft2 array for 9690lbs total, or 323lbs per
attachment. This may not seem bad as basic 90 mph wind on 15 ft high building,
but know put 4 or 5 attachments on the same structural member and now that
member is carrying 1292 - 1615 lbs. This is not an evenly distributed load but
multiple point loads. Will it crash a common house? No, but it does point out a
distributed load can be miss leading. Using this lets say the installer spreads
the attachment spacing out from 4' to 8'. The point load will double and if he
does not stager the attachments from rail to rail the point loads will now be
on less structural members increasing the member loads into 2584 - 3230lbs. All
these options are still based on 3.3 psf array load at a 300 ft2 array. Now
think of other point loads the building may have, HVAC and other mechanical
equipment the closer we get to those the more we point loads can add up.
As a general rule I do not worry much about this, but as you move into higher
wind and snow loads this can really become an issue; even if the roof can
handle the load using a PSF type rating, the point loads can be very
surprising. The dead load of the array is normally not the primary concern. It
is the live load of wind, dead load of snow and the array that can step up and
get us in some rare circumstance.
In my limited dealings with inspectors some seem to realize this so they want
to see that the structure can handle these point loads. In other locations they
seem to accept that if the loads are 500lbs or less then it will be ok, but go
above this and they want engineering. Or they do not even care or think about
it at all.
I hope this makes sense.
Chris Meier
Residental Program Manager
UniRac, Inc.
1411 Broadway Blvd. NE
Albuquerque, NM 87102-1545
Ph: 505-242-6411
Fx: 505-242-6412
Email: chr...@unirac.com <mailto:chr...@unirac.com >
Web: http://www.unirac.com <http://www.unirac.com/>
________________________________
From: re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of wes kennedy
Sent: Friday, July 10, 2009 11:45 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Roof Loading request for help
Hi All,
I am fully in favor of standardized permitting, and streamlining the process of
installing PV. With that said, I do think the "one layer of shingles = PV on
roof thing" is apples and oranges. Though the deadloading in psf is similar,
PV doesn't sit on the roof, it lands on some sort of attachment, standoff,
l-foot, what-have-you.
This leads to pretty high point loading values, doesn't it? If you spread your
1000 lb array around 300 square feet, you get your 3.3 psf, BUT if it lands on
30 L feet, each of 4 square inches you end up with much higher points of
concentrated loading.
Anybody worry about that?
Thanks!
Wes Kennedy
NABCEPian
--- On Tue, 7/7/09, Joel Davidson <joel.david...@sbcglobal.net> wrote:
From: Joel Davidson <joel.david...@sbcglobal.net>
Subject: Re: [RE-wrenches] Roof Loading request for help
To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org>
Date: Tuesday, July 7, 2009, 7:13 PM
Allan,
In most of California (seismic zone 4) residential PV systems with solar arrays
weighing less than 4 lb/ft2 do not require structural engineering if the roof
has one layer of composition shingles. The reasoning is that roofs are allowed
2 layers of shingles (old set and re-roof set) and a layer of shingles weighs 4
lb/ft2 so 1 shingle layer and 1 solar array weighing less than 4 lb/ft2 is
within the dead weight load limit. Hope this helps.
Joel Davidson
----- Original Message -----
From: Allan Sindelar
To: 'RE-wrenches'
Sent: Tuesday, July 07, 2009 5:26 PM
Subject: [RE-wrenches] Roof Loading request for help
Wrenches,
We are currently facing a city permitting bureaucracy that has recently
discovered solar - that is, suddenly each department in the permitting and plan
review departments is coming up with standards for PV systems. Some of the
standards, of course, make no sense.
PV systems typically add about three pounds per square foot to the
loading on a roof. We are facing a city requirement for structural engineering
work for standard roof attachment if the mounting approach is to make
penetrations into the roof structure. This is a typical requirement that will
only add considerable cost to each PV system, and we're looking to have our
ammunition to fight this well stocked in advance. Specifically, are building
authorities in other jurisdictions requiring structural engineering work for
this type of roof attachment?
Thanks
Allan
Allan Sindelar
al...@positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe , New Mexico 87507
505 424-1112
www.positiveenergysolar.com <http://www.positiveenergysolar.com>
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