Hi Wes,
I agree, but for lack of someone else's better (or any) standards, I'll use my
standards. Point dead-loading at fasteners is a design consideration as is
fastener wind-loading. A well designed PV array will disperse dead load points
when wind loading is practically dealt with. I'm all for standardized designs
instead of having to get each rack reviewed and possibly burdened with a
structural engineering fee. I have done boilerplate rack designs as have other
designers, but that does not help you in your jurisdiction. I think UniRac has
done a good job of blazing the way toward standardized rack designs. Perhaps
wrenches can spring-board off their work to establish a standard.
For a while, one jurisdiction in Southern California did not require structural
engineering calculations for roof-mounted commercial PV arrays that weighed
less than 6 lb/ft2 on flat roofs less than 75 feet above grade. Then they
changed it to 4 lb/ft2. Now it is up to the Planning whether a structural
engineering stamp is required. The fundamental problems are (1) no uniform
nationwide requirement and (2) arbitrary and continuous AHJ requirement
changes. We can have a national standard and local AHJs can have specific
requirements, but please please please let's get them to stop changing the
rules for a couple of years so we can spend time on refining designs that we
now have.
Joel Davidson
----- Original Message -----
From: wes kennedy
To: RE-wrenches
Sent: Friday, July 10, 2009 10:44 AM
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 <[email protected]> wrote:
From: Joel Davidson <[email protected]>
Subject: Re: [RE-wrenches] Roof Loading request for help
To: "RE-wrenches" <[email protected]>
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
[email protected]
NABCEP Certified Photovoltaic Installer
EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe , New Mexico 87507
505 424-1112
www.positiveenergysolar.com
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