[RE-wrenches] Roof Loading request for help
Joel Davidson
joel.davidson at sbcglobal.net
Fri Jul 10 17:13:22 PDT 2009
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 <joel.davidson at sbcglobal.net> wrote:
From: Joel Davidson <joel.davidson at sbcglobal.net>
Subject: Re: [RE-wrenches] Roof Loading request for help
To: "RE-wrenches" <re-wrenches at 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
Allan at 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
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