Uplift Wind Loading Values [RE-wrenches]

Mon Mar 24 21:15:26 PST 2003

Joel,

I'm talking about the exact Solec system that was installed on over a
hundred homes in Sacramento. Identical mounting technique, identical
hardware, same Ananda junction box, identical everything. This was not a
SMUD design--it was the design that came as a result of the second Pioneer 1
solicitation--this is, in fact, the design you were referring to.

Bill.

-----Original Message-----
From: Joel Davidson [mailto:joeldavidson at earthlink.net]
Sent: Monday, March 24, 2003 6:51 PM
To: RE-wrenches at topica.com
Subject: Re: Uplift Wind Loading Values [RE-wrenches]

Hi Bill,
I had nothing to do with any systems at PVUSA. SMUD probably decided to use
the
screws-into-plywood method because they liked it. Now that UniRac offers
slotted
rails so you can hit the rafters with sliding feet, lag bolts are a lot
easier
and stronger when they hit a nice solid rafter on center.
Best regards,
Joel Davidson

Bill Brooks wrote:

> Joel,
>
> You did not personally do the system at PVUSA, but the same folks who
> installed the SMUD systems did the one at PVUSA for SMUD. It was part of
> SMUD's small system's testing. This was real 1/2" plywood, not OSB.
>
> I've looked at every residential roof mounting technique to date, and 3.5"
x
> 5/16" stainless lags in roof structure beats them all hands down.
>
> Bill.
>
> -----Original Message-----
> From: Joel Davidson [mailto:joeldavidson at earthlink.net]
> Sent: Sunday, March 23, 2003 7:58 AM
> To: RE-wrenches at topica.com
> Subject: Re: Uplift Wind Loading Values [RE-wrenches]
>
> Dear Bill,
>
> I did no systems at PVUSA.
>
> Tom Lane is right about the pre-chewed substitutes used in place of
plywood.
> OSB
> can be used in glue-ups but has poor fastener properties. Second and third
> growth framing lumber is also pretty wimpy. In the 1960s, I started using
> hotdip
> galvanized nails to frame up buildings after reading a study about box and
> common nails  losing 50% of their holding power in green framing lumber in
> the
> first year. Some builders use glue and screws like cabinet makers to build
> solid
> homes, but most residential and commercial buildings are not as well built
> as
> the PV arrays on them.
>
> Best regards,
> Joel Davidson
>
> Bill Brooks wrote:
>
> > Joel,
> >
> > The only one of those system's I'm intimately aware of was mounted at
> PVUSA
> > on 1/2" plywood. Over half the screws had lifted 1/4" to 1/2" out of the
> > deck providing for a nice path for water into the roof over time. They
> were
> > all gooped with a copious amount of sealant, but the problem was the
> > fasteners were unscrewing because there was insufficient friction force
on
> > the plywood.
> >
> > This was not a pullout issue. I believe that 1/2" plywood is strong
enough
> > for the application even though, as Tom points out, it is almost never
> used
> > anymore--only OSB for the last decade. The problem is that the screws
> > unscrewed, either through thermal cycling or wind vibration. Many
> > jurisdications in California will not accept any fastener method without
a
> > minimum of 1" into the wood. I'm not trying to rag on the system too
bad,
> > but I would never recommend that method. If they haven't leaked yet, get
> > ready---it will happen.
> >
> > That experience, along with similar problems with other
> wood-screws-in-deck
> > schemes have brought me to the conclusion that lags into rafters or
> trusses
> > is the ONLY viable roof mounting technique. There are several good
designs
> > out now that allow for proper installation---use them. I don't mean to
> rain
> > on your parade, but.....
> >
> > Bill.
> >
> > -----Original Message-----
> > From: Joel Davidson [mailto:joeldavidson at earthlink.net]
> > Sent: Saturday, March 22, 2003 4:17 PM
> > To: RE-wrenches at topica.com
> > Subject: Re: Uplift Wind Loading Values [RE-wrenches]
> >
> > Jeff,
> >
> > In 1994, SMUD was the world's biggest PV customer. The challenge was to
> > design
> > and install the lowest cost grid-tied PV systems in the world on
multiple
> > sites.
> > Siemens Solar and other bidders had little or no construction
experience.
> I
> > knew
> > that just about anything could be fastened to plywood if enough screws
are
> > used.
> > I discussed my design with an American Plywood Association engineer and
> then
> > I
> > paid a registered engineer to review my design. We installed over one
> > hundred 4
> > kW PV systems on homes and 288 kW on churches totaling over 71,000
square
> > feet.
> > Installations were limited to gable roofs in good condition with 1/2
inch
> or
> > thicker plywood and composition shingles less than 10 years old. Solar
> > panels
> > with 7 Solec SQ80 modules (4.33 ft x 13.5 ft) were fastened to roofs
with
> 24
> > each 1.5-inch #10 stainless steel wood screws through 0.25-inch thick
> > aluminum
> > mounting feet and into the plywood. We hired local electricians and
> trained
> > them
> > how to make water-tight roof attachments. We paid a construction
> specialist
> > (retired general contractor with over 50 years experience) to fly to
> > Sacramento,
> > review our design and observe installers at work. To date, there have
been
>
> > no
> > roof leaks despite the worse rainy season in 100 years in 1994-95,
several
> > storms since then, winds over 80 mph at some sites and a few minor
> > earthquakes.
> >
> > I used the same screw mounting on my own PV system, but I don't
recommend
> > fastening PV arrays with screws into plywood or other "out of the box"
> > designs
> > unless you are willing to do your homework, get stamped drawings and
calcs
> > from
> > licensed engineers and pay a fair price for experienced supervision and
> > quality
> > work.
> >
> > Peace,
> > Joel Davidson
> >
> > ****snip****
> >
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