Uplift Wind Loading Values [RE-wrenches]

[Joel Davidson]

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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