Uplift Wind Loading Values [RE-wrenches]

[Joel Davidson]

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


"Jeffery Wolfe, Global Resource Options" wrote:

> Keith,
>
> When I don't get things really engineered, I tend to build a battleship.
> (Maybe an unfortunate analogy tonight.)
>
> I'd consider installing a long piece of angle below the roof purlins,
> spanning several purlins or beams (depending upon design of the roof
> structure). I'd then put the same type of angle above the roof, and clamp
> the two together with bolts. Doesn't need to be real heavy angle, the object
> is simply to create a solid sandwich.
>
> Short of that, I've also taken 5/16" all thread rod and bent it into a "U".
> Both ends stick through the roof and hold a foot in place, with the U going
> around a rafter. (I was on a wood roof.) This particular construction
> survived some pretty good midwestern thunderstorms, but was not subjected to
> any hurricanes.
>
> I still haven't got the courage, or the engineering back up, to simply use
> surface screws for any panel installs. Joel had some stuff he sent around a
> while ago on some screw attached racks he had engineered for SMUD (I think).
> But then you're trusting that for 20 or more years the plywood doesn't
> degrade, and the screw material doesn't corrode, and that's not a great bet
> with small diameter fasteners. Bad enough with lags into 2x's!
>
> My 2 cents.
>
> Jeff Wolfe
> Global Resource Options
> Selling Solar and Working for Peace

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