Uplift Wind Loading Values [RE-wrenches]

[Bill Brooks]

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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