Uplift Wind Loading Values [RE-wrenches]

[Matt Lafferty]

Keith asked:

is it possible to use self tapping screws into a corrugated roof that is
supported by z bar purling for pv racking support?  Or is it advisable
to drill the 90 degree angle iron that makes the z bar purling assembly
and have threaded rod through the roof? I feel this might be better to
limit the compression on the corrugation and the roof.  What is the wind
rating or perhaps a mechanical engineer would know how many self tappers
x width of screw or just go for threaded rod and sleep better!

All:

It seems to be the "method du jour" of late in California to use Tek
screws to fasten a PV Rack atop a corrugated metal roof with Z-purlins
as the sub-structure.  One installation currently underway that I know
of is literally on the beach and, therefore subject to the highest
Category of Wind Design.  (I'm not involved in the project, for the
record).

The attachment is made directly through the roof material and into the
Z-purlin.

I have looked at the "engineering" for the uplift calculations and am
left scratching my head.....Why is it this engineer uses such a low #/sf
uplift (approx 7 psf) when all the requirements I've read say 25 psf for
this Wind Category?  What will happen in a year or three?  Using a
single #12 Tek per 10 s.f. of glass to hold this all together?  Are you
sure you have E&O insurance, Mr. Engineer?

This is not the only example (quite a few as a matter of fact) of
shoddy/irresponsible engineering that I've seen applied to Roof
Attachment.  I digress from expounding on those at this time, but
suffice it to say, Installer Beware if you are relying on this kind of
crap!

I have further issues with this method....Keith is right.  Compression,
and the resulting tearing of the sheet metal around the screw is a leak
waiting to happen.  It's one thing if it's a barn (that doesn't make
leaks right, just less costly to repair) but it's an entirely different
issue if it's a finished ceiling below.

What about the black-eye that results when a publicly accessible and
viewable system "leaves the roof"?  The quality of labor and lack of
quality control I've seen installing in this manner has not demonstrated
to me, at least, a level of care that I would have representing me,
especially if I were relying on "low-ball" engineering to provide my 5
year warranty.  "Spinners", snapped screw heads (read this as ZERO
attachment), non-perpendicular screw alignment, etc.  My term for all of
these is "Air Screws".

I am with Keith.  Bolt the darned thing down.  Seal it well.  Sleep
tight!

-Matt Lafferty
mlafferty at universalenergies.com

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