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RE: Roof top Mech. Screen

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Rich,

I assume you meant to say you need to design a screen for a roof top HVAC
unit or exhaust blower or some other mechanical system?  And as an architect
is involved, I also assume this screen is to "hide" the unit instead of
"protect" it from winds or blowing snow or snow/ice sliding down the roof or
etc., and that this in not in the middle of a chemical plant or some
otherwise highly corrosive environment.  Not that all chemical plants have a
highly corrosive environment, but...

Anyway, any paint system will require repainting from time to time.  Hot
dipped galvanizing is pretty well maintenance free for many years.  If the
screen is intricate in design, you can weld it together and touchup a
painted system pretty well, but a galvanized system should be bolted
together for maximum corrosion resistance.  You can weld galvanized systems
and use a spray paint, "cold galv", on the welded areas, but in my
experience, they will eventually rust after some time and you will end up
with red streaks on the silver/gray galvanized surfaces.  A paint system can
match any color you want, but galvanizing is silver/gray and doesn't really
hold paint very well.  Besides, if you paint a galvanized surface you are
back to a maintenance issue for aesthetic reasons at least.  You didn't
mention whether this was a solid screen, alternating slats, expanded metal
screen, or what.  Galvanizing will warp structural members due to the
temperatures required and delicate designs do not always come out as nice as
they went in.  But you can galvanize the tubular steel "frame" and then bolt
in lighter weight screening material, aluminum or plastic perhaps.

There has been an issue with "low/no lead" galvanizers.  Some galvanizing
shops have lead amounts in their galvanizing that are unacceptable to some
companies with a lead abatement program, so this may be something else to
check on if you go with galvanizing.  And if you do just happen to be in the
middle of a chemical plant, then you do need to check the environment as
some materials can attack the galvanizing and painting ends up being a much
better corrosion protection system.

Michael E. Rissell, P.E.
Project Manager
Emerson - PC&E, Inc
mike.rissell(--nospam--at)pceinc.com

-----Original Message-----
From: richard lewis [mailto:rlewistx(--nospam--at)juno.com]
Sent: Friday, February 15, 2002 1:43 PM
To: seaint(--nospam--at)seaint.org
Subject: Roof top Mech. Screen


I have been asked to design a screen for roof top mechanical screen.  The
screen is about 4 feet high.  The architect prefers to use tube steel.
My question is about corrosion protection.  Should the exposed steel be
galvanized or just painted with a zinc-rich primer and finish coat?  I
know painting is cheaper.  What is the advantage and disadvantage of
each?

Rich
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