From the Gold Coat FAQ:
"In the past few years, the manufactures of wood preservative chemicals
changed the formulations they use for most residential and commercial
applications. New formulations, including ACQ and copper azole, contain
more copper than the historically used CCA. Because dissimilar metals will
react with each other, the increase in copper content has created a more
corrosive environment for the steel in the structural connectors and
After some googling, it sounds like stainless is the best solution,
followed by 'hot dipped' fasteners (and I've actually got some hot dipped
staples on the shelf which do indeed say they're OK for PT-lumber).
I'll have to see if any of the local supply places have something suitable
later on today...
(some sites do seem to suggest that the corrosion's such a slow process
that it's not a big deal to use 'regular' fasteners in anything
non-structural, and to just replace as/when/if needed, but I think I'd
rather do things properly from the outset)
It's not always a slow process. I've seen drywall screws go in a year
or two. Of course, using drywall screws in lumber is dumb, but lots of
people do it. I've seen regular screws that didn't last but a few years.
And the regular screws stain the wood.
We just replaced the aluminum cladding on several 6x6 porch posts for a
customer. They were wrapped in the aluminum coil stock that is not painted on
the inside, just a mill finish. They had terrible blisters completely through
the cladding in less than 4 years.
"Gold" fasteners are usually cadmium plated and not galvanized.
Because they are 'plated' rather than 'dipped", they do not have
enough coating to avoid the corrosive effects of ACQ pressure treated
However MCQ pressure treated lumber is now on the market and is
claimed to be much better suited to standard exterior fasteners. As I
understand it, the "M" in MCQ stands for 'micronized" meaning that the
copper compound is much finer and penetrates the wood much better,
therefore they can use less of it. If you look at the wood it only
has a slight hint of green copper color vs the predominant green color
of ACQ. MCQ lumber is sold by Home Depot and "Yella Wood"
distributors. It has a 40yr warranty like ACQ & CCA PT lumber.
Aha. That sounds like the stuff I have; it's certainly a lot less green
than the P-T stuff I remember of a few years ago (enough that I was a
little skeptical about it actually being treated at all when I picked it
up due to the lack of any strong color ;)
According to the end-tags it's branded as MCA - Micronized Copper Azole
(I never realised until now that the tags have a whole bunch of info on
the reverse side - duh).
It still says to use hot-dipped galvanised fasteners, though. That's
interesting because it is indeed the same stuff that Home Depot sells -
yet I couldn't see any hot-dipped screws listed on HD's website (although
I've found that their website's a mine of misinformation before, so who
knows). There's a better supply place in town that I'm sure will do what I
need, but I might swing by HD too just to see what their advice is; it'd
be strange if they sell the wood but not the manufacturers' recommended
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