Recommended fastener materials for preservative-pressure-treated wood
The problem
Up until January 1, 2004, the most common chemical used
as a preservative for pressure-treated wood was CCA
(Chromated Copper Arsenate). CCA is no longer allowed as
a treatment for wood used as foundation sills (mudsills)
in residential construction. Some of the new chemicals
used for preservatives include ACZA (Ammoniacal Copper
Zinc Arsenate), CA (Copper Azole), CC (Copper Citrate),
ACQ (Ammoniacal Copper Quat). All of the preceding chemicals
bond with the wood cells, giving a "waterproof" treatment.
ACZA, CA, CC and ACQ are much more corrosive
than CCA because they contain more copper. Copper and
steel react chemically, and the steel corrodes. The building
codes require hot-dipped galvanized, stainless steel, silicon-bronze,
or copper fasteners for connecting treated wood. This would include
all fasteners that contact the treated wood, such as
sill anchors, framing clips, nails used to connect studs or
sheathing to the sills, anchor rods for shear wall tie-downs,
and so forth. The codes go on to say that you can use only steel or stainless
steel for structural connections (such as shear panel nailing).
The major hardware manufacturers (Simpson and USP)
recommend STAINLESS STEEL fasteners for connecting wood
treated with any of the above chemicals. Hot-dipped galvanized
fasteners, even "triple-dipped," are not adequate!
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The following is a quote from a preservative industry
document regarding corrosion: "It is generally recognized that the potential for fastener
corrosion in forest products based building materials used in an interior exposure
environment is minimal because the equilibrium moisture content of the wood
is maintained at a level that does not support corrosion reactions." Note that foundation
sills ("mudsills") are required to be treated wood specifically because they will have
elevated moisture levels due to moisture wicking up through the concrete from the ground. You
do not want your project or home to be a trial laboratory for testing fastener corrosion.
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A better alternative
Another class of treatment chemicals is Borates, such as DOT,
or Disodium Octaborate Tetrahydrate.
Of the current Borate treatments, DOT does not corrode
fasteners as much as CCA did, and therefore does not require
special fasteners. Borates will disolve in water, so
borate-treated wood must be protected from liquid water.
Borate-treated wood is perfectly acceptable for use in
foundation sills. Borates are not toxic to humans
(this is the same class of chemicals as borax that you
use to wash your clothes or hands, and is a major ingredient
in dishwasher detergent). Borates are much less harmful
to the environment than copper and arsenic-containing
wood treatments.
For more information on borates, visit or search for:
"Timbor" by US Borax
"Sillbor" by Arch Chemicals
"Advance Guard" by Osmose
EnviroSafe Plus, a product to help make DOT
more water resistant
More information on nails
Not all "galvanized" fasteners are created equal. Most nail gun
manufacturers use galvanized wire to make their "galvanized" nails.
The heads and points of such nails have no galvanizing. The head is the
last part of the nail that you want to corrode, yet nails manufactured
from galvanized wire have the least protection at their heads. If you
need nails that will last, you can get collated (gun) nails that are
hot-dip galvanized after manufacture. One manufacturer of such nails is
Maze Nails.
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ATTENTION!
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| The following is an excerpt from a letter that appeared in the July, 2009 issue of
Journal of Light Construction. This is a very grim report on the effects of ACQ lumber
on galvanized nails and anchor rods. Other contractors have had similar experiences.
shearwalls.com
recommends stainless steel fasteners for contact with lumber treated with anything besides borates.
"Hardware Corrosion From PT Lumber: Looking for Answers"
I've been a professional builder and contractor for 20 years and am glad somebody finally has enough
sense to develop a less corrosive treatment ("Pressure-Treated Wood: The Next Generation," Journal of
Light Construction, April, 2009). In 2005, I built my own house on San Juan Island here in Washington
State. ACQ lumber had not been on the market for long, and I'd already had some experiences with the extreme
corrosion it causes. Using lumber that corrodes metal made no sense to me...
During the framing of the house, I had to move a door opening; it was in a shear wall and had a big
hold-down (HD8A) next to it that needed to be moved, so it required engineering approval. I reframed the
doorway and installed a new epoxy-grouted anchor and hold-down. I cut off the existing anchor rod for the
old hold-down--a 7-8-inch diameter galvanized rod. The anchor rod had been in contact with the ACQ mudsill for about
two months--it was the rainy spring season, so it had been almost constantly wet--but already it looked like
it came from a 200-year-old shipwreck! There were no threads left where it passed through the mudsill, and I
estimated that the corrosion had eaten away 20 percent of the diameter. (Per Code, if a bolt is over 1/2-inch
in diameter, you don't even need galvanizing.)
Recently, after reading your article, I climbed into the crawlspace and pulled some toenails out of a kneewall.
The 10-penny nails had been driven through untreated 2x6 studs into ACQ-treated mudsills. The nails were
hot-dipped galvanized, as recommended at the time. What I found was very scary, since the shear strength
of a nail is a function of the diameter: These nails had maybe 25 percent of their diameter left.
Do I have some weird super-corrosive condition going on here, or are all the houses built since 2004 now
ticking time-bombs? Not only are the studs no longer effectively nailed to the mudsills, but the anchor-bolt
shear strength is shot, and the plywood nailing into the mudsills along the base of the shear walls is
worthless. When The Big One hits, all the engineering and effort to strengthen a house for earthquakes
is no better than the connection to the foundation--which, based on my observations of this house, will
be nonexistent in a few more years.
Does anybody have any answers for this? Is anyone in the industry really looking at the ACQ corrosion issue
as it concerns the multitude of houses, remodels, and decks that have been built using treated lumber
since it was forced down our throats?
Steve Mittendorf
Mittendorf Quality Construction
Seattle
For the full text of this letter, see:
HTML Version of Letter
or:
PDF Version of Letter
(Note: this file may only be available to JLC Online members...)
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