What’ll you have with that wood preservative: Hot-dip
galvanized, polymer coated, or stainless steel?
Deck building used to be simpler. At the lumberyard, you’d
load up on CCA-treated 2-by stock for the floor system, 6x6s for
the posts, and whatever the budget allowed for the decking —
anything from 1x6 pressure treated to more-expensive 1x4 Doug fir.
Buying hardware and fasteners was straightforward too. Inside the
lumberyard, you’d load up on nails, nuts, bolts, screws, and
maybe joist hangers. And you’d be good to go.
Buying lumber and fasteners is no longer so straightforward. Since
CCA was withdrawn from the residential market in 2004, new
preservatives have taken its place. The corrosiveness of some of
these chemicals has in turn spawned new types of
corrosion-resistant hardware, which have left deck builders
wondering which ones work best and if the best ones are worth the
A Little Chemistry
According to Dr. Pascal Kamdem, professor of wood science and
technology at Michigan State University, chromated copper arsenate
(CCA) pressure-treated wood was phased out because European
countries objected to the chromium, while concerns in the United
States centered around the arsenic. “Chemical companies
wanted a pressure-treating formula that would be acceptable
worldwide, so they got rid of both objectionable
One of the benefits, however, of the chromium and arsenic in CCA
was that they slowed the corrosion of fasteners and hardware.
Though the two main replacements for CCA — alkaline copper
quaternary (ACQ) and copper azole (CA) — work about as well
as their predecessor at resisting rot, problems with corrosion of
deck hardware surfaced early on (Figure 1). Not only do ACQ and CA
lack corrosion-inhibiting chromium and arsenic, they have much
higher concentrations of copper than CCA — not a bad thing to
ward off most fungal growth, but just awful if you happen to be a
less noble metal than copper. Also, some ACQ and CA formulas
contain ammonia; those are even more corrosive.
Figure 1. Many new wood treatment chemicals are
more corrosive than the old CCA. Hardware that once could be
depended on to last for decades now may degrade in only a few
The “retention level” of treatment chemicals is another
factor to look out for — wood that’s treated for ground
contact contains more preservative than wood that’s intended
for aboveground use, and therefore is more likely to cause
This chemical stew boils down to one thing: The pressure-treated
wood you’re using wants to eat your hardware for lunch. A
study done by Simpson Strong-Tie found that ACQ is twice as
corrosive to metals as CCA, and some other studies have found it to
be three times as corrosive.
Copper actually isn’t corrosive in a dry, inert environment,
but what deck is never wet? And when water contacts ACQ- or
CA-treated wood, it lifts copper ions from the preservative. As the
copper-tainted water seeps onto metal hardware, a galvanic reaction
causes the hardware to corrode. Without revisiting high school
chemistry, suffice it to say that a galvanic reaction is good for
only one of the two metals in the mix — and in the case of
copper and steel, galvanized or not, the latter loses (Figure 2).
In other words, on your carefully built deck, the
copper-impregnated wood wins, the steel fasteners lose, and —
possibly — your deck falls down. To protect fasteners and
hardware from corrosion, manufacturers are taking several
Figure 2. In the presence of an electrolyte, the
copper in treated wood chemically attacks steel. This nail was
bright and shiny when placed in a salt solution with a piece of
copper. Twenty four hours later, the water is tinted and rust has
settled to the bottom of the container.
Galvanizing works sort of like the candy coating on a Tootsie Roll
pop. As you suck on the pop — no biting allowed — the
candy that surrounds the chocolate nugget in the center dissolves.
Once the candy coating is gone, it’s curtains for that
nugget. On galvanized-steel fasteners and hardware, zinc is like
the candy coating; it gets eaten away by the copper in treated
The way that corrosion resistance is measured on galvanized
products is to weigh them before and after submergence in a
corrosive medium to find out how much of the zinc has oxidized.
Because of the sacrificial nature of the zinc, thicker coatings
give longer-lasting protection to steel than thin ones.
The thinnest zinc layers are found on bright, shiny electroplated
hardware (Figure 3). The copper in pressure-treated wood will eat
through electroplated zinc almost as fast as you scarfed down that
bag of Doritos at lunch. Electroplating is a quick and inexpensive
process that does little more than keep the rust off the stuff
while it glints at you from the bins saying, “Buy me. Buy
Figure 3. Shiny, electroplated fasteners (top, in
photo) don’t hold up to the more-active copper in
preservatives such as ACQ and CA. Look for their dull-gray, hot-
dip galvanized cousins (bottom, in photo).
Steel fasteners that have been hot-dip galvanized, on the other
hand, are not shiny, or at least they aren’t by the time you
see them. Instead, they are a dull gray sharkskin color, and
sometimes the surface is rough, almost crusty. Rough and crusty is
good; it means the zinc layers are thicker.
Galvanizing can occur at different times in the manufacturing
process: Pre-coat dipping is done to sheet steel before it’s
cut and stamped into hardware — joist hangers, for instance.
And post-coating is done after the piece is formed, most often on
heavier-gauge steel with thicker edges. While it may seem as though
post-coating would do a better job covering the stamped-and-cut
edges, it’s not necessary on the light-gauge steel used for
joist hangers and the like (Figure 4). According to Ed Sutt,
manager of engineering, fastener development, at Simpson
Strong-Tie, “The zinc coating will chemically migrate to bare
steel and give protection to bare, thin edges.”
Figure 4. Framing hardware is stamped from
galvanized sheet, and the edges are raw steel. Galvanization offers
some protection to the edges, but it’s imperfect, as
evidenced by the light layer of rust on this joist
If you’ve ever tried to spin a nongalvanized or even an
electroplated nut onto a hot-dip bolt, you probably didn’t
succeed — because the hotdip coating increases the size of
the threads. To allow for the thickness of the coating, the threads
on the nuts are tapped oversize before they’re dipped. Only a
hot-dip nut will work on a hot-dip bolt (Figure 5).
Figure 5. Coated in molten zinc after
they’re threaded, hot-dip bolts grow to a larger diameter. To
accommodate this, hot-dip nuts are threaded oversize. Standard nuts
don’t fit hot-dip galvanized bolts.
A final note on zinc-galvanized fasteners and hardware:
Manufacturers won’t guarantee how long their products will
last in pressure-treated wood, for two reasons. The first is that
copper will never stop corroding the sacrificial zinc. It’s
always a one-way death match. Second, other environmental (meaning
chemical) conditions can accelerate the corrosion. Take two
identical decks — one on the sunny, south side of a house in
Nebraska and the other on the damp, shady side of a house on the
coast of Rhode Island, across the bay from an acid-manufacturing
plant — and you can guess which one will have the
longer-lasting hardware. In fact, the presence of lawn fertilizer
and pet urine can have a significant effect on longevity.
And if you do find fasteners with a so-called lifetime warranty,
read it carefully: It probably states that the manufacturer will
replace corroded fasteners or hardware for free. It would be cold
comfort to return to the site of a collapsed deck armed only with a
box of free bolts.
Organic Polymer Coatings
Hardware and fastener manufacturers have developed polymer coatings
as an alternative to hot-dip galvanizing to protect against
corrosion. Sold under a variety of brand names, such as USP’s
Gold Coat, GRK Fasteners’ Climatek, or Simpson
Strong-Tie’s Quik Guard and TufCote, these proprietary
formulas feature organic polymers that, unlike zinc, don’t
react chemically with copper ions and so don’t sacrifice
themselves (Figure 6). Instead, they provide protection by keeping
water — and dissolved copper — from touching the steel.
Think of the polymer coatings as being like the plasticized paper
wrapper that covers the Tootsie Roll pop; it keeps your saliva from
touching the candy coating.
Figure 6. An alternative to hot-dip
galvanization is to polymer-coat electro-galvanized steel. The
polymer on this connector keeps water off the steel, preventing the
galvanic reaction that causes corrosion.
Most polymer coatings are applied on top of a layer of galvanizing.
Manufacturers use the coatings because they are less expensive than
thicker and more numerous layers of zinc galvanizing, according to
wood science professor Dr. Pascal Kamdem, who has tested numerous
polymer coatings for USP, Phillips, The Home Depot, and Crown Bolt,
among others. He says, “The important thing to look for is
the number of layers of waterproof polymer coatings. More are
always better.” Citing a conflict of interest, Kamdem
refrained from revealing which coating held up best in his
university corrosion tests.
Polymer-coated screws have a few advantages over hot-dip galvanized
ones. Some have an especially slippery top coat to make them easier
to drive into wood. Also, most threaded fasteners are heat-treated
to increase their strength, but when they’re subsequently
hot-dip galvanized, the high temperature (around 860°F)
of the molten zinc in the galvanizing tank takes the temper out,
lessening their strength. Not being subjected to those extreme
temperatures means that polymer-coated decking screws and the new
style of polymer-coated, thinner-diameter, self-drilling lag screws
(Figure 7) — getting popular for attaching ledgers to rim
joists — retain their strength.
Figure 7. The new generation of thinner ledger
screws gain their strength from heat treatment. Hot-dip
galvanization would negate this, so most such screws are polymer
A final plus is that polymer coatings don’t tend to clog the
drive hole in the head of a screw. How many old-style galvanized
screws have you chucked after pulling them out of your nail bag to
find the Phillips slots clogged with a little ball of zinc?
Stainless Is Painless, Until You Have to Pay
No one denies that stainless steel is the gold standard for the
hardware and fasteners used in today’s pressure-treated
woods. Nickel and chromium are alloyed with steel to make stainless
steel resistant to corrosion throughout; thus it doesn’t
depend on a coating. Simpson Strong-Tie, a company that makes
various galvanized and polymer-coated — as well as stainless
steel — hardware and fasteners, defers to stainless as the
go-to material when you’re unsure about the wood you’re
using or when the fastener will be exposed to a harshly corrosive
environment, such as near an ocean. Their literature says:
“If you suspect this or are uncertain, use stainless
Easy to say when you’re selling the hardware, but not as easy
when you’re paying for it. Any manufacturer’s stainless
steel products will cost more than its coated-steel ones. For
instance, Titan Metal Werks, a fastener manufacturer, charges about
$25 for 400 of their ACQ-rated (zinc coated with a polymer topcoat)
#6-by-2-inch SplitStop screws. In contrast, the same size and
number of stainless screws costs about $66. Greg Greenlee, director
of engineering at USP, says, “Depending on the hardware or
fastener, stainless steel can cost five to ten times as much as
Simpson’s Ed Sutt says that while stainless steel does cost
more, the price may be worth it. “When you consider the total
cost of a deck,” he says, “the hardware, even when
it’s stainless, is a small percentage.”
Don’t Mix and Match
Mixing and matching metals and coatings is not a good idea. If you
choose hot-dip zinc-galvanized connectors, use fasteners and
hardware with the same coatings. The same goes for polymer coatings
or stainless steel. If you use one type with another, there’s
a possibility the different metals will start their own corrosive
reactions — the exact thing you’re trying to prevent.
For instance, because stainless steel will degrade galvanized
steel, especially in a marine environment, don’t install
galvanized joist hangers with stainless steel nails.
Greenlee recommends buying the top-rated hardware for the job at
hand, regardless of whether you’re using different types of
pressure-treated wood on a deck. “If all your fasteners can
handle ground-contact wood (the most corrosive of all types because
it has the highest levels of copper), it won’t matter if you
use them on less-corrosive material, even if it’s a little
overkill.” Still, how do you know what the wood is rated for
and whether a fastener is compatible?
Manufacturers of the new chemicals and coatings are required to
label their products. All pieces of pressure-treated wood should
have a label (usually stapled to the end grain) that indicates its
intended usage as well as the chemicals used as the preservative.
And the same goes for the hardware and fasteners. Manufacturers
with proprietary brand names for their coatings label their boxes
with statements about the corrosion resistance of their
Unfortunately, you need to be careful about what information you
rely on. Glenn Mathewson, a former deck builder who is now a
building inspector in Westminster, Colo., tells a story of a
builder who used lumber treated with a micronized-copper
preservative. Micronized copper uses a less soluble form of the
metal that isn’t supposed to leach out of the wood, and is
therefore said to be less corrosive. The wood-treating company told
the builder he could use hardware that was G-90 galvanized, instead
of G-185. Mathewson balked: “When you’re trying to
decide if a certain piece of metal will withstand the chemicals in
a certain type of pressure-treated wood, look at what the hardware
manufacturer states, not what the wood treater says.”
Codes and Standards
For years, hardware and fastener makers have been diligent about
testing their products for structural integrity (Figure 8). Greg
Lindsay, general manager at Portland Bolt in Portland, Ore., says,
“It’s real unlikely that you’d ever find
fasteners out in today’s market that wouldn’t pass a
Trust the hardware manufacturer, not the wood treater, to provide
information on whether a product will work with a particular
preservative. Hardware manufacturers continually perform
accelerated weathering tests to ensure durability.
And now there are criteria for corrosion resistance as well. The
thickness of a galvanized coating is measured by an American
Society for Testing and Materials (ASTM) standard, based on the
amount of zinc by weight that coats the steel.
For ACQ- and CA-treated wood, you need hardware with a G-185
coating (Figure 9), which signifies 1.85 ounces of zinc per square
foot. (The measurement “1.85 ounces per square foot” is
somewhat misleading. It actually applies to a 1-foot-square piece
of steel sheet, which of course has two faces, meaning that half
that amount of zinc is on each side.) Metal meeting past standards
of G-90 and lower just won’t stand up to the corrosiveness of
ACQ and other chemicals.
Figure 9. On galvanized steel, copper attacks the
zinc coating instead of the underlying steel, but this only buys
time. Eventually the zinc will be used up and the steel exposed.
Thicker zinc coatings last longer and are indicated on hardware
labels by the phrase G-185.
Fasteners also are subject to standards regarding thickness,
adhesion, and finish of galvanized coatings. The applicable
standards for fasteners are ASTM A153 or the newer ASTM
The International Code Council-Evaluation Services (ICC-ES)
develops tests for building products’ code compliance, which
are then administered by ICC-accredited, third-party testing
laboratories. Manufacturers can now have their products
corrosion-tested based on ICC-ES AC-257: Acceptance criteria
for corrosion-resistant fasteners and evaluation of corrosion
effects of wood treatment chemicals. It’s a sort of
umbrella standard for fasteners and hardware that are meant to be
used with treated lumber.
AC-257 references the ASTM A153 standard as a benchmark. So, if a
fastener meets ASTM A153, it also satisfies AC-257 and the local
inspector should accept it. Several manufacturers have already
gotten approvals and will usually mention it on their Web sites and
in their literature.
Michael O’Reardon, a regional manager for ICC-ES, says,
“By the middle of summer ’09, you’ll be able to
go to www.icc-es.com and search by the hardware company’s
name to see which of its products have an Evaluation Service Report
that refers to AC-257.”
But what about the bins of nuts, screws, and bolts at the
lumberyard with no specific brand name? When it comes to galvanized
fasteners, you know to avoid anything shiny (because it might be
just electroplated zinc) and to look for the dull, gray, crusty
fasteners. To be sure, Mathewson says to ask a salesperson because
“the fastener should have come in some kind of packaging that
will say whether it has a zinc coating in accordance with ASTM
It’s likely that your building department is going to be a
stickler about the metal you want to use to hold your deck
together. On the question of whether a piece of hardware will pass
code, Mathewson’s points out that it’s up to your
building inspector. “It doesn’t matter what the
pressure-treating wood company says, and it doesn’t matter
what the hardware maker says. The codes all say that ‘final
acceptance is up to the building official.’”
Jefferson Kolle is a writer in Bethel, Conn.,
and a former professional carpenter.