This article explores the corrosive nature of ACQ pressure treated lumber which has been commonly used since 2002 or so.  Special thanks to the engineers at Simpson Strong-Tie and the American Wood Protection Association. 


This bolt is only 5 years old



Hot Dipped














Galv Corroed Screws


Even though I will be looking at much of the same material, this article as well as this article have good synopsis of information about ACQ pressure treated wood.

The second half of this article is complicated.  It discusses chemical reactions between nails and a type of pressure-treated wood used in California shear walls called ACQ-B, an acronym for Alkaline Copper Quaternary. It is manufactured by injecting wood preservative into the wood under pressure.  It can be identified by 1/2 to 3/4-inch incision marks through which the preservative was injected.

In 2002 the pressure treated lumber industry switched to ACQ lumber. It became widely used because the previously used pressure treated wood had arsenic in it and people were getting sick.

There are three types of ACQ lumber, depending on how it is treated.  ACQ-B is used in the Bay Area because it effectively penetrates otherwise difficult-to-penetrate Douglas Fir.  In order to save you the tedium of reading a complicated description of the different types of ACQ and its chemical composition and reactions, I have listed below the practical things you need to know about ACQ pressure treated lumber.

Only use ACQ lumber in a shear wall if you use stainless steel nails in the mudsill.  The G-60 and G-90 hot-dipped coatings are commonly used, but based on observations of short-term corrosion, Simpson Strong-Tie suggests stepping up to the heavier G-185 coatings for hot-dipped galvanized products, Currently, G-185 is the best galvanized protection that you can buy. Simpson’s ZMAX line and USP Structural Connectors’ Triple Zinc line both are rated at G-185.  ZMAX is expensive and stainless steel is even more expensive, so it is much better to use borate pressure treated wood because it does not corrode nails or hardware.  The information about hot dipped coatings is only important if you must use hardware in existing ACQ lumber.

Borate, also called borax for those who remember the old western “Twenty Mule Teams” will remember soap commercials like this one, is safe.  It is found in laundry detergents, stain removers, all kinds of cleaning products, cosmetic creams placed directly on the skin, bath salts, and eye drops, and other household uses.  I don’t think you would want to use skin cream made with didecyldimethylammonium chloride, one of the ingredients of ACQ.

Because evidence given to me by the American Society of Home Inspectors has shown even bolts can also corrode in ACQ lumber, sill plates should ALWAYS be borate-treated lumber.

Water causes borates to dissolve, so don’t use borate lumber if it will get wet, such as decks.  ACQ is for wet applications.  Given shear walls and other earthquake resisting components are only used in dry locations there is no reason to consider ACQ at all.

That is all you need to know.  Follow this advice and that is all you need to do.

More photos are featured below.

If a 5/8-inch diameter bolt can look like this after a few years you can imagine what a smaller diameter nail will look like over time.  This bolt was galvanized to resist deterioration.

The Science Behind ACQ Lumber

The chemicals in ACQ enable lumber to resist decay and infestation from insects.  The main active ingredient in ACQ is soluble copper, which is one of the most cost-effective wood preservatives available.  Quaternary, normally truncated to Quat, is another biocide in the ACQ preservative and provides additional protection from fungi and insect attack that copper alone would not control. Alkaline simply means the pressure treated wood is alkaline as opposed to acidic. ACQ is made by injecting copper and quaternary aluminum into the wood under high pressure.  It is common to see short incisions in the wood which are made to inject the preservative as deeply into the wood as possible.  These incisions immediately identify the wood as being pressure-treated.

There are 12 different types of pressure treated wood, depending on the application.  These vary from wood used in children’s furniture to wood that can be embedded in mud and used as a foundation.  For our purposes, we are concerned with ACQ-B pressure treated lumber because it can be found in all builder supply yards in the San Francisco Bay Area and are the number one choice for mudsills.

Corrosion Caused by ACQ

There are 3 varieties of ACQ in common usage, types B, C, and D.  The copper in each of these varieties of ACQ causes corrosion to nails, though some to a greater degree than the others depending on three variables: the amount of copper injected into the wood, the chemical used to facilitate the injection process known as the carrier, (in the case of ACQ-B the carrier is ammonia), and the moisture content.  Moisture is the conductor or medium that allows molecules in the nails to migrate to the copper that has been injected into the lumber.  This migration causes the corrosion.  Obviously, corrosion can cause nails and performance of shear wall to degrade.

Contributing Factors in Deterioration

Retention level is also a consideration.  Retention level refers to the amount of chemical preservative that remains in the wood after pressure treatment.  The higher the retention level, the greater the tendency to degrade metal.  The amount retained is measured in Lbs. per cubic foot (pcf).

Besides the type of carrier and chemical retention, another factor is the presence of moisture in and around the ACQ lumber because galvanic corrosion can only occur when it has a conductor or electrolyte.  Water is a conductor.  Without this water, electrolyte corrosion cannot occur.  A water electrolyte is analogous to the water needed for a hair dryer to shock someone when it falls in the bathtub.

When galvanized fasteners and ferrous metals are in contact with each other and these three variables are present with ACQ-B lumber, corrosion occurs.  The higher the moisture level, such as with exterior decks, the faster the rate of galvanic corrosion.  In those areas not exposed directly to water, relative humidity has a significant impact on this third variable because of its effect on moisture content.  The higher the relative humidity, the higher the moisture content of the wood. According to the National Oceanic and Atmospheric Administration, the Bay Area has an average relative humidity of 84%, which is quite high.  Douglas Fir lumber in existing buildings in the East Bay has a high moisture content, averaging 13%; in San Francisco it is even higher.  This further exacerbates galvanic corrosion.


Using the Proper Fasteners

So which nail to use with ACQ-B lumber and plywood in retrofit shear walls?  It depends on the carrier, the preservative retention level, and the moisture content.
Various methods of combating this corrosion have been developed, the most common ones being the use of galvanized and stainless-steel nails.  In the table below, fasteners are rated in terms of their resistance to ACQ corrosion, from Low corrosion resistance to High resistance.
The table is from Simpson Strong-Tie’s technical bulletin Barrier Membrane and Pressure Treated Wood which recommends types of fasteners to be used in pressure treated lumber.  A barrier membrane is simply a metal coating such as zinc.
According to footnote 1, not visible in this image, the column “Higher Chemical Content” refers to any type of ACQ lumber with a retention rate of 0.4 or greater. According to this table, ACQ with a retention level of 0.4 or greater must use stainless steel nails no matter what the environment.  This includes virtually all of the ACQ lumber used for shear walls in the San Francisco Bay Area.  Moisture content of the wood is reflected in the Environment column to the far left which in turn relates to relative humidity.  The greater the relative humidity, the greater the tendency for nail corrosion.
 The only leeway can be found in the “ACQ with Ammonia” column, designated below with a green arrow.  In this case, Simpson’s ZMAX double hot dipped galvanized nails are approved for ACQ lumber so long as the lumber is in an interior dry location and has a retention level < 0.4.  Now, 0.25 and 0.4 retention levels are the only two retention levels available in San Francisco Bay Area lumber yards, and 0.25 is hard to find.  This means ALL shear walls built with ACQ lumber with an ammonia carrier and ammonia quat, no matter what the moisture and retention levels, should use stainless steel nails with the single exception of 0.25 ACQ used in an interior dry location.
In summary
1-ACQ-B lumber has a high concentration of ammonia.
2-Stainless steel nails should be used when ammonia is present.
3-The Bay Area has high relative humidity.
4-Readily available pressure treated lumber has a retention level of 0.4.
5-High relative humidity and ammonia promote corrosion of galvanized nails.
6-To avoid corrosion, stainless steel nails should be used in retrofit shear walls built with ACQ lumber.
7-Galvanized nails can be used in lumber with a retention level of 0.25 in a dry interior environment (such as a kitchen).
8-Retrofit shear walls are almost never built in interior dry environments with low relative humidity.
9-Even when built in interior environments, they still need stainless steel nails.
10-Shear walls using pressure treated wood should always use stainless-steel nails.
 In short, stainless-steel nails, regardless of the retention level, carrier, indoor, exterior, wet service, dry service- should be used with ACQ lumber except on the rare occasion that a shear wall is built in an interior-dry environment with a o.25 retention level.  If a shear wall is built in an interior dry environment, you should not need pressure treated lumber anyway. When looking at the table below, remember ACQ-B lumber always has ammonia. That is why it begins with the letter “A”

ACQ Top and Bottom Table

Practical Application of Stainless-Steel Nails in Residential Seismic Retrofits

Screenshot 2014-12-07 23.51.42

Prevalence of Stainless-Steel Nails in Shear Walls built with ACQ-B

After calling a few Bay Area lumber yards, I discovered that very few of them sold stainless steel fasteners suitable for shear wall construction except as a special-order item.  This is apparently because for many years after ACQ-B was introduced, the only recommendation by the pressure treating industry was to use galvanized nails.  This in turn influenced the Building Code to do the same.

Section 2304.9.5 of the 2001 and 2007 California Building Codes sums up their recommendation in one sentence.

“Fasteners for pressure-preservative treated wood shall be of hot-dipped galvanized steel, stainless-steel, silicon bronze or copper.”

This code section does not recognize other variables such as environment, retention, or carrier.

This means galvanized nails, which will almost always be the fastener of choice because of price, have been used in shear walls with ACQ-B pressure treated lumber on virtually every house built from 2004 until the present.

Section 2304.9.5.1 of the 2010 and 2013 California Building Code reflect a weak and minor change

“Fasteners in contact with preservative-treated wood shall be of hot-dipped zinc-coated galvanized steel, stainless-steel, silicon bronze or copper.  Connectors that are used in exterior applications and in contact with preservative treated wood shall have coating types and weights in accordance with the treated wood or connector manufacturer’s recommendations. ” 

In short, there is a very high likelihood shear walls built with pressure-treated lumber after 2004 will have a very high failure rate in the next big earthquake due to nail corrosion.  The use of galvanized nails has and will unfortunately undermine the life and safety of many people relying on shear walls to protect them.

Study of ACQ in New Zealand

The most exhaustive exploration of the effects of ACQ lumber on fasteners can be found in this Research Report funded by New Zealand’s equivalent of the International Code Council.

The photos below come from that report.  These galvanized fasteners were found to be in this condition after being used in exterior applications for 3 years.

Galv Nalls Corroded


Galv Corroed Screws



Hot Dipped















I have taken the liberty to redact those portions of this Research Report that do not apply to shear walls in the San Francisco Bay Area.

 “ACQ (alkaline copper quaternary) and CA (copper azole) water-borne preservatives are an acceptable treatment of exterior-grade timbers. However, their use necessitates achieving a significantly higher retained copper concentration than other pressure treated wood.  Simple electrochemistry suggests timbers containing a higher water-soluble copper concentration are more likely to initiate serious corrosion of susceptible metallic components embedded or in contact with these timbers. Previous laboratory-scale accelerated tests conducted by BRANZ and other research institutes confirmed the potential for an increased corrosion risk for mild steel and galvanized steel with CA and, particularly, ACQ treated timbers.

“This research investigated the corrosion performance of fasteners made from mild steel, galvanized (zinc) steel and stainless steel inserted into Pinus radiata treated ground contact ACQ using a three-year, non-accelerated field exposure testing scheme at two sites in Wellington, New Zealand.

“Testing results clearly showed that galvanized and mild steel fasteners embedded into timbers treated with ACQ and CA preservatives, particularly at the ground contact preservative level, are significantly more degraded through corrosion than other treated timbers such as CCA, which was banned in the United States in 1994 because of its arsenic ingredient. The corrosion acceleration of ACQ and CA treatment over CCA could be four and nine times for mild steel and zinc-coated steel after one year of exposure at Judgeford, respectively, and be around four times after three years of exposure at Oteranga Bay.”

The ACQ resulted in severe corrosion of mild steel and zinc-coated fasteners, leading to heavy iron stain on the timber after only three years. This also made the retrieval of fasteners, particularly screws, difficult. This causes a significant loss of strength and structural integrity. “Nail sickness” is a term which has long been used to describe this phenomenon.

The higher copper retention in ACQ and CA treatment is believed to be responsible for this significantly increased corrosive attack. However, it is unlikely to be the only mechanism operating.  It was also noted that timbers treated with ACQ, or CA had a higher moisture content than those treated with CCA under identical exposure conditions. These two differences may also affect metal deterioration to some extent.

Atmospheric corrosivity of a geological environment affects fastener corrosion. At Oteranga Bay, a severe marine environment represented a harsher atmosphere than Judgeford.  The exposed section of the fastener was attacked more quickly, leading to higher corrosion rates, thus shorter service life.  It is expected that within longer exposures, the influences of different climatic conditions on fastener performance will be demonstrated markedly since the decayed timbers will provide more pathways for ingress of airborne pollutants that can accelerate corrosion on the embedded sections.

It is quite certain that long-term durability cannot be achieved for mild steel and galvanized steel fasteners when inserted into these timbers. The extremely fast corrosion in the initial stage of exposure will severely damage the integrity of the coating and attack the underlying steel substrate.

Performance of the exposed head and the embedded body of a galvanized (zinc) fastener can be quite different, especially when it is embedded into ACQ or CA treated timbers exposed to a relatively benign climate such as Judgeford. After three years of exposure, the coating on the head of some nails still appeared to be fine, although surface oxidation was observed, while the coating on their shaft had been seriously damaged. This large performance difference between the head and the shaft makes the identification of any premature failure of fastener and/or timber joint very difficult.

Stainless-steel nails and screws did not show any sign of significant deterioration on their body section when embedded into CCA, CA and ACQ treated timbers even after three years of exposure at Oteranga Bay.

Based on this study, it is doubtful that zinc-coated fasteners, including hot dip galvanized nails and mechanically plated screws, will be able to meet the New Zealand Building Code.  The use of either AISI 304/316 grade of stainless steel, or durable equivalents such as silicon bronze, for structural components and connections, would appear to be justified.