Pressure-Treated Timber FAQs

What is pressure treatment?

Pressure treatment is a process that forces chemical preservatives into the timber. Timber is placed inside a closed cylinder, then vacuum and pressure are applied to force the preservatives into the timber. The preservatives help protect the timber from attack by termites, other insects, and fungal decay.

What types of pressure treatments are used?

Waterborne, Creosote, and Oil-borne (penta) are the three broad classes of preservatives typically used when pressure-treating timber.

Timber treated with waterborne preservatives is typically used in residential, commercial and industrial building structures. Creosote is primarily used for treating railroad ties, guardrail posts, and timbers used in marine structures. Oil-borne (penta) is most often used for treating utility poles and cross arms.

Several typical waterborne preservatives used in building applications include: Chromated Copper Arsenate (CCA-C), Alkaline Copper Quat (ACQ-C, ACQ-D, ACQ-D Carbonate), Micronized Copper Quat (MCQ), Copper Azole (CBA-A & CA-B) and Sodium Borates (SBX/DOT).

These treatments are often referred to by trade names such as: Wolmanized Natural Select™ (Copper Azole), Preserve and NatureWood® (ACQ), MicroPro™, Smart Sense™ (MCQ), and Advance Guard® (Borate). Each preservative usually has a number of variations available so care should be exercised when specifying treated timber.

Some different oil-borne preservatives that are used are Chlorpyrifos/IBPC, Copper Naphthenate and Pentachlorphenol. One advantage of these treatments is that they do not create swelling in the timber, but there is generally an added cost over water-borne treatments as well as availability in some regions.

What does retention level mean and what retention level should I use?

Retention level refers to the amount of preservative that remains in the timber after the treatment process is complete. It is measured on a weight basis and is typically expressed as pounds of preservative per cubic foot (pcf) of timber.

The metric equivalent is kilograms per cubic meter (kg/m3)

There are a number of typical retention levels available. Generally, the harsher the condition the timber is exposed to, the higher the retention level must be.

For Above Ground use/exposures the following retentions are typical: (i.e. timber not in contact with soil)

0.25 pcf (4.0 kg/m3) for ACQ, CCA-C, MCQ
0.20 pcf (3.2 kg/m3) for CBA-A
0.10 pcf (1.6 kg/m3) for CA-B

For Ground Contact use / exposures the following retentions are typical: (i.e. timber in contact with soil)

0.40 pcf (6.4 kg/m3) for ACQ, CCA-C, MCQ
0.41 pcf (6.56 kg/m3) for CBA-A
0.21 pcf (3.36 kg/m3) for CA-B

For timber with actual retention levels greater than 0.40 pcf (6.4 kg/m3) for ACQ MCQ, 0.41 pcf (6.56 kg/m3) for CBA-A, or 0.21 pcf (3.36 kg/m3) for CA-B (Ground Contact), Stainless Steel connectors and fasteners are recommended. Verify actual retention level with the timber treater.

For retention levels at or below ground contact use, consult our Anchoring and Fastening Systems Catalogue.

See treated timber industry web sites for other use/exposure categories and retention levels.

What types of timber can be pressure treated and what are the differences in corrosion rates?

Trees / logs from which commercial timber is cut have a number of different layers. The two primary layers are called heartwood and sapwood. Heartwood provides most of the "structural" strength to the living tree while the sapwood transports the sap from the base of the tree up to the leaves.

Timber preservatives penetrate sapwood easier than heartwood. As a result, wood species such as Southern Pine, which have a high percentage of sapwood, are predominately used in pressure treating.

Wood species such as Douglas Fir have more heartwood so modifications are typically required to the preservative to achieve adequate penetration and retention levels. The modification that is usually made is to change the "carrier" used in the preservatives. Often this carrier uses an ammonia base, which improves the penetration but also tends to increase the corrosivity of the preservative. (The carrier used to treat sapwood species usually has an amine base.) This increase in corrosivity may be short term or long term. Hybrid carriers, a mix of amine and ammonia bases, may also be used to treat heartwood species.

Incising (perforating the timber with small slits) may also be utilized to increase the penetration of preservative in heartwood species.

Tell me a little about CCA.

Chromated Copper Arsenate (CCA) has been used successfully for a number of decades for pressure treating timber. Several types of CCA have been used, however, CCA-C (type C) has been the predominant preservative used for timber likely to come in contact with the products Simpson manufactures.

Why was the use of CCA discontinued for residential and general consumer use?

In recent years pressure treated timber received negative publicity mainly focused on the use of arsenic in CCA. The increasing pressure to eliminate the use of CCA resulted in the treated wood products industry voluntarily transitioning from CCA to alternative preservative systems.

CCA is no longer being produced for residential or general consumer use.

What products are still manufactured using CCA?

CCA treated timber products are still produced for use in some industrial, highway, and agricultural applications. These uses will include timber used as poles, piles, guardrail posts, and timber used in saltwater marine exposures.

What products took the place of CCA-C?

A number of alternative preservatives are available. These include ACQ-C (Alkaline Copper Quat Type C), ACQ-D Carbonate (Alkaline Copper Quat Type D, Carbonate formulation), CBA-A and CA-B (Copper Azole Types A and B), as well as SBX/DOT (Sodium Borate) and Zinc Borate preservatives. As mentioned earlier, each preservative usually has a number of variations available so care should be exercised when specifying treated timber.

It is expected that the formulations used in these products will continue to undergo periodic modifications so once again, care should be exercised when specifying any treated timber.

Are these alternative products more corrosive than CCA-C?

Testing has indicated that some of the alternative products are more corrosive to steel and some protective coatings applied over steel than Chromated Copper Arsenate (CCA-C). Contact the treated timber chemical supplier for more information and see the Preservative Treated Timber Technical Bulletin (PDF).

What metals and protective coatings does the Pressure Treated Timber Industry recommend for use with these newer products?

Hot-dip galvanised or stainless steel fasteners, anchors and hardware are recommended by the Pressure Treated Timber Industry for use with treated wood. This has been the position of this industry for years and their position has not changed with the transition to the alternative copper-based products. In the past this industry did not address the required levels of galvanizing, however most of those in the industry now provide information regarding the minimum level of galvanizing that should be used.

The thicker the galvanised coating the longer the expected service life of the fastener, connector, anchor, or other hardware will be.

Electroplated / electro galvanised and mechanically galvanised coatings should not be considered to be hot-dip galvanised. (Class 55, or higher, mechanical galvanizing provides galvanizing equivalent to the hot-dip galvanizing used on timber connectors and fasteners. Ref. ASTM B695 for additional information.)

It is also worth noting that the galvanised coating thickness varies depending on the galvanizing process used. Remember, the thicker the galvanised coating, the longer the expected service life of the steel will be.

Refer to the different chemical manufacturers and/or treaters as well for their recommendations. A list of trade names is included at the bottom of this page.

Are all stainless steels acceptable for use with pressure-treated timber?

All stainless steels may not be acceptable for use with pressure treated timber. Testing has shown that Types 304 and 316 stainless steels perform very well with CCA-C, ACQ-C, ACQ-D Carbonate, CBA-A, and CA-B treated timbers. Type 316 stainless steel contains slightly more nickel than other grades, plus 2-4% molybdenum, giving it better corrosion resistance in high chloride environments prone to cause pitting such as environments exposed to sea water.

What is hot-dip galvanizing?

Hot-dip galvanizing is a process of providing a protective coating (zinc) over bare steel. The bare steel is cleaned, pickled, fluxed, then dipped in a molten bath of zinc and allowed to cool prior to inspection and shipping. Additional information is available at www.galvinfo.com. Some anchors and fasteners can be hot-dip galvanised. Steel connectors can be hot-dip galvanised (See below: "What is the difference between Simpson's Hot-Dip galvanised (HDG) products and products that are hot-dip galvanised after fabrication?" for additional information.)

Terms such as G90 & G185 reflect the galvanised coating thickness that meet the ASTM A653 specification for sheet steel.

What is Mechanical Galvanizing?

Mechanical galvanizing is a process of providing a protective coating (zinc) over bare steel. The bare steel is cleaned and loaded into a tumbler containing non-metallic impact beads and zinc powder. As the tumbler is spun, the zinc powder mechanically adheres to the parts. The zinc coating has "good" durability, but has less abrasion resistance than hot-dip galvanised zinc coatings since it does not metalurgically bond with the steel. Some anchors and fasteners can be mechanically galvanised.

What is ZMAX®?

ZMAX® is a thicker coating (galvanised per ASTM A653 with a G185 coating) than the standard hot-dip galvanised coating (galvanised per ASTM A653 with a G90 coating). This thicker coating is available on select Simpson Strong-Tie connectors. Due to this increased galvanised thickness these connectors would be expected to have a longer service life than standard galvanised connectors. See our flier F-3FINISHES (PDF) for additional information.

What is the difference between Simpson's Hot-Dip galvanised (HDG) products and products that are hot-dip galvanised after fabrication?

There are two processes used to hot-dip galvanize parts: (1) "continuous" hot-dip galvanizing ("Continuous HDG") and (2) "batch" or "post" hot-dip galvanizing ("Batch/Post HDG").

Continuous HDG is generally used to galvanize steel coils at various speeds (up to 183 meters per minute) on thinner gauge steels (galvanised per ASTM A653). Simpson uses this type of steel to fabricate most thinner gauge products.

Batch/Post HDG is generally used to galvanize heavier individual parts not capable of being fabricated from galvanised steel coils (galvanised per ASTM A123 for connectors and ASTM A153 for fasteners). After being fabricated, the parts are dipped into molten zinc for a longer period of time than Continuous HDG, usually resulting in a thicker coating and more protection against corrosion. Batch/Post HDG can be used on Simpson connectors, such as CC and CB column caps and bases and thicker gauge hangers(14 gauge and thicker).

What products does Simpson have available in stainless steel, ZMAX®, and hot-dip galvanised after fabrication (Batch/Post HDG)?

Simpson has a broad selection of these products. Download our flier F-3FINISHES (PDF). For availability, contact your local Simpson Strong-Tie representative, or call the Sydney office at 02 98317700

What Simpson products should I use with the new alternative pressure-treated timbers?

Stainless Steel is always the most effective solution to corrosion risk. However, it is also more expensive and sometimes more difficult to obtain. To best serve our customers, Simpson is evaluating the options to identify the safest and most cost-effective solutions. Based on our testing and experience there are some specific applications that are appropriate for ZMAX®/HDG or G90 connectors (see page 22 of the Anchoring and Fastening Systems Catalogue to select the appropriate connector for your situation.) Click here for recommendations for Quik Drive fasteners.

See the Preservative Treated Timber Technical Bulletin (PDF).

What is a fastener?

When the term "fastener" is used it is typically referring to a nail, screw, bolt, or anchor.

What is a connector?

When the term "connector" is used it is typically referring to a manufactured device that is used to connect two or more timber members together, or to attach a timber member to concrete or concrete masonry. Fasteners are used in conjunction with the connector to effectively join the members.

Joist hangers, post bases, hurricane ties, and mudsill anchors are all examples of a connector.

Can I use galvanised fasteners with stainless steel connectors?

No. You should not use galvanised fasteners with stainless steel connectors. Stainless steel fasteners should be used with stainless steel connectors. Galvanizing (zinc) and stainless steel are considered to be dissimilar metals which can cause the zinc to corrode when placed in contact with the stainless steel. This would result in the galvanised fasteners losing their protective coating faster than expected.

Hot-dip galvanised and stainless steel components should not be placed in contact with each other.

What fasteners should I use with pressure-treated timber?

The Treated Timber Industry recommends the use of hot-dip galvanised or stainless steel fasteners for use with treated timber. (See above: "What metals and protective coatings does the Pressure Treated Wood Industry recommend for use with the "new generation / alternative" products?" for additional information.) You should not mix the use of hot-dip and stainless steel fasteners and connectors. Click here for recommendations for Quik Drive fasteners.

Hot-Dip galvanised (HDG) or double-barrier coat fasteners must be used with ZMAX® and HDG connectors. If hot-dip galvanised lag screws are used in connectors instead of the required galvanised Simpson Strong-Drive screws, contact Simpson for applicable load reductions.

Refer to the different chemical manufacturers and/or treaters as well for their recommendations. A list of trade names is included at the bottom of this page.

How long will my fasteners, connectors, and anchors last when used in pressure-treated timber?

Many environmental and other factors affect the service life of fasteners, connectors, and anchors. The test standards do not enable the test results to be translated into expected service life information. It is impossible to accurately predict the service life of any specific installation.

At the time of installation my pressure-treated timber was installed dry (moisture content less than 19%) and it was installed in a condition where it will remain dry throughout its service life. What connector coating finish or material should I use?

Simpson's standard G90 galvanised connectors may be used with the following timber treatments: ACQ-C, ACQ-D (Carbonate), CBA-A, CA-B, SBX/DOT, and Zinc Borate if the timber was installed dry and will remain dry throughout its service life. If the chemical retention level in the treated timber exceeds the Ground Contact level then Stainless Steel connectors are recommended. (Ground Contact level is: 0.40 pcf (6.4 kg/m3) for ACQ, 0.41 pcf (6.56 kg/m3) for CBA-A, and .21 pcf (3.36 kg/m3) for CA-B). Verify the actual retention level with the timber treater. SBX/DOT and Zinc Borate treated timbers are less corrosive than CCA-C treated timber -- as a result the retention level of these timbers is not of much concern and standard G90 galvanised connectors can be used.

I have pressure-treated mudsills. Should I be using stainless steel anchor bolts? What about hot-dip galvanised?

The mudsill is a location that is considered dry in comparison to a deck, for example. For timber that is installed and remains dry, the corrosion potential will be comparatively low. Regarding code issues, section R319.3 of the 2006 International Residential Code (IRC) addresses fasteners for pressure-, preservative-, and fire-retardant-treated timber; Bolts of 1/2" (12.7 mm) and greater do not need to be hot-dip galvanised steel, stainless steel, silicon bronze or copper.

What are these white deposits on my connectors and are they bad?

The white powdery substance which can form on the surface of a connector is commonly referred to as "white rust" or wet storage stain. This presence of this substance on the connector can vary from unsightly to problematic. Simpson connectors are coated with zinc to help protect against base metal corrosion (G90 has 0.90 oz/ft2 [0.27 kg/m2] and G185 has 1.85 oz/ft2 [0.54 kg/m2]). The nature of zinc, as a metal, is that it is relatively reactive, insofar as oxide layers form on the surface, some of them quickly. The formation of these oxide layers is a byproduct of the environment in which the connector is placed. These oxide layers often are seen in the form of white deposits or white rust on the surface of the connector. White rust may be caused by closely packed, newly-galvanised products that are stored in damp or poorly ventilated conditions. If, after steel manufacturing, the steel is coiled while still wet, a zinc hydroxide layer will form that may prevent other protective oxide films from forming on the surface. The time of the year can also have an impact on "white rust" appearance. It is also possible that the surface irregularities associated with the thicker G185 coating might hold more moisture to the surface of the steel and show more white rust. The extent of film coverage is a factor of the exposure to moisture, the temperature during storage and the presence of accelerating corrodents. White rust can exist without contact with pressure treated timber, but the problem may be exacerbated by the chemicals in the timber.

It is possible that other white substances can develop on the surface of connectors. Some environments such as those with high chloride/salt content can form white deposits. Appearance aside, it is unlikely white rust has any negative impact on the protection that zinc provides.

For more information on this topic, refer to the link below:

What about applying a barrier or epoxy around the anchor bolts to prevent corrosion?

This is an application that Simpson cannot comment on, as it is unclear whether or not this is suitable.

What should I do if I have existing fasteners and connectors installed in contact with preservative treated timber that is not CCA-C?

If possible, you should verify the specific type of preservative treated timber you have - including the chemicals used, retention level and the wood species. Much of this information is often on a tag stapled to the end of the boards, or may be ink stamped on the boards.

You should also perform periodic inspection of your connectors, fasteners, and anchors to ensure their strength is not being adversely affected by corrosion. It may be necessary to have a local professional perform the inspections.

The treated timber industry has recommended the use of stainless steel and hot-dip galvanised (HDG) fasteners and connectors for years. Their recommendation is still the same for use with the "alternative" treatments that are replacing CCA-C. (See above: "What is the difference between Simpson's Hot-Dip galvanised (HDG) products and products that are hot-dip galvanised after fabrication?" for an explanation on the different types of hot-dip galvanizing.)

If your fasteners, connectors, or other items in contact with the treated timber do not follow the treated wood industry recommendations and/or Simpson's recommendations then you should seriously consider replacing them. A local professional civil or structural engineer should be used to provide the necessary guidance when replacing these types of structural items.

What experience has Simpson had with the alternative treatments that are replacing CCA-C?

Simpson first became aware of some CCA-C alternatives being used when we received reports of increased connector and fastener corrosion when installed with some treated timbers. These reports led us to explore the phenomena and expand our corrosion warning in our catalog to include specific language related to treated timber.

As time progressed and the public concern over the safety of CCA-C intensified, the treated timber industry decided to voluntarily phase out CCA-C.

Simpson has tested more than 1,800 samples of various combinations of steels, coatings, preservative treatments, wood species, and over 3,000 fasteners to try to better understand the issues. Using the American Wood-Preservers Association test standard E12-94 for many of these tests, our results indicate there are some significant differences between the corrosion rates of CCA-C and some of the "alternative" products. The E12-94 test standard is an accelerated test method. The test results may or may not have a relation to actual service life. It is worth noting that many variables can affect the test results, including the characteristics of the timber as well as a number of chemical additives that can be added to the treatments.

The connector industry's standard hot-dip galvanised specification of G60 has been used successfully with CCA for several decades. This level of galvanization now needs to be increased as a result of the use of newer "alternative" replacements for CCA-C. Generally, the thicker the hot-dip galvanised coating is, the longer the expected service life of the connector or fastener should be. We offer many products in ZMAX®, hot-dip galvanised (HDG), mechanically galvanised and stainless steel for use with the treated timbers of today. (ZMAX® is our branded name for continuous hot-dip galvanised steel with a G185 specification that provides more than three times the zinc of the industry standard G60 galvanised coating.)

The suitability of fasteners and connectors is now not only dependant on the long list of environmental factors that always applied, but to a much greater extent than before, is also dependant on the base treating chemicals, additives, and wood species used. When anticipating the use of steel connectors and fasteners with treated lumber, users should consult our Preservative Treated Timber Technical Bulletin (PDF), the treated lumber chemical supplier, and the timber treater for specific recommendations.

Due to the many variables involved, many of which are controlled by the chemical supplier and the timber treater, Simpson cannot make an unqualified recommendation of any galvanised or other coating for use with treated timber. Additionally, because of the many variables involved, Simpson cannot provide estimates on service life of connectors, anchors or fasteners.

How often should I inspect the fasteners and connectors on my deck or other accessible exterior projects?

The rate of corrosion is dependent on many factors including environmental and project specific conditions. Your periodic inspections should be related to the severity of the exposure and other project-specific conditions.

How do I properly specify pressure treated timber?

At a minimum you should specify the wood species, the preservative, retention level and which options, if any, you require. (One option example would be to add a water repellant to the preservative being used.)

You should also talk with the treater to determine what "carrier / base", and what other additives are being used so you can assess the corrosive potential of the treated timber. Formulations with an ammonia carrier may be more corrosive than those with an amine carrier. See above, What types of timber can be pressure treated and what are the differences in corrosion rates? for more information.

Once you know the specific treatment parameters that are being used, then you need to ensure you have specified appropriate fasteners, connectors, and other hardware.

Can I use "barriers" between my Simpson connectors and pressure treated timber to prevent corrosion?

See our technical bulletin on the use of barriers, Barrier Membranes and Preservative Treated Timber (PDF).

What testing has Simpson performed?

See the Preservative Treated Timber Technical Bulletin (PDF) and Barrier Membranes and Preservative Treated Timber Technical Bulletin (PDF).

What is the difference between Salt Spray corrosion testing and Treated Timber corrosion testing?

These two types of tests use entirely different test standards and test environments. Salt spray tests are intended to provide relative corrosion information of items exposed to exterior environments and use salt solutions along with elevated temperature to accelerate corrosion rates. Treated timber tests are intended to provide relative corrosion information of items in contact with preservative treated timbers and typically use humidity and elevated temperature to accelerate corrosion rates. Neither test can be translated to determine the expected service life of a specific installation.

There is no apparent correlation between Salt Spray and Treated Wood corrosion tests. The fact that one item performs well in one of these two tests does not necessarily mean that it will perform well in the other test.

What information can Simpson Strong-Tie share about claims that some newer treated timber chemical formulations are less corrosive than some currently available products?

Simpson Strong-Tie periodically conducts testing of various preservative treated timber materials which come into contact with our structural connectors and fasteners. With these results, information is conveyed through catalogs, technical bulletins, fliers and training sessions. Simpson will continue to test materials until treated wood and potential corrosion problems are resolved. As always, users should educate themselves by consulting with the chemical manufacturer of the product and other sources about claims made about the viabilty of a particular product.

Is it safe to use G90 connectors with Micronized Copper Quatenary (MCQ™ or Smart Sense-MicroPro™) pressure-treated timber?

MCQ is one of the newer pressure-treated timber formulations on the market, which claims to be less corrosive to metal connectors and fasteners.

Simpson Strong-Tie tested Smart Sense-MicroPro (MCQ) and ACQ CarboQuat pressure-treated timbers (0.25 pcf [0.4 kg/m3] and .40 pcf [0.64 kg/m3] retention levels were used for both treated timbers) to measure corrosion rates. Simpson Strong-Tie followed the general requirements of the AWPA E12 test criteria with minor exceptions. Simpson went beyond the minimum 10-day requirement established by AWPA E12 and took measurements of G90 and G185 coupons at 30, 60, 90 and 120 days. (Note: This is our standard protocol for corrosion testing and provides additional data points to better establish the trend in corrosion rates.)

Our test results indicated that the potential corrosion rate of Smart Sense-MicroPro (MCQ) treated timber is somewhat less than the ACQ timber that was tested. However, the MCQ corrosion level was not reduced enough to warrant recommending the use of G90 in all applications.

Therefore, for MCQ treated lumber, our minimum recommendation is to use G90 for Interior Dry Applications where the timber is dry (moisture content less than 19%) when installed and will remain dry in-service. For Exterior Applications, wet or dry (that are not Higher Exposure), Simpson recommends ZMAX®/HDG connectors as a minimum. For Higher Exposure environments, such as along coastlines, Simpson recommends Type 303, 304, 305 or 316 Stainless Steel connectors and fasteners. (Note: When using Stainless Steel connectors, use Stainless Steel fasteners. When using ZMAX®/HDG connectors, use fasteners galvanised per ASTM A153.)

See our Quik Drive Fastener Minimum Coating or Material Recommendation page for additional information.

Note: Links to third party sites are provided as a convenience. Simpson Strong-Tie Australia PTY. Limited does not review or control the linked sites and these links do not constitute any endorsement or assumption of responsibility for the contents of these sites.

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