Pressure treated lumber is lumber that has undergone a process that forces chemical preservatives into the wood. Wood is placed inside a closed cylinder, then vacuum and pressure are applied to force the preservatives into the wood. The preservatives help protect the wood from attack by termites, other insects, and fungal decay.
Type of Pressure Treatment[]
Waterborne, Creosote, and Oil-borne (penta) are the three broad classes of preservatives typically used when pressure-treating wood.
Wood 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.
Water-Borne Preservatives[]
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 (CA-B & CA-C, μCA-C) 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 wood.
Oil-Borne Preservatives[]
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 wood, but there is generally an added cost over water-borne treatments as well as availability in some regions.
Wood Types[]
Trees / logs from which commercial wood 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.
Wood 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 wood with small slits) may also be utilized to increase the penetration of preservative in heartwood species.
Fasteners and Connectors for use with Pressure treated lumber[]
Hot-dip galvanized or stainless steel fasteners, anchors and hardware are recommended by the Preservative Treated Wood 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 galvanized coating the longer the expected service life of the fastener, connector, anchor, or other hardware will be.
Electroplated / electro galvanized and mechanically galvanized coatings should not be considered to be hot-dip galvanized. (Class 55, or higher, mechanical galvanizing provides galvanizing equivalent to the hot-dip galvanizing used on connectors and fasteners. Ref. ASTM B695 for additional information.)
It is also worth noting that the galvanized coating thickness varies depending on the galvanizing process used. Remember, the thicker the galvanized 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.
All stainless steels may not be acceptable for use with preservative treated wood. Testing has shown that Types 304, 305 and 316 stainless steels perform very well with woods that hay have excess surface chemicals. Type 316 stainless steel contains slightly more nickel than other grades, plus 2-3% molybdenum, giving it better corrosion resistance in high chloride environments prone to cause pitting such as environments exposed to sea water.