Pressure-Treated Wood Preservatives 101
Wood can be subject to attack by insects, microorganisms, and decay fungi, especially in high-moisture conditions. That’s why many builders opt for pressure-treated wood with preservatives to help ensure long-term structural performance by protecting lumber used in a wide range of service conditions. Pressure-treated Southern Pine wood products are an economical and renewable building material.
There is a variety of processes used to protect wood products, including pressure, non-pressure, thermal, nonbiocidal, and fire-retardant treatments. There is also a variety of preservatives used for each treatment process. The best preservative for any project largely depends upon the product, species, and end-use application.
Pressure treatment with preservatives protects wood exposed to high-moisture conditions and potential deterioration. Treated wood must meet minimum requirements for preservative penetration and retention for use in a particular service condition.
But not all wood treats the same. Most wood species do not easily accept preservatives and must first be “incised,” or perforated along the surface, reducing the wood’s strength and stiffness.
Southern Pine’s unique cellular structure, meanwhile, easily permits the deep penetration of preservatives, rendering the wood useless as a food source for fungi, termites, and micro-organisms without the need to incise. Its superior treatability has made it the preferred species when pressure treatment with wood preservatives is required.
TYPES OF WOOD PRESERVATIVES
There are three broad classifications of preservatives used to pressure treat wood products:
- Waterborne preservatives serve a variety of uses including residential, commercial, marine, agricultural, recreational, and industrial applications. They are most often specified because they are clean in appearance, odorless, paintable, and registered with the Environmental Protection Agency (EPA) for interior and exterior use without a sealer.
- Creosote preservatives, including creosote/coal tar mixtures, protect railroad ties, marine pilings, and utility poles.
- Oilborne preservatives are used primarily for utility crossarms and poles, piling, posts, and glulam.
Wood preservatives are registered with the EPA for their safe use. There are numerous preservatives available in the marketplace today, many approved by the American Wood Protection Association (AWPA) while others have ICC-ES Evaluation Reports (ESR). These preservatives can be expected to provide excellent protection for wood products when appropriate preservative retention levels are applied.
Waterborne Preservatives, Copper-Based
Copper has been used for centuries to control fungi. Copper-based preservatives rely on the well-established effectiveness of copper to provide most of the protection to the wood; co-biocides complete the formulations by adding protection against copper-tolerant fungi. Copper-based preservatives generally can be used in interior and exterior applications, even in the most severe conditions.
There are two main methods to prepare copper-based preservatives for pressure treatment:
- Dissolved copper-based preservative solutions are effective in penetrating the cells in the wood fiber during the pressure-treatment process and provide long-term protection from fungal decay and termite attack. They were developed to protect wood products exposed to exterior conditions.
- Copper Azole (CA) and Alkaline Copper Quat (ACQ) commonly are used to treat Southern Pine wood products in a variety of exterior residential, commercial, and agricultural construction applications.
- Chromated Copper Arsenate (CCA) is no longer used in consumer applications, but it remains an EPA-approved preservative for use in non-consumer industrial, commercial, marine, and agricultural applications.
- Typical applications for CCA-treated Southern Pine include:
- Agricultural timbers and poles
- Foundation pilings
- Highway construction
- Marine (brackish and saltwater)
- Permanent wood foundations
- Utility poles
- Micronized copper-based preservatives are formulated with micron-sized particles of copper suspended in water. They were developed to reduce the impact on aquatic and terrestrial environments and are less corrosive to fasteners. Micronized Copper Azole is commonly used to treat Southern Pine wood products in interior and exterior applications.
Waterborne Preservatives, Non-Copper-Based
Preservatives without copper, which were developed for use in less-severe service conditions, provide a lower environmental impact, a natural appearance, and are not corrosive.
Carbon-based preservatives (PTI and EL2) are approved for interior and exterior above-ground uses. This includes the bulk of exterior wood products used in residential construction – everything except ground-contact applications.
The most common borate preservative is inorganic boron (SBX). SBX is standardized by AWPA for interior uses, such as sill plates, furring strips, joists, studs, roof trusses, blocking, rafters, beams, and other framing applications. ESR-2667 provides acceptance for exterior construction, above-ground, coated, and rapid water runoff uses, such as fascia, siding, and trim, when continuously protected from direct wetting with a minimum of one coat of primer and two coats of finish paint.
Creosote is a tar oil distillate and one of the oldest wood preservatives. Creosote is an effective preservative for severe exposure conditions in commercial, industrial and marine applications, including marine borers in salt water. It is also commonly used by the railroad industry to treat crossties and switchties. Creosote is not generally used in consumer applications due to its odor and oily surface appearance. Creosote-based preservatives include Creosote Preservative (CR), Creosote Solution (CR-S) and Creosote-Petroleum Solution (CR-PS).
Oilborne preservatives include Pentachlorophenol (Penta), Copper Naphthenate and Oxine Copper. Penta is commonly used to treat utility poles; it is approved for above-ground and ground contact end uses but is not generally used in consumer applications because of its odor and oily surface appearance. Copper naphthenate is used to treat field cuts and holes per AWPA Standard M4; it is also used to treat glulam (after gluing) and utility poles.
Oxine copper is low in toxicity and approved for above-ground end uses, including glulam beams (after gluing) and certain applications where food is harvested, transported, or stored. Oxine copper can also be used to treat field cuts and holes per AWPA Standard M4.
WOOD PRESERVATIVES FOR LONG-TERM PERFORMANCE
Wood products are preservative treated to provide the required longevity for their intended use. This includes consideration of the following:
- Using the right fasteners and connectors for corrosion protection
- Re-drying after treatment with waterborne preservatives for dimensional stability
- Structural design, including using the latest design values and appropriate adjustment factors
- Onsite job storage for moisture protection
- Treatment for field-cut ends, notches, and drilled holes
- Proper installation, finishing, and maintenance
Fasteners and Connectors
Metal used in construction must be properly protected from corrosion. The potential for corrosion of fasteners, such as nails, screws, bolts, nuts, and washers, and connectors, including joist hangers, straps, hinges, post anchors, and truss plates, in contact with pressure-treated wood varies by preservative system and end-use exposure.
For example, wood treated with noncopper based waterborne preservatives, such as SBX, PTI and EL2, is less corrosive overall than wood treated with copper-based preservatives. Meanwhile, wood treated with micronized copper preservatives, such as MCA and μCA, is less corrosive overall than wood treated with dissolved copper preservatives, such as ACQ, CA and CCA.
It’s also worth noting that interior, dry environments are less conducive to corrosion than exterior, wet environments. Particularly severe environments include swimming pools, below-grade foundations, saltwater exposure, trapped water, and other environments that allow for constant, repetitive, or long periods of wet conditions.
It is important to follow specific recommendations from preservative and hardware manufacturers.
General guidance includes:
- Model building code requirements are provided in Section 2304.10.5.1 of the 2018 International Building Code® and Section R317.3 of the International Residential Code®.
- In general, the codes require fasteners to be of hot-dipped, zinc-coated galvanized steel in accordance with ASTM B695, type 304 or 316 stainless steel, silicon bronze, or copper.
- An exception allows the use of plain carbon-steel fasteners for wood treated with borates and used in an interior, dry environment.
- Connectors should conform with ASTM B695.
- Fasteners and connectors used together must be of the same metallic composition to avoid galvanic corrosion. For example, use hot-dip galvanized nails with hot-dip galvanized joist hangers and stainless-steel nails with stainless-steel joist hangers.
- Aluminum fasteners and aluminum building products, such as window and door trim, flashing, siding, roofing, and gutters, should not be placed in direct contact with wood products treated with dissolved copper preservatives. Direct contact with aluminum may be allowed for wood products treated with borates, carbon-based and micronized copper preservatives when used for interior and exterior, above-ground applications that do not trap moisture or allow the wood to be exposed to standing water or to water immersion.
- Stainless steel fasteners are required for permanent wood foundations below grade and other severe environments.
- Electroplated galvanized fasteners and connectors typically are not accepted for use in exterior applications, regardless of the preservative system.
- Other coatings or metal products may be available, so follow the hardware manufacturers’ recommendations for compatibility with wood treated with various preservative systems and end-use exposures.
- Construction adhesives are not a replacement for metal fasteners but can be added for extra holding power. Ensure the product label states the adhesive is “for use with treated lumber” and follow the adhesive manufacturer’s instructions.
Dried After Treatment
As the name implies, waterborne preservatives use water to carry preservatives deep into the wood’s fiber. Treating standards require wood products to be dry before treatment, with the moisture content percent requirement varying by product and size. For example, the moisture content for dimension lumber must be 19% or less before treatment and can exceed 50% when removed from the treating cylinder.
Most pressure-treated Southern Pine is delivered without being redried, but drying after treatment – either KDAT (kiln dried after treatment) or ADAT (air dried after treatment) – can be specified to return the wood to its dry condition. Drying after treatment may be required for structural strength or dimensional stability. For example, AWPA Standard U1 requires pressure-treated lumber and plywood for permanent wood foundations to be dried after treatment.
Building codes also may require drying after treatment where wood treated with waterborne preservatives is used in enclosed locations where drying in service cannot readily be achieved. Examples include interior products and framing such as cant strips, flooring, joists, millwork and trim, roof and floor trusses, sill plates, and studs.
Drying after treatment is recommended when dimensional stability and tightness of fit are important to the end-use application. Examples include decking, playground equipment, porch flooring, and siding where long-term performance is desired.
TREATED WOOD: THE LOWER ENVIRONMENTAL IMPACT OPTION
Life Cycle Assessment is an essential part of green building because it offers an objective and consistent way to measure the environmental impacts of building materials and assemblies. LCA confirms the environmental and other advantages of wood construction. Study after study in Europe, North America, and elsewhere has shown that wood outperforms other products when considered over its complete life cycle.
But how does wood treated with preservatives compare to other building products used for the same application? Members of the Treated Wood Council wanted to know and published a series of life cycle comparisons on a range of treated products, from decking to marine piling. Regardless of the application, analysis shows preservative-treated wood has a lower environmental impact when compared to steel, concrete, or plastic composites.
Wood is the responsible choice for building professionals. Why? Because wood products require less energy to manufacture – meaning fewer greenhouse gasses, like CO2. And trees use CO2 to grow, changing greenhouse gases into the building blocks we know as wood. That’s why wood products are increasingly being recognized as tools to combat climate change.
How does wood reduce carbon? The lifecycle looks like this to reduce emissions:
- As trees grow, they remove and store carbon from the atmosphere.
- Managed forests increase carbon storage by harvesting slower-growing trees and replacing them with younger trees.
- Wood products, such as lumber and furniture, store carbon during their life cycle. They are 50% carbon by weight.
- Recycling wood products allows them to continue to store carbon indefinitely.
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