Non-Pressure Treated Wood
For most treated wood, preservatives are applied in special facilities using pressure. However, sometimes this isn’t possible, or the need for treated wood was not apparent until after construction or building occupancy. In those cases, preservatives can be applied using methods that do not involve pressure vessels.
Some of these treatments can only be done by licensed applicators. When using wood preservatives, as with all pesticides, the label requirements of the Pest Management Regulatory Agency (in Canada) or the EPA (in the USA) must be followed.
Five categories of non-pressure treatments
Treatment during Engineered Wood Product Manufacture
Some engineered wood panel products, such as plywood and laminated veneer lumber (LVL) are able to be treated after manufacture with preservative solutions, whereas thin strand based products (OSB, OSL) and small particulate and fibre-based panels (particleboard, MDF) are not. The preservatives must be added to the wood elements before they are bonded together, either as a spray on, mist or powder.
Products such as OSB are manufactured from small, thin strands of wood. Powdered preservatives can be mixed in with the strands and resins during the blending process just prior to mat forming and pressing. Zinc borate is commonly used in this application. By adding preservatives to the manufacturing process it’s possible to obtain uniform treatment throughout the thickness of the product.
In North America, plywood is normally protected against decay and termites by pressure treatment processes. However, in other parts of the world insecticides are often formulated with adhesives to protect plywood against termites.
This is anticipatory preservative treatment applied by dip, spray or brush application to all of the accessible surfaces of some wood products during the construction process. The intent is to provide a shell of protection to vulnerable wood products, components or systems in their finished form. One example would be spraying house framing with borates for resistance to drywood termites and wood boring beetles in some cases. Such treatments may also be applied to lumber, plywood and OSB to provide additional protection against mould growth.
Sub-surface pre-treatment (Depot treatment)
This is preservative treatment applied at discrete locations, not to the entire piece, during the manufacturing process or during construction. The intent is to pro-actively provide protection only to the parts of the wood product, component or systems that might be exposed to conditions conducive to decay. One example would be placing borate rods into holes drilled in the exposed ends of glulam beams projecting beyond a roof line.
This is preservative treatment applied at discrete locations to treated wood in service to compensate for either incomplete initial penetration of the cross section, or depletion of preservative effectiveness over time. The intent is to boost the protection in previously-treated wood, or to address areas exposed by necessary on-site cutting of treated wood products. One example would be the application of a ready-made bandage to utility poles that have suffered depletion of the original preservative loading. Another example is field-cut material for preserved wood foundations.
This is preservative treatment applied to residual sound wood in products, components or systems where decay or insect attack is known to have begun. The intent is to kill existing fungi or insects and/or prevent decay or insects from spreading beyond the existing damage. One example would be roller or spray application of a borate/glycol formulation on sound wood left in place adjacent to decayed framing (which should be cut out and replaced with pressure-treated wood).
Formats of non-pressure treatments
Non-pressure treatments come in three different forms: solids, liquids/pastes, and fumigants. Unlike pressure-treatment preservatives, which rely on pressure for good penetration, these rely on the mobility of the active ingredients to penetrate deep enough in wood to be effective. The active ingredients can move in the wood via capillarity or can diffuse in water and/or air within the wood. This mobility not only allows the active ingredients to move into the wood but can also allow them to move out under certain conditions. This means the conditions within and around the structure must be understood so the loss of preservative and consequent loss of protection can be minimized. Borates, fluorides and copper compounds are particularly suitable for use as solids, liquids and pastes. Methyl isothiocyanate (and its precursors), methyl bromide, and sulfuryl fluoride are the only widely used fumigant treatments. Methyl bromide was phased out, except for very limited uses, in 2005.
The major advantage of solids in these applications is that they maximize the amount of water-soluble material that can be placed into a drilled hole, due to the high percentage of active ingredients contained in commercially-available rods. The major disadvantage is the requirement for sufficient moisture and the time needed for the rod to dissolve. The earliest and best-known solid preservative system is the fused borate rod, originally developed in the 1970s for supplementary and remedial treatment of railroad ties. These have since been used successfully on utility poles, timbers, millwork (window joinery), and a variety of other wood products. A mixture of borates is fused into glass at extremely high temperatures, poured into a mould and allowed to set. Placed into holes in the wood, the borate dissolves in any water contained in the wood and diffuses throughout the moist region. Mass flow of moisture along the grain may speed up distribution of the borate. Secondary biocides such as copper can be added to borate rods to supplement the efficacy of the borates against decay and insects. While all preservatives should be treated with respect, many users feel more comfortable dealing with borate and copper/borate rods because of their low toxicity and low potential for entry into the body.
Fluorides are also currently available in a rod form. The rod is produced by compressing sodium fluoride and binders together, or by encapsulation in a water-permeable tubing. Fluorides diffuse more rapidly than borates in water and may also move in the vapour phase as hydrofluoric acid.
Zinc borate (ZB) is a powder used to protect strand-based products. It is blended with the resins and stands during the manufacturing processes for OSB and other strand based products becomes well dispersed throughout. Zinc borate has very low water solubility and can protect strand based products from decay and termites.
Liquids, Pastes and Gels
Liquids can be sprayed or brushed on to surfaces, or poured or pumped into drilled holes. Pastes are most often brushed or troweled on, then covered with polyethylene-backed kraft paper creating a “bandage.” Pastes can also be packed into drilled holes or incorporated into ready-to-use bandages for wrapping around poles. Borates and fluorides are commonly used in these formulations because they diffuse very rapidly in wet wood. Copper moves more slowly because it reacts with the wood. For dryer wood, glycols can be added to borate formulations to improve penetration. Over-the-counter wood preservatives available for brush application are based on either copper naphthenate (a green colour), or zinc naphthenate (clear). Both are dissolved in mineral spirits-type solvents. In addition, water-borne borate/glycol formulations can also be purchased over-the-counter as roll-on liquids.
These treatments are typically delivered as liquids or solids; they change to a gas upon exposure to air, and become mobile in the wood as a gas. Some solid and liquid fumigants are packed in permeable capsules or aluminum tubes. Methyl isothiocyanate (MIT), and chemicals that produce this compound as they break down, are used for utility poles and timbers. This compound adsorbs to wood and can provide several years of residual protection. Sulfuryl fluoride and methyl bromide are used for tent fumigation of houses to eradicate drywood termites.
Repairing Cuts in the Treated Shell
Pressure-treated wood in the ground can undergo significant internal decay within just six or seven years if cuts, bolt holes and notches are not brush treated with a field-cut preservative. Common over-the-counter agents for this purpose include copper naphthenate (a green colour), or zinc naphthenate (clear). Both are dissolved in mineral spirits-type solvents. Other brush-on agents include water-borne borate/glycol formulations which can also be purchased at building supply outlets.
Forgetting this critical step will almost certainly shorten the life span of the product and will void any warranties on the product. Although brush-on application of wood preservatives isn’t nearly as effective as pressure-treatment, the field-cut preservatives are usually applied to the end grain, whereby the solution will soak in further than if applied to the side grain.
In FPInnovations’ field tests of these preservatives, copper naphthenate performed best. Zinc naphthenate (2% zinc), which is colourless, was not as effective but may be suitable for above-ground applications where the decay hazard is lower and if the dark green colour of copper naphthenate is undesirable. Note that the dark green of the copper-based product will fade after a few years.