When the intended application for a wood product is known to present a hazard, specify pre-treated wood. In Canada, reference a CSA O80 standard. In the US, reference an AWPA standard. Also specify that all field cuts must have a field-cut preservative applied. Ideally, all trimming, boring, and other cuts in the pieces should be done prior to pressure treatment. In addition, there are complexities involving choices in species, types of preservatives and the need for incising the wood – it’s best to consult with an experienced specifier of treated wood.
For do-it-yourselfers: Ideally, the treated wood you buy at your local building supplies store will have an end tag to help you choose the right product. The tag identifies the type of preservative used, the amount retained, the appropriate use for this piece of wood, the treatment standard (i.e. CSA O80 for Canadian products) to which it was treated (if any), and the treatment plant name and location. The most important information to look for is the use class. If the piece is going in the ground (e.g. a fence post), you need the piece to be treated for “ground contact.” All other uses (such as fence boards, deck boards and shingles) can be labeled “above ground.” The piece may also be tagged with consumer safety information. You might also find this information in the store, either posted or as a technical brochure.
Using common sense and standard safety equipment (personal protection and wood-working machinery) applies when working with any building products. Gloves, dust masks and goggles are appropriate for use with all woodworking. Here are a few key points specific to treated wood:
- Pressure-treated wood is not a pesticide, and it is not a hazardous product. In most municipalities, you may dispose of treated wood by ordinary garbage collection. However, you should check with your local regulations.
- Never burn treated wood because toxic chemicals may be produced as part of the smoke and ashes.
- If preservatives or sawdust accumulate on clothes, launder before reuse. Wash your work clothes separately from other household clothing.
- Treated wood used for patios, decks and walkways should be free of surface preservative residues.
- Treated wood should not be used for compost heaps where free organic acids produced early in the composting process can remove the fixed chemicals. It is, however, safe to use for growing vegetables in raised soil beds. If, after reading this, you are still concerned, place a layer of plastic sheet between the soil and the treated wood wall.
- Treated wood should not be cleaned with harsh reducing agents since these can also remove the fixed chemicals.
Selecting the Right Fasteners
Fasteners for use in treated wood that will be exposed to the weather should be selected to withstand weathering as long as the treated wood itself. Stainless steel fasteners are by far the ideal, but hot-dipped galvanized fasteners will normally provide the desired service life. Hot-dipped galvanized nails should not be fastened using a high pressure nail gun due to the risk of damage to the coating during firing. The protective coating on electroplated galvanized fasteners is too thin and will perform poorly, and common nails will corrode rapidly after fastening most copper-based treated wood. As a general rule aluminum fasteners should not be used with treated wood, except new generation products (MCA treated) specifically tested, approved and labelled as suitable for contact with Aluminum. Do not use stainless steel fasteners in saltwater applications.
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.
Click here for more information on fasteners.
Click here for more information on performance tests done with treated wood.
Click here for more information on wood preservatives that can be applied on site.
Click for consumer safety information on handling treated wood (Canada).