Pressure-treated (PT) lumber is the most common structural material for residential deck construction in Canada. Its durability advantage over untreated wood comes from preservative chemicals driven into the wood cells under pressure — but not all PT lumber is treated to the same level, and using below-specification material in the wrong location is a source of premature structural failure.

The treatment standard most commonly referenced in Canadian residential construction is the AWPA (American Wood Protection Association) Use Category system, which was adopted in Canada following the phase-out of chromated copper arsenate (CCA) in the early 2000s. Understanding this system helps when reading lumber end tags at the yard or reviewing a contractor's material specifications.

How Pressure Treatment Works

In the treating process, lumber is loaded into a sealed cylindrical vessel. Air is removed from the wood cells under vacuum, and then preservative solution is forced in under pressure. After pressure is released, a final vacuum draws out excess solution. The result is a concentration of preservative within the wood measured in kilograms of chemical per cubic metre of wood — this is the retention level.

The type of preservative used affects both the chemistry and the appearance of the finished product. The most common types used for residential decking in Canada include:

  • ACQ (Alkaline Copper Quaternary): A copper-based preservative widely used since CCA's phase-out. More corrosive to standard steel fasteners than CCA was, which is why stainless or hot-dipped galvanized fasteners are now standard practice.
  • CA (Copper Azole): Another copper-based treatment, similar performance profile to ACQ. Also requires compatible hardware.
  • Micronized copper (MCQ, MCA): Copper in particle rather than dissolved form; slightly less corrosive to hardware than ACQ or CA but still incompatible with standard carbon steel fasteners.
  • Borate treatments (SBX/DOT): Used only for interior, above-ground applications where the wood will not be exposed to rain. Not appropriate for outdoor decks.

Use Categories and Retention Levels

The AWPA Use Category system assigns treatment requirements based on how much biological hazard the wood will be exposed to:

Use Category Exposure Typical Application Min. Retention (ACQ)
UC3B Above ground, repeated wetting Decking boards, railings, horizontal surfaces 0.15 kg/m³
UC4A Ground contact, general use Posts in soil or concrete, bottom plates 0.40 kg/m³
UC4B Ground contact, severe hazard Posts in high-decay-risk soils, coastal areas 0.60 kg/m³
UC4C Ground contact, critical structures Posts where replacement would be difficult 0.60+ kg/m³ with specific preservative types

The end tag stapled to PT lumber at the mill identifies the use category, the treating company, the preservative type, the retention level, and the certifying agency. This tag is the controlling specification, not the green colour of the wood.

Common Errors in Material Selection

Using UC3B lumber for in-ground posts

Above-ground rated lumber (UC3B) used for deck posts set in concrete or soil fails significantly faster than ground-contact grade. In BC's wet coastal climate, this difference can mean a post that deteriorates in five to eight years instead of the 25-plus years that UC4A or UC4B lumber can achieve.

Ignoring the end tag

PT lumber is sold in standard residential grades (e.g., No. 2 and Better, Select Structural) regardless of treatment level. A piece of UC3B No. 2 S-P-F looks identical to UC4A No. 2 S-P-F except for the end tag. Reading the tag before purchase, and verifying tags on delivered material, is the only reliable check.

Using incompatible hardware

Copper-based preservatives accelerate corrosion of standard bright common nails and most zinc-plated hardware. The relevant standard for fastener compatibility is AWPA M6. Hot-dipped galvanized (HDG) fasteners meeting ASTM A153 and stainless steel fasteners (Type 304 or 316) are compatible. Electro-galvanized or mechanically galvanized fasteners are generally not considered adequate for direct contact with ACQ or CA treated lumber in wet applications.

Practical note: Some post base hardware manufacturers publish specific guidance on their products' compatibility with current preservative types. Verifying compatibility with the hardware manufacturer — not only the preservative supplier — is worth doing for load-bearing connections.

Working Safely with Pressure-Treated Lumber

Modern copper-based preservatives are considerably less hazardous than the CCA (chromated copper arsenate) used through the late 1990s. Nevertheless, reasonable precautions apply:

  • Wear a dust mask and safety glasses when cutting PT lumber. Preservative-laden sawdust is a respiratory irritant.
  • Wash hands before eating and after extended handling.
  • Do not burn PT lumber offcuts. Burning concentrates preservative chemicals in ash and smoke.
  • Dispose of PT offcuts according to local municipal solid waste guidelines — most jurisdictions accept them as ordinary construction waste, not hazardous material, but regulations vary.
Backyard landscaping with paving stone patio in Nanaimo, British Columbia, Canada

Backyard patio construction in Nanaimo, BC. In the wet Pacific Northwest climate, material selection for ground-contact components is particularly consequential.

Alternatives to Pressure-Treated Wood

PT lumber is the standard choice for structural framing, but several alternative materials are used for deck surfaces and railings:

  • Composite decking: Wood-plastic composite (WPC) and capped composite boards do not rot and do not require sealing. However, they have different thermal expansion characteristics than wood, require specific fastener spacing, and still need sound structural framing beneath them.
  • Cedar and redwood: Naturally decay-resistant species suitable for above-ground decking in moderate climates. Not appropriate for ground contact without treatment.
  • Thermally modified wood: Wood modified at high temperatures to reduce moisture uptake. Growing in availability but not yet as standardised in Canadian residential codes as PT lumber.
  • Steel framing: Galvanized or powder-coated steel framing systems are an option in marine or high-decay environments where even ground-contact PT lumber performs poorly.

Useful References