Pressure-Treated Wood Decking

Key Takeaways:

• Pressure-treated wood is the most affordable decking material at $2–$5 per sq ft for materials
• Total installed cost for a 300 sq ft deck: $6,000–$12,000 in most Canadian provinces
• Requires annual staining/sealing — skip this and the wood deteriorates within 3–5 years
• Treated with ACQ or CA-C preservatives (CCA banned for residential use since 2003)
• Lifespan: 15–20 years with proper maintenance, 5–8 years without

What Is Pressure-Treated Lumber?

Pressure-treated (PT) lumber is softwood — typically Spruce-Pine-Fir (SPF) or Southern Yellow Pine — that has been infused with chemical preservatives under high pressure in a sealed cylinder. The treatment prevents rot, insect damage, and fungal decay that would destroy untreated softwood within a few years of outdoor exposure.

In Canada, residential PT lumber uses ACQ (Alkaline Copper Quaternary) or CA-C (Copper Azole) preservatives, regulated under CSA O80 Series standards. The older CCA (Chromated Copper Arsenate) treatment was phased out for residential use in 2003 due to arsenic content.

Treatment Levels for Decking

Use Category	Retention	Application
UC3A (Above Ground)	6.4 kg/m³	Deck boards, railings — not touching ground
UC4A (Ground Contact)	9.6 kg/m³	Posts, beams, joists near soil
UC4B (Below Ground)	9.6 kg/m³	Buried portions of posts

Always check the end tag stapled to each piece — it shows the treatment level and preservative type.

Cost Breakdown

For a 300 sq ft deck (the most common residential size in Canada):

Component	Cost Range
PT deck boards (5/4 × 6)	$600–$1,500
PT joists, beams, posts	$400–$800
Fasteners (ACQ-compatible)	$100–$250
Railings	$1,200–$3,000
Concrete footings	$300–$600
Labour	$3,600–$6,000
Total installed	$6,200–$12,150

This makes PT wood 40–60% cheaper than composite and 50–70% cheaper than PVC for the same deck size.

Maintenance Requirements

Here's the reality most PT deck owners discover too late: the upfront savings come with ongoing maintenance costs.

Year 1 — Do Nothing

New PT lumber needs 3–6 months to dry before it can accept stain. The wood arrives from the treatment plant still wet with preservative. Staining wet wood traps moisture inside, leading to premature rot. Test readiness by sprinkling water — if it soaks in, the wood is ready. If it beads, wait longer.

Year 1 (After Drying) — First Stain Application

Apply a penetrating deck stain — semi-transparent oil-based stain is the best choice for first application. This protects against UV and moisture. Budget $1–$3 per sq ft for materials, or $3–$6 per sq ft if hiring a professional.

Years 2–20 — Annual Maintenance Cycle

• Every spring: inspect for loose boards, popped screws, and signs of rot
• Every 1–2 years: power wash and re-apply stain/sealer
• Every 5–7 years: replace any boards showing significant checking, splitting, or rot
• Annual cost: approximately $200–$400 in stain, sealer, and replacement boards

Performance in Canadian Climates

Freeze-Thaw

PT wood absorbs moisture despite the treatment. When that moisture freezes, it expands, causing checking (surface cracks) and raised grain. After 100+ freeze-thaw cycles per winter in Ontario, Quebec, and the Prairies, PT boards develop a rough, splintery surface.

UV Exposure

Untreated PT wood turns grey within 6–12 months of sun exposure. The greying is cosmetic, not structural, but most homeowners prefer the original colour. Deck stain with UV blockers prevents this.

Warping and Cupping

PT boards can warp, twist, and cup as they dry — especially if installed while still wet from treatment. Best practice: let boards acclimate on-site for 5–7 days before installation, and install crown-side up.

Who Should Choose PT Wood?

Pressure-treated is the right choice if:

• Budget is the primary concern — no decking material costs less per square foot
• You're building a large deck (400+ sq ft) where material savings are substantial
• You enjoy DIY maintenance — staining can be meditative, and the results are satisfying
• You want the option to change stain colour every few years (composite colour is permanent)

It's not the right choice if:

• You want zero annual maintenance — choose composite or PVC
• You're building a vacation property you can't maintain regularly
• You're in a coastal salt-air environment — PT wood requires marine-grade fasteners and more frequent treatment