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
