Ottawa homeowners know the drill: you build a deck, a fence, or a garden bed using pressure-treated lumber because it's supposed to last. But a few years in, the wood starts to soften, crack, and crumble at the edges — and suddenly that "rot-resistant" material doesn't seem so resistant after all.
Home improvement specialist Steve Maxwell, writing for the Ottawa Citizen, breaks down why this happens and what you can actually do about it.
The Misconception About Pressure-Treated Wood
Pressure-treated lumber is infused with chemical preservatives — most commonly copper-based compounds — that protect against fungal decay and insect damage. The operative word, though, is against. The treatment slows rot; it doesn't eliminate it entirely.
The preservatives penetrate only so deep into the wood fibres. When you cut, drill, or notch a piece of pressure-treated lumber, you expose untreated interior wood. That's your entry point for moisture and decay.
Ottawa's Climate Is Especially Tough on Wood
Our freeze-thaw cycles are brutal. Ottawa's winters push temperatures well below -20°C, and the spring thaw brings prolonged moisture. Wood expands and contracts with each freeze-thaw cycle, opening up tiny cracks that collect water. Once water sits inside those cracks, the conditions for rot are ideal: moisture + oxygen + wood fibre = fungal decay.
Decks, fence posts set in soil, and raised garden beds are particularly vulnerable because they're in constant contact with ground moisture. A post buried in earth — even pressure-treated — will eventually rot from the bottom up if not properly protected.
What You Can Do About It
Seal all cut ends immediately. Any time you cut pressure-treated lumber, apply an end-cut preservative (available at most Ottawa hardware stores). This step is skipped constantly and it's one of the biggest mistakes DIYers make.
Use post bases instead of burying wood. Metal post bases anchor deck posts above ground, keeping them out of direct soil contact. This single change can dramatically extend the life of a deck.
Check your drainage. Water pooling against wood — whether at the base of a fence post or under deck boards — is the fastest path to rot. Grade soil away from structures, and make sure deck boards have adequate spacing (about 3–6mm) for drainage and airflow.
Apply a penetrating water repellent. Even pressure-treated wood benefits from a quality water repellent applied every two to three years. Look for products that penetrate rather than just coat the surface — they last longer and keep moisture out more effectively.
Inspect annually. Poke suspicious-looking areas with a screwdriver. If it sinks in easily, rot is already present. Catching it early means you can cut out and sister in new lumber rather than replacing entire sections.
When to Replace vs. Repair
If rot is localized — a single board, the end of a joist — you can often cut away the damaged section and sister in new wood. If structural members like beams or posts are compromised, replacement is the safer call. Rotted structural wood is a safety issue, not just a cosmetic one.
With Ottawa's short building season, fall is actually a great time to assess your outdoor structures so you're ready to repair or replace before spring. Don't wait until the deck wobbles to take action.
Source: Ottawa Citizen / Steve Maxwell, Houseworks column
