Composite decking is sold on a single promise: zero maintenance. The warranty paperwork says 25 years, the brochure photos show pristine boards in eternal late-afternoon light, and the contractor quotes you $20,000 mostly on that promise. Then year three arrives in southern Ontario, you walk out in early April after the snow has finally cleared, and there are dark streaks running across half your deck. The streaks are mold.
It happens to essentially every composite deck in the GTA. The boards themselves don’t rot — the cap is plastic, water beads off it, the warranty is intact. But mold doesn’t need the board to decompose. It just needs a thin film of organic material (pollen, decomposed leaf litter, tree-tar drip) to land on the cap and stay wet long enough to colonize. Ontario springs do exactly that.
Why composite mold is an Ontario-specific problem
Three things have to line up: organic deposit, moisture retention, and slow drying. Southern Ontario delivers all three:
- Organic deposit.The GTA’s tree canopy is one of the densest in any North American metro. Maple keys, oak tannin-rich leaves, locust pods, willow drift, and conifer needles all break down on the deck surface and feed mold spores.
- Moisture retention.Composite is hygroscopic at the surface even when the cap is intact — the cap is chemically smooth, which means water films hang around longer on it than on splintery cedar. Add the GTA’s spring humidity (typically 70–85% RH in April–May) and the film never fully evaporates.
- Slow drying.A north-facing or tree-shaded deck in Toronto or Mississauga gets two to four hours of direct sun a day in spring. That isn’t enough to clear surface moisture before the next dew cycle. Mold colonies establish in 4–7 days under these conditions.
This is also why composite mold is more aggressive on north-facing Toronto and Hamilton decks than on south-facing Burlington and Oakville decks. The microclimate matters more than the brand of board.
What it actually looks like
There are three patterns to recognize, and they have different causes:
- Dark linear streaks running with the board grain. This is Aureobasidium pullulans, a black mold that follows the microscopic ridges in the composite cap. Most common on three-to-six-year-old decks. Surface-only — doesn’t enter the board.
- Fuzzy green or grey-green patches. Algae, not mold. Grows in cool, shaded, wet conditions. Most common under chairs, planters, and along the house edge of the deck.
- White or pinkish surface bloom. Mildew, the early stage of mold growth. Wipes off easily if caught in the first season; bonds to the cap if left into a second.
None of the three is a warranty defect. All three are environmental. Trex, TimberTech, and Fiberon all explicitly disclaim mold and algae in their published warranties; they cover board failure (cracking, delamination, fading beyond a specified range), not surface biological growth.
What actually removes it
The most-recommended product on contractor forums — pressure washer at 1,500–2,000 PSI — is also the most common cause of warranty voids. Trex’s own cleaning guide caps pressure at 1,500 PSI with a fan tip held no closer than 12″, and Fiberon’s is 1,500 PSI / 25° tip / 8″ minimum. Most homeowners overdo all three and end up with permanent etch marks in the cap that no amount of scrubbing reverses.
The order of operations that works:
- Dry sweep first. Pollen, leaf bits, and decomposed organic film come off easier dry than wet. A stiff push broom in the direction of the board grain. Five minutes.
- Apply a deck-specific composite cleaner. Composite-rated cleaners (Trex DeckCleaner, Corte Clean, Olympic Composite) are formulated to break the mold cell wall without attacking the cap polymer. Avoid bleach-based household cleaners — they’ll fade the colour and the cap warranty excludes chemical damage.
- Scrub with a soft-bristle deck brush. Wire brushes leave embedded metal fragments that rust into the cap. Soft-bristle nylon, with the grain, with enough pressure to feel the brush actually flex.
- Rinse with a garden hose, not a pressure washer. If you absolutely must pressure-wash, stay under 1,500 PSI, use a fan tip wider than 25°, and keep the wand 12″+ off the deck. Move with the grain. Never against.
- Let it dry 24 hours before furniture goes back on. Trapped moisture under furniture re-seeds the colonies you just cleaned.
For deep streak patterns that survive the standard wash, oxygen bleach (sodium percarbonate, the active in OxiClean) at the manufacturer-recommended dilution is the strongest treatment the warranties tolerate. Apply, wait 15 minutes, scrub, rinse. Repeat once if needed. Do not exceed a single seasonal treatment — repeated oxygen bleach exposure will dull the cap finish.
How to slow it down between cleanings
- Annual prevention wash in early May. Catches spore growth before the colony establishes. One round of cleaner + scrub + rinse, 20 minutes for a 200 sq ft deck. The single highest-ROI maintenance action you can take.
- Clear the leaf litter twice in October. Half the mold problem starts in autumn when leaves sit on the deck for two weeks under rain. Sweep them off weekly through leaf season.
- Move furniture every three weeks during summer. Stationary furniture creates damp spots. Even small position changes prevent the localized growth patterns you see under chair feet by August.
- Prune the canopy. If a mature tree drops debris directly on the deck, the per-year mold load is roughly double what an unshaded deck sees. A one-time pruning costs less than two seasonal washes.
When it’s actually a defect
Three patterns are worth contacting the manufacturer about because they may be warranty-covered:
- Mold that returns within four weeks of a proper cleaning. May indicate a cap-integrity defect on that batch of boards.
- Mold that has bonded into a visible texture (not just a stain). If you can feel the mold through the surface, the cap may have failed. Photograph it before any cleaning attempt.
- Differential growth on boards from the same lot. If half the deck is fine and half is overrun under identical conditions, the affected boards may be from a bad cap run.
The manufacturers all require documentation: original purchase receipt, lot numbers from the board ends (often hidden under fascia), photos before any cleaning, the cleaning products attempted. Get the documentation together before you call. The warranty teams reject 90% of mold claims on first call because the homeowner already pressure-washed the evidence away.
What it costs to live with
A composite deck that gets the prevention wash once a year and a full cleaning every three years costs roughly $30–$60 a year in cleaner and your own labour. A homeowner-hired professional cleaning runs $200–$400 for a 200–300 sq ft deck in the GTA, typically billed in May or September.
That number is meaningfully smaller than the maintenance load of cedar or pressure-treated — both of which need annual re-staining or re-sealing on top of the same mold management. The five-year cost comparison we ran for cedar versus composite on a typical Toronto deck still comes out in composite’s favour once mold maintenance is included on both sides.
Composite is “low maintenance”, not maintenance-free. The brochures elide that distinction and the warranties exclude it. But forty-five minutes of work in May, repeated annually, keeps the deck looking right for the entire 25-year warranty window. That’s still the best maintenance economics of any deck material sold in Ontario.
Pricing your build: the calculator uses 2026 GTA installed rates for capped composite, including the maintenance line items above. The per-city pages set your local default material based on what dominates in that municipality.