A wet Ontario summer can leave a deck speckled with black, grey and green mildew. You will learn how to confirm that is what you have, how a low-pressure soft wash lifts it without tearing the wood, and what to do afterwards so it does not come straight back.
In This Article
Why a wet summer leaves decks speckled
Mildew needs three things: moisture, warmth and something to feed on. A humid July supplies the first two. A film of pollen, leaf bits, dust and spilled drinks supplies the third.
Wood makes it easy. Boards soak up rain and dew, then dry slowly wherever air cannot move. The usual trouble spots are the shaded side of the house, the strip along the wall, the gaps between boards that fill with debris, and the ground under planters, mats and furniture legs. Composite decking is not immune either, since mildew grows on the dirt that settles on it rather than in the boards.
Confirm it is mildew before you treat it
Not every dark mark on a deck is growth, and cleaning will not fix the ones that are not.
- Black, grey or green speckles that spread, mostly in shade: likely mildew or algae. It feels slick when wet and smears under a damp cloth.
- Even silver-grey across sunny boards: sun weathering. The colour is the same everywhere, and the wood is not slick.
- Dark rings around screws and nails: a fastener stain. It follows the metal and does not spread.
- Brown patches, especially on cedar: tannin bleed. It shows after rain and lightens with a brightener.
- Dark, soft or crumbling spots: rot. A screwdriver tip sinks in, and the board needs repair, not cleaning.
To be sure, treat a hidden patch with the cleaner you plan to use, following its label. If the dark spot fades within minutes, you are dealing with growth.
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Why low pressure suits mildew on wood
A hard jet strips dirt from the surface, but mildew sends threads into the top layer of the wood, and the jet tears soft grain while it works. A soft wash reverses the job. The cleaner kills the growth, a brush loosens what is left and gentle water carries it away.
Two cleaner families are common. Oxygen-based products are kinder to wood fibres and nearby plants, and they suit light to moderate mildew. Sodium hypochlorite mixes act faster on heavy growth, but they can lighten wood and harm plants, so they must be diluted, rinsed thoroughly and kept off the garden. Follow the manufacturer's label for strength and dwell time, and never mix cleaners. Bleach combined with ammonia or an acid releases toxic gas.
A deck soft wash in seven steps
- Clear and sweep. Remove furniture, planters and mats, then pick debris out of the gaps with a putty knife.
- Protect the plants. Soak nearby beds with fresh water and cover delicate shrubs. Keep pets and children off the deck.
- Apply the cleaner with a pump sprayer, one section at a time, covering rails and the underside of the top cap as well as the floor.
- Let it dwell for the label's time. Do not let it dry on the boards.
- Brush along the grain with a soft to medium brush wherever dark patches remain.
- Rinse generously at low pressure, rails first and floor last, pushing water away from the house.
- Rinse the plants again, then leave the deck alone until it is fully dry.
Choose a calm, overcast day. Sun dries the mix before it works, and wind blows it onto the neighbour's tomatoes.
After the wash: dry, restore and protect
Ontario humidity slows drying, so give the boards a day or two, and longer after rain. Finishes need dry wood, and a coat applied over damp boards can peel.
- Grey or brown patches left behind? These are often tannin or weathering, and a wood brightener treats them better than another wash.
- Check the old finish. Flick water onto a board. If it beads, the finish is working. If it soaks in, the wood needs a new coat.
- Choose a finish with care. Some deck stains contain mildew inhibitors. Read the label for minimum application temperature and drying time.
Bare, thirsty wood drinks moisture all season, which is why a deck that goes another year without a coat tends to mildew sooner.
Keeping mildew from coming back
- Sweep weekly through pollen, seed pod and leaf season, and flush the gaps with a hose.
- Lift planters on feet and skip rubber-backed mats, which hold moisture against the boards.
- Trim branches and shrubs that shade the deck or drip onto it, so the boards dry faster.
- Point sprinklers at the lawn and make sure the downspout does not empty onto the deck.
- Store cushions dry, and do not leave wet towels or toys on the boards.
Timing matters too, and the guide to when to wash a deck covers the season. Both jobs are quoted for free at D&D, and soft washing and pressure washing each start from $350 for a typical Kitchener-Waterloo home.
Key Takeaways
- Mildew grows where moisture, warmth and a film of pollen or debris meet, which is why a wet summer and a shaded deck are a bad mix.
- Check that dark marks are growth, since rot, tannin and fastener stains need different fixes.
- A low-pressure soft wash with a label-directed cleaner and a soft brush lifts mildew without tearing the grain.
- Dry the boards fully, renew a failing finish and keep debris, mats and shade off the deck to slow the return.
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