The Best Deck Cleaner UK (2026): What Actually Shifts Algae on Old Timber

Algae, and the black spot fungus that follows it, live on the film of moisture sitting on your boards every morning from October to May. That single fact decides which bottle to buy. What matters is not how aggressive a cleaner is but whether it stays wet long enough to kill what is growing in the grain — which is why a £14 bottle from a DIY shed sometimes beats one at three times the price.

We clean and treat forty to sixty decks a season, mostly tanalised softwood. What follows is where your £25 is best spent, what the small print is really telling you, and why a deck bolted to the back of a Victorian terrace needs a different approach from one on a 2015 estate.

This assumes a timber or composite deck of roughly 10 to 30 m² and one of two goals — clean enough to sit on, or clean enough to take a finish. For safe, non-slippery boards through the summer, a biocidal wash and a stiff brush is the whole job: an afternoon and about £25 in chemicals. For prep before oil or stain you also need the silvered layer and any failed coating gone, which means a cleaner plus a brightener and a three-day dry window. Our step-by-step decking clean covers the method; this is about what goes in the bucket.

Composite owners should read the warranty before the label. Most capped boards tolerate dilute oxygen bleach and mild detergent, but nearly all void cover if you use sodium hypochlorite or a solvent, and some exclude pressure washing above 80 bar. If you are still choosing materials, our ten-year cost comparison is more useful than any cleaner.

Honest disclosure: this is expert selection based on repeated trade use across several British seasons, not a controlled laboratory comparison. We have not measured spore kill rates, and anyone claiming to have done so on a garden deck is guessing. What we can tell you is how these products behave on real boards a year later. The criteria we apply:

• •A named active ingredient. If a bottle will not say whether it is hypochlorite, percarbonate or a quaternary ammonium biocide, we do not buy it.
• •Required contact time. Anything asking for under ten minutes before rinsing is a detergent with marketing on it, and the deck is green again by the first heavy autumn dew.
• •Whether it furs the timber. Hypochlorite attacks lignin and raises the grain — you notice a fortnight later, when the boards feel like a nail file.
• •What the run-off does — borders, ponds and old drainage all care, and in a terrace you cannot choose where water goes.
• •Cost per square metre at the stated dilution, not the bottle price.

These four cover almost every deck in the country. Full pros and cons below; the short version:

• •Softwood, green and slippery, no staining planned — Barrettine Mould & Mildew Cleaner. Long contact time, no bleaching, kills rather than lifts.
• •Silvered hardwood such as iroko or balau, or black iron stains around old fixings — Owatrol Net-Trol. Oxalic acid does what no biocide can.
• •Small budget, boards being restained anyway — Ronseal Decking Cleaner & Reviver. Cheap, honest, available on a Sunday.
• •Cannot rinse, or must not rinse — Wet & Forget. Apply, walk away, let the weather finish it.
• •Rotten or lifting boards — none of the above. Cleaner does not fix structure; work through our deck maintenance checklist first.

This is where most buying advice falls apart. Decks on Victorian and Edwardian terraces are rarely freestanding — they sit hard against a solid nine-inch brick wall with lime mortar, no cavity, and either a slate damp-proof course at 150 mm or nothing at all. Solid walls wick moisture inward, so forty litres of chlorinated water at the base of one is how a dry back wall becomes a damp patch on lime plaster and lath by Christmas. The rules we work to on anything pre-1919:

• •No sodium hypochlorite within a metre of lime pointing or soft handmade brick. It leaves a white salt bloom that takes two years to weather off, and eats lime mortar faster than algae.
• •Keep any pressure washer under 100 bar with a 40-degree fan tip, and never aim at the pointing — a lance strips lime joints in seconds. Our pressure washer guide covers which machines turn down properly.
• •Sweep and scrape debris out first. Victorian yard gullies are narrow, often silted, and usually run to a combined sewer — a season's leaf mulch flushed down one is a £180 drain-jetting call.
• •Old hardwood decks — iroko, balau, reclaimed pitch pine — go grey rather than green and stain black around cut nails. That is iron tannate; only oxalic acid shifts it.
• •Check the socket — pre-1970s wiring in an outbuilding often lacks RCD protection, so use a 30 mA RCD adaptor.
• •Decks over old brick or flagged yards have no ventilation gap beneath. Soak them heavily and the underside stays wet for a fortnight, rotting joists while the top looks immaculate.

Four active ingredients cover the whole market, and knowing them beats any star rating. Benzalkonium chloride is a quaternary ammonium biocide: it kills algae, moss and spores, leaves a residual effect for a few months, and does not bleach the wood. Sodium percarbonate is oxygen bleach — it breaks down to soda ash and hydrogen peroxide, and is safest near borders. Oxalic acid is a brightener, not a cleaner: it removes tannin and iron staining but kills nothing. Sodium hypochlorite is fast, cheap and the one we use least.

Check the dilution rate before comparing prices. A 5 L concentrate at 1:4 gives 25 litres of working solution, and at a realistic 1 litre per 4 m² that covers around 100 m² — so the dearer bottle is often the cheaper job. Then read the recoat guidance: some biocidal washes must be fully dry and neutralised before a finish goes on. Read our decking stain comparison first.

Buying patio cleaner for a deck is the commonest. Hard-surface products are made for stone, are far more alkaline, and will bleach and fur timber — they belong on slabs, covered in our patio cleaning guide. Close behind is buying on price without reading the dilution, then applying it neat because the deck still looks green.

The rest are timing errors. Cleaning in October, when nothing dries and you have handed the algae a damp surface to recolonise. Cleaning in hot sun, when the solution evaporates before it has done ten minutes of work — overcast and still is ideal, which Britain supplies generously. And cleaning straight before staining without a brightener, so the finish sits on a grey oxidised layer and lifts in sheets next spring. Allow two dry days, brush along the grain, and the same £25 bottle buys two clean summers instead of one.