At home
Four ways to clean your deck
Warren Judd
Barbecue season is approaching, but if your house has a deck chances are that winter has left behind a slippery coating of algae, fungi, mosses, moulds and lichen. So if you’re planning on spending more time outside, you might need to give it a good clean, or risk literally hitting the deck after a few outdoor wines.
Here are four ways you can restore the appearance – and grip! – of weathered wood.
Option 1: Chemical-free
(Best for the environment)
Water-blasting only uses water and a modest amount of electricity, so it’s the best option for the environment. However, some deck aficionados dislike water blasting because they claim it damages the wood fibres, leaving you with a rough surface that may be easier for unwanted organisms to latch on to in future. It’s also the option that takes the most effort – expect several hours of work for a normal sized deck. If using an electric water blaster, make sure you plug it in via an RCD safety device or similar.
How to
Pass the high-pressure nozzle over the deck, stripping off slime and dirt.
Need to know
Go too fast and you won’t remove the slime! This option takes time – and you’ll get wet doing it.
Price guide
You can hire a water blaster for a day for around $65 – see www.hirepool.co.nz.
Option 2: Quick fix
The deck-cleaning chemical that has been around the longest, and is also the least expensive, is bleach or sodium hypochlorite. The active ingredient is usually 5 per cent sodium hypochlorite (which is only slightly more concentrated than household bleach brands).
Bleach also generally contains some sodium hydroxide (caustic soda), which is a very strong base or alkali, and can burn or damage skin (especially eyes). So always treat strong hypochlorite solutions with respect – it should be promptly and thoroughly washed from skin and also desirable plants.
Also, don’t mix it with any other chemicals! A friend of my mother once mixed Janola and Harpic to clean her toilet better. Harpic contains hydrochloric acid which releases deadly chlorine gas from hypochlorite. She almost died and suffered permanent lung damage…
How to
Dilute with water and spray on when the deck is dry. The solution is fast-acting (minutes) and should be hosed off after use.
Need to know
Critics of bleach-based cleaners say they badly damage wood fibres and give timber a washed-out (or bleached) look. It’s also corrosive to metal nails and screws. Additionally, bleach can sterilise soil by killing helpful fungi and bacteria, and changes its acidity, though a few litres getting rinsed into the unproductive dirt beneath your deck is unlikely to have much effect on your garden.
Price guide
About $19 for 5 litres. A litre of diluted solution should cover 4m2 of deck.
Common brand
- 30 Seconds Outdoor Cleaner
Option 3: Spray and leave
Another class of deck cleaners use benzalkonium chloride or something similar called quaternary ammonium compound (QAC). These are chemicals derived from ammonia, but unlike that substance, are not strong bases, have no strong smell and will not burn your skin.
Nonetheless, they will kill a wide range of organisms including bacteria, viruses, algae, moulds, and fungi, so are effective deck cleaners. They’re also found in disinfectants and have a soap-like cleaning effect.
This class of cleaner is slow acting so you just walk away and leave it to work, which happens over a few weeks. Hosing and scrubbing the deck after a time will likely give a better-looking result.
How to
Dilute with water, then spray on when the deck is dry and leave to work. Hosing and scrubbing a few weeks later may also help.
Need to know
Although QACs are considered quite mild (they are used in nasal sprays and in contact lens cleaners), they are toxic to fish and aquatic invertebrates, even at very low concentrations, so must be kept out of streams, drains and ponds. They’re also somewhat damaging to plant leaves.
Price guide
From $30 to $99 for 5 litres, depending on the brand. A litre of diluted solution should cover 5m2 of wooden deck, and 10m2 of concrete.
Common brands
- 30 Seconds One Step
- Wet and Forget
Option 4: The one-hour job
(Best for the environment)
The other main type of deck cleaning chemical uses sodium percarbonate. This comes as a white powder which is dissolved in warm water immediately before use, releasing hydrogen peroxide and eventually oxygen.
Oxygen? Isn’t that the stuff we breathe? Indeed, but it’s also a dangerously reactive chemical. Fortunately, deck moulds and fungi are vulnerable to oxygen attack – and sodium percarbonate cleaners are good for badly discoloured, seriously slimy decks.
Environmentally, percarbonate is the most benign of the three chemical cleaners and is also gentle on wood. However, the fizzing solution is still best kept off desirable plants.
How to
Dissolve about 60g of percarbonate per litre of warm or hot water, then spray liberally on the deck. Leave it for an hour, by which time it will be inactive. The deck should then be scrubbed with a stiff broom and hosed off.
Need to know
The breakdown products are oxygen gas and sodium carbonate, which will do your soil nothing but good.
Price guide
About $65 for 1.7kg of powder. On Trade Me you can buy 5kg of pure sodium percarbonate for $34.50.
Common brands
- Kemsol
- Wet and Forget Hit the Deck