How To Clean Timber Decks

Including cleaning of:

  • Hardwood Decks
  • Softwood Decks
  • Timber Fences

Water blasting a timber deck to remove fungal, moss and lichen growths will severely damage the surface. The pressure required to remove moss, mould and lichen will damage the timber fibres leaving deep gouges and a furry striped finish.

The best method for cleaning timber is to use a mild exterior cleaner like Bio-Shield®.

Treating a deck with Bio-Shield will kill and gently remove the growths then keep the surface clean for extended periods. 
Regular lighter applications of Bio-Shield at the first sign of re-growths will effect a rapid clean up and keep the surface clean again for an extended period.
A deck that is kept clean with Bio-Shield® will also prevent the build up of black fungal and lichen growths that make a deck slippery and dangerous to walk on when its wet.

Should I water blast clean or use an exterior cleaning solution to clean a timber deck?

Cleaning a timber with high-pressure water will damage the surface, shredding the timber fibres and cause a furry look when the timber dries, with the severe damage to the timber fibres. The pressure required to remove algae and lichen is just too much for the timber to withstand, even on hardwood timbers.

Instead, the use of a mild exterior cleaner is recommended to ensure that no damage is done to the surface of the deck. A good exterior cleaning solution will kill and remove the growth while also keeping the surface clean for extended periods. This can be maintained by using regular lighter applications of the cleaning solution at the first sign of regrowth. This means that it is quicker and easier to clean up the deck the next time any growths show up.

A deck that is kept clean with an exterior cleaning solution will also prevent the build up of black algae and lichen growths. These growths can make a deck slippery and dangerous when wet.


Preparation is the key to a successful do-it-yourself project. Following these easy measures will ensure that your timber deck has a nice, professional appearance and long-lasting protection against algae and moss regrowth.

Remove any pet toys and feeding dishes from the area to be cleaned and treated. Make sure to keep children and pets off the treated surface until it dries and don't feed pets on the treated surface for 2 to 3 weeks.

Make sure the deck is clear of loose debris such as tree leaves, and dirt. It pays to also cut back any overhanging trees to help keep the deck clean and provide the exposure to rain the treatment requires.

The exterior cleaning solution is most effective on a completely dry timber deck.

Check the weather forecast, or rain radar - you are going to need 3 to 4 hours of no rain to effectively treat timber. Furthermore, air temperatures above 10 degrees C° assists in accelerating the biocidal action.

Follow your chosen exterior cleaning solutions mixing instructions to prepare the solution for the application.


If you have a respirator, now is the time to put it on along with a pair of gloves and gumboots or long pants. Try to stay upwind during application if you don't have a respirator. On a warmer day, evaporation of the solution will produce vapour that should be avoided as much as possible.

Slow evaporation speed aids the treatment process, so the best time to treat a timber deck is on an overcast day. If this isn't possible, early in the morning after the deck has dried out or late afternoon are good options too.

Fill your sprayer with your chosen mild exterior cleaning solution and pump it up to pressure.

2 to 3 applications of the exterior cleaning solution are required to saturate a timber deck and keep it wet for 10-15 minutes for the treatment to work.

Apply slowly and carefully so that the following stroke overlaps the previous by 50 percent to guarantee uniform and thorough coverage. Angle the spray head into the gap in between boards to prevent missing the lichen growth in the gaps.

Always treat the entire deck and apply enough coats to ensure the deck stays wet for 10 to 15 minutes. If you fail to get complete coverage, the treated areas will appear lighter than the untreated areas, giving your deck a patchy appearance.

As the exterior cleaning solution takes action, the deck may become orange-brown - this is a positive indication that the solution has broken down the cell walls of the algae.

The surface of heavily blackened timber will require some additional effort to assist with cleanup.

After 3 to 4 weeks, hose and brush down the deck with a stiff yard broom to agitate and loosen the surface, before flushing the deck with clean water to remove any sludge. Run the broom along the boards, with the grain. Doing this in the rain is better as the timber will be softer and require less effort to agitate.

This extra clean up is only needed on the worst decks and will only be required for the initial clean up, and not for the maintenance cleans, provided the outdoor cleaners are applied regularly. Apply your exterior solution at the first sign of any regrowth to easily maintain the appearance of a clean timber deck.


Provided the treated surface is exposed to rainfall it will continue to clean up over the next 3 to 12 months. The more rain the surface receives, the faster it will clean up.

Treatment should be reapplied at the first sign of regrowth. This is usually every 6- 12 months for shady areas, and up to 3-4 years for other areas. Applying a treatment at the first sign of regrowth will ensure that the clean up is easier, faster and requires less solution. New growths will die off and clean up quickly.

Once the deck surface is cleaned the first time, maintenance cleaning with an exterior cleaning solution will be fast and efficient.