How to soft wash timber decking & wooden buildings

August 25, 2023 3 Comments

How to soft wash timber decking & wooden buildings

There are many species of wood included under umbrella terms such as oak and pine
. And the density of the wood can vary dramatically, even though we may call it by the same name.

This varying density influences the ability of wood to absorb softwash chemicals. The denser the wood the less chemical will be absorbed.

Because there are so many varieties of wood – and so many sub-species within each variety – it is impossible to know all the effects that different chemicals will have on "wood".

For example, Pitch Pine is extremely dense, which makes the wood hard and heavy, whereas Scots Pine is the opposite – yet they are both called "Pine".

Only a company specialising solely in timber treatment could build up such an experiential database. And even then treating a test patch before soft washing the entire surface would be highly recommended.

We've found substantial differences in the effect of treating wooden sheds, fences, decking, garden furniture and cladding because, even though on a cursory first glance they may look similar, the wood used in their construction may be quite different.

So the rule, as always, is treat a small, out of the way test patch first. Get your customer's approval of the result they can see on the test patch, and their permission to continue, before soft washing the entire wood surface.

Quick tip: Scroll to the foot of this page if you are looking for information on how to soft wash outdoor wooden garden furniture.


Pressure (jet, power) washing wooden buildings & timber structures

When treating light green algae on easily accessible timber structures such as decking, fences and wooden sheds – many soft washing contractors will lightly pressure wash and then treat the wood with Benz Bio Cleanze.

This provides the customer with an excellent result and is an acceptable approach to soft washing many wooden structures.

However ...

Imagine trying to do that on the property below, in which you can see Ben soft washing cedar tiles, which are badly contaminated by green and black algae, with Bio Cleanze:

Soft wash cleaning cedar tiles - shingles

It would likely not be financially viable to pressure wash this building prior to soft washing because of the extra time it would take and the need to use a cherry picker or similar high reach equipment. There's also the risk of damaging the tiles (shingles) by pressure washing to be considered.

We treated the cedar tiles on this job with Bio Cleanze at 30:1 (we used this relatively high concentration because of the severity of the infestation). We first concentrated the spray on the flat "face" of the tile to kill the worst of the green and black algae.

The following day we sprayed it again, this time concentrating on spraying up into the lapped joints.

By treating twice, allowing 24 hours for the algae to partially break down between treatments, we gave Bio Cleanze the best chance of doing a good job in the time available.

An even better approach would be to wait seven days before applying the second treatment, although the logistics of a contract (travel time, access, and so on) may preclude this.


As with the soft washing business in general, the principles are simple but the permutations of chemical-material compatibility, accessibility, the expectations of our customers and their wilingness to pay, can be complex.

Quick tip: Be aware that pressure washing any wooden building, or timber structure such as decking, can sometimes cause an unsightly "striping" effect.

This can be particularly apparent around hand rails, gate panels, etc. Striping
is usually due the operator using excessive pressure. Skilful use of the pressure washer is therefore called for. We emphasise the use of light pressure washing, carried out with skill and care.


Pressure washing before treating with Bio Cleanze will take longer than just applying the chemical. So every contractor must weigh up the equation of time, cost, need, and quality of result.

There's no hard and fast rule and ultimately we all learn the way that suits us, and each particular contract, through practical experience.

On large cladding jobs, such as the property in the above photo, large fences, etc, pressure washing prior to soft washing with Bio Cleanze will probably not be the most effective approach.

This is because of the increased labour costs and the potential for causing damage (striping and physical damage to the tiles/shingles). And so it's often a better approach to use pure soft washing methods.

How to remove green algae from wood using Benz Bio Cleanze

Our experience suggests that, in general, treating with Bio Cleanze is usually the best way to soft wash wooden structures such as decking, fences, sheds and cladding that are contaminated by green algae.

Simply spray and leave the algae to die and fade away over time through natural weathering.

Offer your customer "Clean & Maintain" annual treatments to keep their property permanently clean – and give you a nice source of repeat business.

Cleaning wooden decking and walkways such as boat jetties

We've found that, generally, first giving the surface that will be walked on a light pressure wash removes the worst of the heavy algae and produces the best final result.

Pressure washing will remove the majority of heavy green algae from the decking surface. This means that Bio Cleanze has less organic growth to kill. And therefore there will be more unused chemical left to provide long-term residual biocidal protection against re-colonisation.

Quick tip A sales point for annual maintenance treatments of any wooden walkways, e.g. timber decking and timber jetties, is that of health and safety. Timber covered in green algae can be very slippery and presents a very real safety hazard.

Any commercial establishment such as a cafe, restaurant, or boat club will know they must do everything they can to stop their customers hurting themselves, or face claims for injuries. So you can show a genuine financial advantage to these establishments from using your soft washing service.


Bio Cleanze can be applied directly to heavy green or black algae without first pressure washing; but you need to consider the following factors:

  • Much of the soft washing power will be used up in killing the green or black algae. This means there will be less remaining in the wood to provide the valuable long-term residual biocide protection against re-colonisation.
  • The green or black algae may turn white, leaving a whitened appearance to the wood that some customers don't like, although this will often fade in time.

    It can usually be removed by pressure washing but we don't recommend this approach because jet washing after applying Bio Cleanze will remove some of the active ingredient from the timber surface, lessening the residual biocidal effect that helps prevent re-contamination.
  • Quick tip: After the algae has been removed by treating with Bio Cleanze – or Benz Lightning Cleanze – it may expose previously coated (i.e. with paint, varnish, or other products) sections of timber that may then look unappealing. These can be re-coated or left for the remaining coating to wear off over time.
  • Be aware that there may be grey stains or residue remaining after application, although this is rare. The best practice is to let your customer know about this in advance, so they are prepared if it happens.

    At the risk of sounding repetitive, always first treat a small, out of the way test patch if you are at all uncertain about the most effective treatment before soft washing the entire property.

In the photo of a test area below you can see the effect on cedar wood tiles that were colonised by green algae, after only three days following a treatment of Benz Bio Cleanze at 30:1

Benz Biocidal Wash remove green algae from cedar tiles - shingles

Long-term protection against re-colonisation – a big advantage of using Bio Cleanze to treat timber

  • The active ingredient in Bio Cleanze – ddac – is well-known to be a powerful wood preserver, and is used in wood preservative products to protect timber against wet and dry rot.
  • Thus, by using Bio Cleanze to treat your customer's timber structures you are offering them a valuable additional service above and beyond the initial cleaning.
  • You can make this feature of Bio Cleanze a powerful sales point when tendering for a contract, which may be sufficient for the customer to choose your service over the competition.
  • Remember that other cleaning companies are likely only offering jet washing or sodium hypochlorite treatments. So tell your customer that neither of these approaches will help preserve their timber, nor offer long-term residual biocide protection against re-colonisation by green and black algae.

How to remove green and black algae from wood using Benz Lightning Cleanze biocide

As when treating with Bio Cleanze, we've found that, generally, first giving very heavy contamination a light pressure wash removes the worst of the heavy green or black algae and produces the best final result. Alternatively, be prepared to give the surface two chemical treatments.

Quick tip: If there's only light algae contamination a single treatment with Lightning Cleanze may be sufficient by itself.


There are several reasons why, in general, we do not recommend using a sodium hypochlorite product as a first choice to soft wash wood:

    1. Chlorine is used by pulp mills to break down the fibres of wood in the paper-making process. So treating wood with sodium hypochlorite is never going to be the best way of cleaning wood because of the potential for inflicting long-term damage on the wood fibres.

    2. The colour that remains after soft washing wood with sodium hypochlorite can have a "bleached out" look. Some customers may not mind this; some may even like it, but many will not. So proceed with caution and get your customer's approval of a small test patch for the result they are likely to end up with before cleaning the entirety of their timber property.

    3. Sodium hypochlorite kills the green and black algae (thus making it a more effective treatment than pressure washing alone) but, simply because of it's chemical nature, cannot provide long-term residual protection against re-colonisation.

    An exception where Benz Lightning Cleanze can be the best choice is when soft washing timber wall cladding, including cedar tiles (shingles):
    Timber wall cladding often turns very black and it may be impossible to to restore the surface effectively using Bio Cleanze alone.

    Also, it may be too expensive or impractical to carefully pressure wash. Fortunately, treating with Lightning Cleanze at 10:1 usually produces a dramatically fast result. So treating timber with Lightning Cleanze can be the most effective approach for some contracts. Then maintain ongoing with Bio Cleanze.

    Quick tip: We recommend thoroughly rinsing the surface immediately before applying Lightning Cleanze and immediately after it has done it's work to minimise chlorine damage to the wood.

    And, assuming your customer is willing to pay a bit extra for the best possible job, after rinsing Lightning Cleanze from the surface treat the timber cladding or cedar tiles (shingles) with Bio Cleanze at 50:1 (40:1 in winter cold).

    Bio Cleanze will provide valuable long-term residual biocide protection against re-colonisation by green and black algae as well as a degree of protection from wet and dry rot.

    The photos below show the before and after effect of treating cedar tiles (shingles) with Benz Lightning Cleanze, which were badly colonised with black and green algae.


    Soft wash cedar tiles - shingles remove green & black algae



    Soft wash cedar tiles - shingles clean green & black algae

    You'll notice that the cedar tiles (shingles) have whitened after treatment. We told the customer about this in advance, so they knew what to expect, and they were very happy with the result.


    Now, having pointed out the possible shortcomings, if the use of Lightning Cleanze is indicated for a particular contract – as was the case for the property in the above photos – let's look at some ways to protect the wood.


    How to protect wood/timber when soft washing with Lightning Cleanze

    There are a some unscrupulous contractors who use very high concentrations of sodium hypochlorite to soft wash wood/timber structures. They achieve a very fast result, get paid, and disappear.

    They are giving the soft washing industry a bad name as their methods can seriously damage their customer's timber property. Using high concentrations of sodium hypochlorite in a softwash solution also carries inherent risks to the health of the operator, their customers, the general public and of course children and animals.

    This is a similar situation to the way unscrupulous caustic soda dippers destroyed vast numbers of valuable old furniture during the paint stripping craze. They used too high a concentration of chemical and left the wooden furniture immersed in it for too long.

    Techniques to protect timber from damage when soft washing:

    • Always pre-soak the wood with fresh water before soft washing wood with Lightning Cleanze. The water will soak into the wood and ensure the softwash solution stays on the surface, where it is needed to kill off the colonisation of green and black algae.
    • Thoroughly rinse Lightning Cleanze from the wood after no more than one hour. Do not leave to soak as the sodium hypochlorite can damage, or at the very least weaken, the structure of the wood.
    • Never use highly concentrated sodium hypochlorite to softwash wood as the potential for causing long-term harm increases proportionately to the strength of the mix.

      Lightning Cleanze is much kinder to wood as it is formulated to offer the highest possible cleaning effectiveness while eliminating the majority of the risk inherent in highly concentrated sodium hypochlorite.
    • We recommend applying LIghtning Cleanze at 10:1, which you'll find sufficiently strong for most of the timber infected with green or black algae that you may come across.
    • Whenever possible choose Bio Cleanze as your first choice to soft wash wood.


    How to treat outdoor wooden garden furniture

    Treat with Lightning Cleanze at 10:1. (not Bio Cleanze, if people will sit on treated chairs or eat at treated tables)

    If the contamination is severe, give the garden furniture a very light pressure wash first.

    The furniture will require regular maintenance treatments going forward if it is left outside, every 6-18 months depending upon local conditions.

    Quick tip: Wood varies in density and it's response to treatment at any given ambient temperature. The algae will disappear from some wood surfaces very quickly, whereas other types of wood it may take longer to go completely.

    Click the link below to read articles related to soft washing wood


    We hope this information is helpful and sincerely wish you great success in your soft washing business,

    Benz Team

    Click here to get more practical soft washing how-to tips

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    Phone: 0800 70 74 222 


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    Phone: 00353 214 622 978


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    Please note: The above numbers are for orders only (click here for tech support)

    PS: Click here to set up your "Clean & Maintain" programme
    . After completing an initial soft wash treatment, put your customer on a "Clean & Maintain" ongoing maintenance programme. Your customers will enjoy a permanently clean property and you will enjoy a steady stream of repeat business.

    This is clearly a win-win arrangement for you and your customers.


    Click here to explore the extensive range of Benz softwash cleaning chemicals & equipment

    Benz Softwash Bio Cleanze DDAC biocide & Benz Softwash Lightning Cleanze SH sodium hypochlorite biocide

    3 Responses


    October 06, 2023

    Wooden artistry showcases the beauty of imperfections. Knots, grain patterns, and natural variations in wood add character and make each piece one-of-a-kind."


    October 29, 2021

    Hi Keiron,

    Tornado Cleanze could be used if the colonisation is extremely severe, what we call a “rescue clean”. But not for ongoing maintenance, for which Bio Cleanze is the product of choice.

    Because the concentration of sodium hypochlorite is so strong in Tornado Cleanze, any treatment will require adherence to our guidelines for pre and post drenching with fresh water to avoid damaging the wood. And the activation time should be as short as possible.

    We would treat with Lightning Cleanze first (starting at 10:1 and moving to 5:1 if necessary) and only using Tornado Cleanze if the results from Lightning Cleanze indicate the need for a stronger treatment.

    We’ve never come across this situation arise ourselves and so have not used Tornado Cleanze on wood. Lightning Cleanze and Bio Cleanze have always proved to produce excellent results.

    Keiron Bradley
    Keiron Bradley

    October 29, 2021

    Hi guys, another interesting artcle as always and one I have read several times before.
    I have a customer with what appears to be red algae on patches of her decking boards. I will, as I always do, thoroughly soak the area first and then I was thinking of trying a test patch, with a limted dwell time to start, using Tornado Cleanze.
    I’m sure that wll be OK and won’t affect the look / colour of the decking, but would be very grateful for any observations you might have.
    Many thanks

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