Paint It Or Peel It? What’s The Best Practice For Algae Covered Decking?

Nothing spoils the look of your nice timber decking like slimy green algae growth in grooves and cracks. It can be tempting to lather on a layer of paint right over the top for a nice clean finish – but out of sight is not necessarily out of mind. 

It’s a common myth that painting over the top of algae will kill off any spores and keep the growth from returning on your freshly painted deck. Unfortunately, this is far from the truth! Painting over the top of algae growth is, at the very most, a temporary solution. While it may cover up the obvious staining and discoloration, the live spores still present will soon make a reappearance. 

And while scraping or stripping off existing paint before repainting is still the best practice, this also won’t eliminate live spores, particularly when algae growth has penetrated through layers of old and peeling paint.

If you’re looking for a long-term solution, removing algae and mould before painting is the best option – but there are a few other tips and tricks that can be used to prevent the problem from returning again and again. 

Most cleaners and treatments effectively remove algae, mould, mildew and moss, covering all your bases and taking the stress out of identifying exactly which uninvited guest has taken up residence on your deck. 

And the good news is that if algae is removed effectively, you may not need to repaint the area at all. 

Does Paint Properly Cover Algae Growth?

Many brands of paint boast that their formulation prevents or slows algae growth, which can be useful for outdoor areas. However, these products don’t actually kill off existing algae or mould present on the surface when it’s painted. 

Deciding whether to paint over algae on your deck? Unfortunately, there’s a long list of cons and only one pro: it may be quicker and easier than properly eliminating algae growth, and a new coat of paint will temporarily hide the telltale green stains. However, the effort you’ve invested into repainting will soon be for naught: the pesky growth will reappear on your newly painted surface in a matter of weeks to months. 

What Types Of Paint Can Be Used To Cover Algae?

For algae-prone areas – particularly covered decks and patios where moisture has trouble evaporating – an algae-resistant variety of paint can be a good investment to reduce the amount of future maintenance needed. Algae and mildew should always be treated before repainting, but if your deck is algae-prone, it can help to choose a variety of paint designed for this kind of damp, shaded area. 

Algae-resistant paint and stain varieties suitable for decking include:

• Cabot DeckCorrect – resists dirt, algae and mildew, fills cracks, skid-resistant 
• Olympic RESCUE IT! MAX Multipurpose Products – primes, seals and corrects in one
• OSMO Natural Oil Woodstain – base and top coat in one, resists algae and mould

Paint also grows more porous over time, meaning older paint is more prone to collecting moisture, enabling the growth of algae or moss. Repainting a surface is a good way to boost its moisture-repelling capabilities, keeping slime at bay for longer. 

Many decking oils, primers and sealants also have algae-resistant properties and can help protect your deck while allowing the natural colour of timber to shine. 

For maximum protection before applying a new layer of paint, a product such as Quantum TS 12 timber steriliser can ensure all possible fungal and algae growth is eliminated before repainting.

However, the best preventative is to address the causes of algae growth and eliminate the conditions needed for algae and mould to thrive. 

There are three main problems to tackle if you want to prevent algae growth:

• Sweep your deck regularly to prevent the build-up of leaves and other debris which encourage algae growth 
• Keep rain and snow away from your deck as much as possible 
• Remove obstacles shading your deck from the sun, or be prepared to invest a little extra time and effort keeping algae at bay 

What Cleaning Products Should I Use To Remove Algae From Decking?

Some guides suggest using water alone to remove algae growth – either through scrubbing and hosing or with a pressure washer – but this isn’t sufficient to remove invisible spores from the deck surface. A bleach or fungicide should always be used before painting to thoroughly kill off any live spores. 

There are specialist deck cleaning products available from hardware and landscaping stores, but many household chemicals will also do the trick when it comes to removing algae. 

Oxygen Bleach

Most commercial deck cleaning solutions use oxygen bleach (eg. sodium percarbonate), the same kind found in the laundry aisle, so a DIY cleaning mixture can be equally as effective when it comes to removing algae growth.

To make your own deck cleaning solution, mix 7.5L of warm water with two cups of oxygen bleach (found in the laundry aisle). You can also add ¼ cup of liquid dishwashing detergent to boost its grime-lifting properties. More intense patches of algae growth can be tackled with a paste of oxygen bleach and a little water, just as you’d apply to clothing to lift stains. 

Any bleach product does run the risk of affecting the colour or stain of your deck, so it’s a good idea to test your DIY cleaner in an inconspicuous place before applying to the entire deck. This may be less of a priority if you’re already planning to repaint after cleaning. 

Chlorine Bleach

Chlorine bleach is commonly recommended by DIYers, but can actually cause damage to existing paint and compromise the structure of the timber itself. Bleach run-off can also harm the health of plants and lawns nearby, which is another good reason to give it a miss. 

Spray-On Fungicides

Commercial spray-on fungicides can be an excellent option to safely kill off algae growth before repainting your outdoor decking. With a non-caustic solution that won’t harm plants or grass, these cleaners kill off the algae spores and help prevent regrowth. Popular brands include ‘Wet & Forget’ and Pro-Kleen ‘Simply Spray and Walk Away’. 

With a spray-on fungicide, you’ll need to leave time for the product to eliminate algae and fungal growth before repainting the deck – typically a few days for green algae and a month or so for black mould growth, though pressure or power washing can speed up the process. 

These gentle fungicides won’t interfere with the application and drying of your paint, so once visible algae has disappeared, you can go ahead and paint the deck as per usual. Of course, it’s always a good practice to clean a surface thoroughly before applying paint, and your deck is no exception. 

What Tools Should I Use To Clean Algae From My Deck?

A long-handled brush or broom is the perfect companion to your homemade or commercial deck cleaner, but a regular scrubbing brush will also do the trick. Either way, synthetic bristles are the best choice when working with chemical cleaners. 

Removing algae from your deck only requires a few simple steps:

1. Spray or pour the cleaning solution onto the deck and allow it to sit for 10 to 15 minutes (or according to the label). Don’t allow the bleach or cleaner solution to dry on the deck. 
2. Scrub thoroughly with your choice of brush. 
3. When dirt and algae is visibly removed, rinse thoroughly with a hose. 
4. If there any spots you’ve missed, feel free to go over them again before leaving the deck to dry naturally. 

The fastest way to clean both algae and chemical cleaners from a deck is undoubtedly with a pressure or power washer – but using such a powerful tool can also cause damage to your deck. Even at the lowest effective pressure setting (500 to 800 PSI) a pressure washer can still cause splintering, stripped paint, raised fibres and lap marks on the timber if handled incorrectly. 

If you’re determined to use a pressure washer, follow these guidelines:

• Use a wide fanning nozzle (40° to 60°)
• Keep the nozzle around 12 inches from the surface
• Keep movements even and consistent to avoid lap marks 
• Clean the entire deck for even colouration 

However, for most homeowners, mastering the safe and effective use of a pressure washer is more hassle than cleaning the deck the old-fashioned and risk-free way, with a long-handled scrubbing brush. 

Looking for a nice in-between option? A low-pressure pump-action garden sprayer can be an effective tool for applying fungicides and cleaners more quickly than a spray bottle. 

Related Questions

Is Algae Dangerous?

Algae growth on a timber deck is fuelled by the accumulation of moisture, often thanks to the build-up of leaves and debris. This moisture is what poses the real threat to your deck, and as it penetrates, can lead to wood rot and structural damage. The presence of algae and mould is more of a warning sign that conditions on the surface of your deck are far from ideal. 

While it’s moisture that does the real damage to the wood itself, algae and moss can also make the deck surface slippery and dangerous to walk on, so it is certainly a safety hazard. And where algae thrives, mould and other fungal organisms also tend to breed, exposing your family to potentially dangerous spores. 

Does Paint Cover Algae Properly?

Paint can cover the visible signs of algae on a surface, but doesn’t kill the live spores. Painting over the top of algae on wooden surfaces such as fences and decking is not effective at permanently solving the problem, and you can expect algae to reappear within a few months. 

Does Algae Rot Wood?

Algae itself does not eat into timber, but the kind of moist environment in which algae thrives also spells trouble for the long-term health of wooden walls, fences and decks. Bacteria and fungi thrive in wet, shaded conditions, particularly where leaf litter and debris offer plenty of fuel for these microorganisms to reproduce. Over time, these factors will lead to wood rot and damage the structural integrity of timber, so it’s vital to treat algae and mould growth wherever it appears.