How to prevent mould on composite decking?

Im not sure what type of composite deck we have as we didn't install it, but black mould seems to grow on it every 2-3 months. It can only be removed by pressure washing which is becoming very annoying and a health hazard...does anyone know if we can apply something to the deck to prevent or slow the mould growth? The deck is next to a pool so it does get wet a lot, but even with direct sunshine daily, it doesn't stop the mould. Thank you for your help!

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EricL

Hello @nickomoffs

Welcome to the Bunnings Workshop community. It's sensational to have you join us and thanks for sharing your question regarding your composite decking.

It appears that your decking panels have a very smooth surface. I suspect that because of its proximity to the pool the evaporation of pool water is causing it to form a film of condensation on the surface of the deck in the late evening. The condensation probably does not last long as it gets evaporated by the morning and afternoon sun, but it is enough to start a chain of events that leads to mould growing on the surface.

I propose trying an experiment, cover your pool in the evenings for the next eight weeks. You should be able to see if mould will start to grow on the surface of your deck. If there is no mould after eight weeks with the pool covered, then you have located the source of the condensation. Another option of course is to wash your deck every six weeks with 30 Seconds 5L Spray And Walk Away Concentrate. You can wash earlier if mould starts to form in a shorter time.

Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.

If you need further assistance, please let us know.

Eric

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/47710#Comment_47710

Afternoon @nickomoffs

This is an interesting one tho think it may be hard to track down the cause.

Does the deck have animals (doggos or cats) that hang out on it or even bird life above? And is that your deck cleaned?

When you pressure wash it do you use a cleaning agent with the pressure wash or just water?

Any chance of an overall shot of the deck area to help us out?

A few thoughts that come to mind with using the pressure washer, if you are just removing the visiable mould but spores are still there then that could explain why it returns. Have you done any grinding of metal around teh area? Yeah may sound like a doppy question but I did on my timber decka nd what looked like a mould case bloomed after a few days (wasnt mould, it was decking oil I was just putting down reacting with the metal I thought id cleaned up)

Dave

nickomoffs

https://www.workshop.bunnings.com.au/discussion/comment/47710#Comment_47710

Thanks Eric! It's more kids jumping in the pool and splashing water over the sides this time of year rather than condensation, and being in SE QLD with the current humidity it takes a long time for it to fully dry out before it gets wet again. The main pain point is the time it takes to pressure wash the whole deck every couple of months, but pouring over the concentrate you mentioned every 6 weeks would be more doable! I have attached a photo for reference. Thank you!

nickomoffs

https://www.workshop.bunnings.com.au/discussion/comment/85889#Comment_85889

Thanks Dave - no dogs or cats and no DIY work being done around the deck. The mould just comes back slowly but surely on the deck boards that seem to cop the most water that runs over the side of the pool...which is sadly unavoidable due to kids and summer fun! 😂

I have attached a photo for reference. The previous photo is a patch of deck away from the pool that doesn't have mould on it.

EricL

https://www.workshop.bunnings.com.au/discussion/comment/108224#Comment_108224

Hi @nickomoffs

Thanks for sharing the photo! That pool area looks wonderful. I can now understand why you are getting mould growth every 3 months especially if the kids are in the pool every day. I still recommend using 30 Seconds 5L Spray And Walk Away Concentrate as it will help control the mould and mildew growth in the area.

If you need further assistance, please let us know.

Eric

nickomoffs

https://www.workshop.bunnings.com.au/discussion/comment/121382#Comment_121382

Perfect, thank you!

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/129633#Comment_129633

Evening @nickomoffs

When you said "SE Queensland" my mind went straight to my sisters houses. Both live up that way and the one that lives a little South has a huge hassel with mould. Even my pillow case that I took up on a roard trip started showing signs. I think @EricL's solution might work, especially for the summer time seasons.

Dave

Nailbag

Hi @nickomoffs

There are quite a few causes of deck mould. One you have already mentioned being next to a pool with splashes etc causing prolonged exposure to water on the deck. Regions of high-humidity can further add to the issue. So, is it only these constantly wet areas getting the mould or all of the deck?

Are the gaps between the boards all clear? Over time these can fill with garden debris etc, thus not allowing water to drain off as well as it could.

Cleaning the deck with a pressure washer, but not actually treating the mould as @EricL mentioned, will only cause it to return sooner than later. A very long time ago I had a house with an outdoor spa that suffered from mould at certain times of the year. I used a mildly diluted white vinegar to treat the mould ofter washing the deck down. This would be safe to use on your composite deck.

Nailbag

https://www.workshop.bunnings.com.au/discussion/comment/85893#Comment_85893

It's worth checking out if you can either get a genuine or after market power head for your pressure washer @nickomoffs Thought there are very effective ones and not so, but a good one will literally save hours off the deck cleaning chore.

Nailbag

Screenshot 2025-01-21 at 10.30.21 pm.png

nickomoffs

https://www.workshop.bunnings.com.au/discussion/comment/47734#Comment_47734

Thank you @Nailbag and @Dave-1 - going to give all of these suggestions a go and see if we can keep the mould at bay for the coming months. Thanks for the help!

diy_hausdesigns

Hi all - saw the thread and thought to share my insights. Mould thrives when the is constant dampness / mosture and no / limited air movement. Board spacing plays a critical role in drainage and air circulation to prevent mould, particularly in an area like that. In addition to the others advice I'd definitely look into the spacing to ensure moisture drains and the deck can breathe, as it looks quite tight.

Cheers.

Nailbag

https://www.workshop.bunnings.com.au/discussion/comment/85916#Comment_85916

Keep us in the loop @nickomoffs would be handy to know for any future enquiries if both my plan and @Dave-1 was helpful.

Nailbag

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