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How to remove mould in subfloor?

Hi there,

Im hoping for guidance on resolving yellow and white mould in a subfloor please?

I recently bought by a 1970’s house that had a concrete slab poured to the entire block minus the subfloor 13 years ago and I believe has caused a water drainage issues, and inevitably a yellow/white coloured mould. The slab has a slight slope towards the front of the property, which causes water that is captured in the backyard, to flow against the back of the house.

Since purchasing two years ago, I have installed ventilation incl. brick vents, four fans and a French drain with agi line and a pit at the back of the house. This has resolved any water issues however the subfloor (clay) soil continues to stay moist after rain and the mould remains.
Any hints or tips would be highly appreciated.

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Comments

JacobZ

Hi @jd15,

Thank you for your question about moisture beneath your subfloor.

You’ve already done a lot of the right work by improving ventilation and drainage under your house. The fact that water no longer pools means the main issue is now residual moisture in the clay soil, which stays damp long after rain. That lingering moisture keeps the air humid and encourages mould to grow back.

To deal with the mould that’s already there, wait until the soil is as dry as possible, then spray with Concrobium Mould Cleaner, ensuring you wear a well-fitting mask while doing so. If the mould is mainly on the soil, lightly rake the top layer before spraying so the product penetrates better. Allow it to sit for a week before moving on to the next step.

I'd then suggest you lay heavy-duty builders’ plastic such as this CAgroup 4 x 40m x 200Um Black High Impact PolyPRO Builders Film across the ground. Cut it to size, overlap the sheets by about 20 cm, and tape the joins with duct tape. Weigh it down with bricks, pavers, or small stones so it doesn’t shift. This barrier stops dampness from the clay evaporating into the subfloor air, which helps the area dry out much faster and makes it harder for mould to regrow.

You can also make small adjustments to keep air moving under the house. Check that your vents aren’t blocked by soil or debris and run your existing fans for longer after rain to help the space dry out. If you can, leave the subfloor access hatch slightly open on warm, breezy days to let out trapped moisture.

Allow me to tag our helpful members @Dave-1, @AlanM52 and @Nailbag to see if they have any other ideas.

Jacob

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/58971#Comment_58971

Good Evening @jd15

I have had the same issue in the past with water and mould. Water issue now solved (cross fingers) but white mould around the edge of the brickwork is still there (it leaves me alone, I leave it alone is kinda my attitude towards it)

Soil will always have a subsurface amount of water wicking through it, The only way I can think of to dry out your top of soil under the house is a more acyive ventilation process. Something that is powered and on, maybe a solar fan that runs during the daylight hours? Having an exhaust fan will also draw in air from outside and that could cause issues so it may not be a straight forward exhaust the air, it may reuire a flitered air intake and then an outlet to draw the moisture away.

Dave

Nailbag

Hi @jd15

From my experience with what you have started and following this suggestions by @JacobZ, personally I would call it resolved. If your land is on a slight slope with dampness under the house, I think you will find pretty normal. You could mix in some dry soi or sand as @Dave-1 suggests, but this wont resolve rising damp from soaked ground. Unless there is water pooling or an increase in dampness, then again following the extra steps by @JacobZ should resolve the scenario

Nailbag.

jd15

https://www.workshop.bunnings.com.au/discussion/comment/58971#Comment_58971

Thank you Jacob, Dave and Nailbag. Highly appreciate the input - I feel like it’s a very common issue for homeowners especially on the east coast with these continued wet periods.

I will take all your advice on and report back with progress after 6 months. I think the guys are spot on with the fact the clay soil is naturally going to soak up water every rain season.

thanks again guys

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