How to fix under-floor dampness?

We acquired an old terrace house from my dad's estate, and there's dampness in the sub-floor soil. It looks like some of the timber also has fungal decay that will need to be replaced.

We're about to identify any potential leaks and fix that up first. There's no smell under house apart from dampness diagnosed during building inspection, so I'm ruling out sewerage. What else should I be looking for when identifying source? Also, what are good options to improve ventilation? we have a small ventilation fan, but likely need more solutions.

solutions to fix and add better ventilation? Screenshot 2025-10-23 at 9.58.30 AM.png

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Dave-1

Good Morning @JustMike7

An old terrace house ould be pretty nice to fix up. Lots of work potentially but wow.

The only thing that comes to mind is better ventilation, tho that could be a double edged sword in promoting dry/damp cycle. I would still install better ventilation but monitor the condition of the area frequently.

Old houses/inner city house may have a lot of moisture accesable under the house from the way they were built, I dont know of an diy way of lessing this. If you want to permantly be able to stop it I would recomend a professional damp expert/builder.

Dave

MitchellMc

Welcome to the Bunnings Workshop community @JustMike7. It's fantastic to have you join us, and many thanks for your question about under-floor dampness.

It's great to see that @Dave-1 has already given a helpful reply.

Many areas under homes naturally retain some dampness, so a slightly moist sub-floor isn’t immediately a cause for concern. Standing water or persistently wet patches, however, do need to be addressed. From your images, there don’t appear to be any overly damp areas, though there is some moisture migration into the timber bearer, as seen on the wall in your photo. Installing a small flashing under the bearer could help prevent moisture from tracking up through the brick.

When identifying the source, check for plumbing leaks, gutter or downpipe issues, subsurface water entry from surrounding soil, and any roof drainage problems that could channel water under the house.

Improving ventilation is key to managing dampness. Upgrading your existing fan is a good start, but adding passive vents along the sub-floor perimeter and ensuring airflow from one end to the other can significantly reduce moisture. If there is no active water pooling, these measures will typically resolve most dampness issues and help protect timber from further fungal decay. Is your current fan venting to the outside through ductwork?

You could also look at adding a waterproof membrane over all the block work, which will help prevent water weeping up through it.

Please let me know if you have any questions.

Mitchell

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