Suspended timber floors are very difficult to survey on a pre purchase survey, if the surveyor isn’t able to lift any floorboards to inspect the sub floor. Lots of properties now have laminate floor coverings, or vinyl floor coverings stuck down in kitchens, or even tiled,  which no vendor would wish to lift to facilitate a full sub floor investigation. I can survey your suspended timber floor using high powered cameras if sub floor vents can be removed, which can help in matters like this.

During a survey the surveyor will often do a bounce test to the floors, where they will jump on the floor to feel how firm it is under the foot, if the floor is carpeted it maybe possible to stick a damp meter into the timber and measure the moisture content of the timber, and look at the levels of the floors to see if the floor has dropped in places etc.

A surveyor will also see how many, and also the location of air bricks to aid sub floor ventilation, and also note ventilation pathways, for example if the property is a middle terrace with a living room and dining room followed by a new build kitchen at the rear incorporating a solid floor, this would be restricting cross flow ventilation pathways if there is no ducting present. It is common for builders not to incorporate ducting from the timber suspended floor through the new build to ventilate out.

See diagram below.


So what can you do to take the survey a step further?

A hammer probe can be used to check the moisture content of the surface of the timber through a carpet and also through into the timber joist. A good place to check to find a loose floorboard to lift is the under stairs cupboard or a built in cupboard typically by the fireplaces. It maybe possible to drill a small hole in a floor board and view the floor with a high powered boroscope. Sometimes it is possible to remove an air brick and insert a CCTV drain camera system, we can also video and photograph the defects for the report.


sub floor inspection / survey

In an ideal world many of the floorboards would be lifted so you can actually visually check the floorboards, joists and plates. Cellar fungus decay typically leaves a thin surface veneer and it’s not until you drive a bradawl into the timber you notice how significant the decay actually is.

Why is ventilation important?

Ventilation is paramount to prevent decay and woodworm infestation, fungal decay and woodworm need moisture to survive, without this they will not survive. Timber is naturally hygroscopic, this means it will absorb moisture from the surrounding air. Suspended timber floors are cold places, with cold temperatures subsequently there will be high relative humidity readings. Older historic properties are more likely to have a hardwood flooring timbers, this is less prone to decay and infestation unlike modern fast grown softwood. Most of the really active woodworm infestations I find are where the floor is a softwood, some of these floors have been dangerous and have collapsed.

The below picture shows a severe active woodworm infestation, this was in a property in Trowbridge Wiltshire.

active woodworm frass

The fungal decay is a dry rot, this is from the sub floor which consisted of new pre treated timber, the ends of the timber were not treated hence thats where it appears the rot has started. This dry rot was in a suspended timber floor in Wiltshire.

dry rot in a suspended timber floor

I have been collecting data from many sub floors over the years to see what is actually happening, and collecting data using data loggers is a relatively simple exercise that helps in the diagnosis and understanding of fungal decay and woodworm problems.

See the graph below that we produce in excel, we also use this data logging for the internal environment typically for condensation and mould surveys. Where the green and the blue line cross, shows when condensation is occurring. This floor was virtually suffering from condensation during the whole time it was being monitored.


Sub floor condensation is where water droplets of water are dripping off the floorboards. The below is a picture of a sub floor condensation issue, and there is also a significant dry rot issue, this was caused because there was a lack of sub floor ventilation in this property situated in Devizes Wiltshire.

dry rot in a sub floor

The below picture shows sub floor condensation issue and early signs of wet rot fungal decay in a property in Trowbridge Wiltshire.

wet rot in a suspended timber floor

The two below graphs show data logging of the sub floor void, this is measuring cross flow ventilation from the front and the back of the house. This is carried out using a calibrated anemometer, looking at the graph you can see how this went up and down because of the wind, even in that short space of time. This showed there was adequate cross flow ventilation in the sub floor void.

measurement of cross flow ventilation in a sub floor void


measurement of cross flow ventilation is a suspended timber floor void

Another thing that some people fail to realise is that the suspended timber floor void is also partly controlled by the internal environment, if you have a high vapour pressure in the building because of poor mechanical ventilation this will go to lower vapour pressure, like the roof void and also the sub floor void.

The majority of suspended timber floor damp issues are because of excess moisture because of cracked and displaced joints in drains, along with blocked and restricted soak-aways, and even high ground levels. Depending on ground conditions if clay for example, you can also find that when the suspended timber floor void floods, that water can stay around for a considerable time. So you can imagine if these defects have occurred, just installing more sub floor vents will not fix the issue.

It isn’t unusual to have issues with damp and musty smells within properties that are associated with damp issues, sometimes these can also be related to lack of sub floor ventilation from  even when cavity wall insulation has blocked any air flow. Again, sometimes defective drains are a common cause of musty smells within a property, this is because properties with earth floors will have a higher vapour pressure if damp, than within the property. This can then help transport the smells to say the living room area.

See this video of me explaining the basics about a suspended timber floor survey. Don’t forget if you need a CCTV drain survey give us a shout to find or eliminate the potential timber decay cause.

If you need a sub floor survey / cctv camera sub floor survey / investigation for woodworm or dry rot – wet rot please contact us and we would be happy to assist you 01225 769215

Posted by Complete Preservation


  1. Not sure what the problem is but we have just purchased a grade 2 listed building in devizes. The removal company noticed the floor was soft when they were moving the furniture around, the chap lifted the floor board and it was wet underneath with yellow fluff that smells earthy. We know this is a serious problem, but what shall we do?


    1. Hi Dawn, sorry to hear this. It seems that you have some form of fungal decay. Give me a ring to discuss everything, but I would recommend a survey to find out what is going on.


  2. This is the perfect blog for anyone who wants to know about this floor topic. Thank you for sharing!


    1. Thanks, just trying to cover basic points so people can have a better understanding of issues 🙂


  3. We own an old end of terrace property that has a permanently damp underfloor soil to the front room. The suspended wooden floor has the condensation/water droplets beneath it as shown above. The property only has a single vent at the centre of the front stone / brick cavity wall to vent underfloor and cavity. All other property walls are solid 9″ brick. We can’t vent rearward as the floor there has very large stone slabs on solid earth. Would 2 additional vents through the side wall venting the underfloor void help at all ?


  4. Hi,

    You could possibly try putting a new vent in the front wall, and connecting a pipe on this, and then taking it as far back as possible. This should help with ventilation. I would also look at getting the internal ventilation looked at, vapour pressure goes down it’s pressure gradient, its likely moisture internally can also be causing issues with the sub floor. I have a blog post to finish on this, once i’ve done it I will send to you email 🙂


  5. Hello I have exactly the situation as set out in your diagram meaning that our dining room smells musty. How can we resolve lack of subfloor ventilation when we cannot fit airbircks due to mid terrace/concrete extension? Is this something you do or is there a company you recommend?

    Thank you



    1. Hi Davina,

      We can help with this if you wish. Email me on


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