ventilation for suspended timber floors

claire_lou1978

In the process of purchasing a house and the homebuyers survey has flagged that the ground at the rear of the property is too high and therefore there is inadequate ventilation under the suspended timber floor.

Has anybody else come across this and if so how easy is it to rectify??

poppysarah

Airbricks - not dear to get put in.
But ground too high means damp proof course could be being bridged?

claire_lou1978

Thank you!

Surveyor didn't mention any issues with damp proof course only ventilation?!?!

Any ideas who I need to speak to for advice - i.e is it worth speaking to a local builder for advice??

poppysarah

You could ring round a few builders and damp proofers.

If you have a friendly builder who you trust that would help.

Wee_Willy_Harris

Might be worth having a chat with the surveyor. They may be able to advise what is needed to satisfy their concerns.

tyllwyd

I think you need to have a better idea of what the actual issue is - are there any airbricks there already? Is it just a case of getting a spade and clearing away a bit of earth that's built up next to the wall, or is there more work involved?

We had an issue where the builders built steps across the airbricks for our suspended floor - the building regs man was quite happy for them to leave a small gap between the airbrick & the step, and just said to keep an eye out in case there was any damp in the future.

David_Aldred

Hi claire_lou,
The function of the air bricks in this case is to provide ventilation beneath suspended timber floors in order to prevent the moisture content of the wood reaching the threshold of decay (20-22%). Dry and wet rot spores are in the air all the time and only require wood of a certain moisture content to germinate and commence decay. If the wood is kept at a moisture content below the threshold of decay then the decay will not occur. If the wood is already at a moisture content above the threshold of decay and in the process of rotting then lowering the moisture content to under 20% will cause the decay to cease irrespective of preservative treatment. The key to keeping wood free from decay is therefore to maintain it at a moisture content of under 20%.

Dampness evaportating up from oversite beneath the suspended timber floors will cause the sub floor void to become a damp environment and the timber floors etc will want to be in balance with such a damp environment such that their moisture content will increase above the threshold of decay in order to be in equilibrium with the damp air and accordingly the wood becomes at risk of decay. Timber that is damp is also at greater risk of wood boring insect attack. In order to prevent this problem vents are placed around the perimeter of the property at low level to allow cross ventilation of the sub floor void thus removing this moisture laden air and hence keeping the moisture content of the timber ground floor below the threshold of decay.

From the above you may appreciate that sub floor conditions can vary from one house to another and some are quite dry whilst others may even be flooded with standing water under floors. In addition the number of vents may have been inadequate when the property was first built or the number may have been compromised by solid floor extensions being added covering original air brick locations. Sometimes the sub floor vents can be blocked with debris or compromised by new paths / soil beds being laid over original levels.

In the first instance the poor sub floor ventilation should send an alarm bell to have all suspended timber ground floor areas checked for moisture content, decay and wood boring insect attack especially those timbers hidden within the sub floor voids. Hopefully the moisture contents will be below 20% and free from decay / wood boring insect attack but this may well not be the case and should be confirmed prior to purchase given the risks identified.

Vents need to be cleared and additional vents provided sufficient to maintain all timbers at a moisture content below 20% once all areas have been inspected / any repairs undertaken as necessary.

New vents should be ducted all the way through the depth of the wall (i.e in the case of cavity walls the vents should not just vent into the cavity of the wall but be 'tubed' all the way from the outside to the inner sub floor void). Where paths / ground is much higher than sub floor areas periscope type vents can be used available form builder's merchants. Hope this helps kindest regards David Aldred Independent damp, wall tie and and timber surveyor.

RenovationMan

We're moving next week into a house where the structural surveyor stated that the exterior ground was too high. You can remedy this by lowering the ground and by angling the pathways or patio stones away from the house, so that water runs away and not towards the house. You can also put in a French Drain, soakaway to take the runoff away, though our surveyor said that in our case it would make matters worse.

claire_lou1978

Thank you so much for all your replies (it really is a minefield!!)

I presume the issue lies with a patio that the vendors have put in blocking the rear vent (front vent is ok).

Will contact a local builder and ask for some further advice :eek: