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April 9, 2019

Suspended Floor

by Geoff Flavell in Damp Proofing Blogs

Poor Subfloor Ventilation causing Timber Decay

Unless subfloor ventilation of suspended ground floor is adequate, decay is very common because the moisture content of the joists and floorboards can become very high, This is caused by the subfloor atmosphere having a persistently very high relative humidity, wood

absorbs water from air of high humidity and if the condition persists the wood can become very wet and start to decay.

The high humidity in the subfloor space is produced by evaporation of water from the soil under the building and from the walls below the damp proof course, This includes both structural walls and any dwarf walls provided to support the timber floor. The soil is often covered with a layer of oversight concrete but this makes little difference unless it is laid on a damp proof membrane.


To prevent excessive humidity in subfloor spaces ventilation below the floor must be increased and air vents positioned to obtain even ventilation of the whole area. Where even ventilation is difficult to achieve the amount of water vapour generated from soil oversite can be reduced by covering with polythene. However it is difficult to do anything to reduce evaporation from walls below the damp proof course, Wherever there is any doubt as to the efficacy the ventilation the timbers should be treated with a dual purpose wood preservative to prevent timber decay.

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