Dry Rot

Dry rot is a fungus that can seriously damage the structure of your house, so make sure you're aware of the dangers


Dry rot is a fungus that thrives in slightly damp, not saturated, conditions. If the moisture content in your house is around 30 per cent then the timber structures are likely to develop dry rot and if it’s higher, wet rot.

The majority of structural timbers in roofs will retain a moisture content around 15 per cent, varying slightly with the seasons. Although fungal spores and risks of decay are widespread, normal standards of heating and ventilation will discourage dry rot and typical damp treated houses will usually emerge unscathed.


The risk areas where drying may be delayed long enough for the rot to take hold and grow are concealed spaces and within floor or roof voids close to damp masonry, which provides a reservoir of moisture to feed the fungus.


The tell tale signs of incipient dry rot are whitish strands or filaments, which can be hard to see initially. Once the timber shows signs of shrinkage and cracking, the rot is more advanced and if the red dust of spores is found then fruiting bodies will be present. (fig 147 & 148) In contrast, wet rot whitens timber and reduces it to a stringy substance.


If in any doubt, advice should be sought from a specialist surveyor or contractor - timber treatment specialists will usually give initial advice without charge.

Chemical solutions can be used to treat the wood, which is the quickest process. Chemical-free treatments are available and although they take longer than more conventional approaches, they can be much less destructive and thereby more economic.

Low toxicity preventative treatments, such as a water-based boron solution, will discourage fungal decay. However, applying this treatment will introduce moisture, so sufficient time must be allowed for treated timber to dry before it’s sealed near to residually damp masonry.

Masonry may warrant treatment also because dry rot is capable of spreading via its filiaments across two metres of masonry in search of more timber.

Useful Contacts

Hutton and Rostron www.handr.co.uk

Words: Jonathan Hetreed Image: Andrew Smith

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