Home Treatment Guidance The control of Wet Rot
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Guidance on:


Wet rot: General information about wet rot and a suggested control strategy

wt-rot-treatmentWR 1.0 Wet rot: General information in brief


Wet rot is a general name given to a whole group of wood rotting fungi that are termed either a brown rot or white rot. The growth of wet rot is limited to the area that has become wet thus the extent of an outbreak is directly related to the source of moisture and its distribution. If for example all joist ends embedded in a wet wall are affected by wet rot then each joist end will be supporting its own individual wet rot outbreak. Wet rot fungi are not able to grow through masonry but in favourable conditions some are able to grow across the surface.


Extensive wet rot decay is sometimes present throughout an entire ground floor affecting floorboards, joists and wall plates. In these situations the source of moisture is usually a damp oversite combined with inadequate or the complete lack of sub floor ventilation. This results in a humid sub floor atmosphere from which the wood absorbs moisture leaving it all susceptible to the germination of wood rotting fungal spores. Good efficient ground floor sub floor ventilation is vital to reduce the risk of wood rotting fungi developing.


Because wet rot is not able to sustain itself on timber that is below 20% moisture content the primary control measure for any wet rot outbreak is to identify the cause of excessive moisture, eliminate it and allow wetted areas to dry out rapidly in well ventilated conditions. This process often takes many months, even years thus a risk period will exist for some time after rectification of the moisture source. Secondary measures are therefore necessary to reduce this risk during this time. Numerous timber and masonry fungicidal products are available to offer protection during this high risk period as part of a wet rot control programme.


Outlined below is a suggested wet rot control regime but as each wet rot outbreak is unique these should only be regarded as suggested control measures. You should always obtain a professional opinion when dealing with wet rot and the primary and secondary control measures outlined in BRE Digest 345 Wet rots: recognition and control should be always adhered to. Failure to do so could result in on going decay.




WR 2.1 Identify the source of moisture responsible for the outbreak and rectify it. Promote rapid drying in well ventilated conditions. If inadequate sub floor ventilation is the cause of the wet rot decay introduce an adequate number of efficient sub floor vents which equates to one 230mm by 150mm air vent approximately every 1.6 metres. The use of terra cotta air vents is not advised as they are only 20% efficient and have the ability to bridge a damp proof course. Ensure air movement in the sub floor void is possible by forming additional openings in subfloor walls as necessary.


WR 2.2 Identify the area or areas of risk using a surface moisture meter to record the moisture content of timbers and to plot the distribution of moisture in adjacent masonry.


WR 2.3 Within the area of risk, by a visual inspection, touch and probing with a sharp instrument identify all timbers affected by wet rot. Carefully assess the condition of structural timbers and in particular any timber embedded in masonry.


WR 2.4 Cut out all structurally weakened rotted wood.


WR 2.5 Following the cutting out of structurally weakened rotted wood apply a fungicidal gel to any newly exposed cut ends of timber that are to remain in situ.


WR 2.6 If wet rot is affecting timbers embedded in wet masonry, design and introduce an alternative means of support to avoid reintroducing timbers back into wet masonry. The use of RSJ's, reinforced concrete lintels, joist hangers, construction of new brick piers etc. individually and collectively often form part of this process.


WR 2.7 Replace all other cut out timber with new industrially pre-treated timber adhering to the appropriate British Standards when necessary. Any newly exposed cut ends of pre-treated timber should be thoroughly coated with a fungicidal gel.


WR 2.8 If the reintroduction of timber into damp masonry is unavoidable then physical isolation between timber and masonry using damp proof course material must be used. The practice of completely wrapping a joist end in a plastic sleeve at ground floor level is not recommended as a micro climate within the sleeve is sometimes created causing condensation and increasing the risk of further decay. The correct use of plastic wedges, an air gap and damp proof course material is advised.


WR 2.9 If considered necessary lay of a damp proof membrane weighed down with sand over a damp oversite to reduce humidity levels of the sub floor void.


WR 2.10 Apply a water soluble fungicidal gel to all other structurally sound timbers considered to be at risk during the drying down phase due to increased moisture content. Subject to the dimensions and accessibility of the timbers involved this may require a drilling and injection procedure.



Wood preservatives and wall treatment fluids are regulated under the Control of Pesticides Regulations 1986. Under these regulations it is an offence to supply, store, sell or use these chemicals unless they have the appropriate approval. Approved products are given an HSE number which must appear on the label. Essential information for the safe use of the products is also given on the label. Specifications for the supply or application of these chemicals should require that only products and processes approved under the regulations shall be used.

©Peter Macdonald 

Contact Peter: Email - info@dampdecay.co.uk - Telephone: 015242 71794