Home Treatment Guidance The control of Dry Rot
PDF Print E-mail


Guidance on;


Dry rot: General information about dry rot and a suggested control strategy:

dry-rot-treatmentDR 1.0 Dry rot: General Information in brief

Dry Rot (Serpula lacrymans) is termed a brown rot fungus which in the right conditions is able to cause extensive damage to timbers that form part of a building structure. A dry rot attack is sometimes secretive but a virulent dry rot outbreak usually makes itself obvious by causing affected joinery timbers to distort and crack and flooring or other structural timbers to collapse. There are also occasions when historic damage caused by dry rot in the past is exposed during alterations; - In other words a leak or moisture penetration occurred at some time in the past and caused dry rot to develop. The leak was repaired and due to lack of moisture the dry rot eventually died without ever being discovered but the damage caused to concealed timbers by the dry rot remained.

Like all fungi, dry rot distributes itself by the production and release of airborne spores which will only germinate (develop, grow) if they land on a suitable food medium containing adequate moisture in a favourable environment. The food medium for dry rot is wood (hardwood and softwood) or any other cellulose containing material such as paper, cork, hardboard etc. A favourable environment is dark, dank poorly ventilated areas.

Dry rot grows most rapidly on wood with moisture content above 30%, but not saturated wood, and it is unable to colonise wood that is below 20% moisture content. In well maintained buildings the moisture content of timbers will be below 20% thus there would be no risk of dry rot or even wet rot developing in these circumstances. Dry rot is sensitive to the level of moisture content in timber and if infected wood dries permanently to below 20% it will stop growing and eventually die.

Dry rot is able to grow over and through inert materials such as brick and plaster and has the ability transmit nutrients to the advancing hyphal edge from wood affected by the outbreak. Dry rot only has a limited capability for further growth once its source of food is removed.

Because dry rot is not able to sustain itself on timber that is below 20% moisture content the primary control measure for any dry 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 control 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 dry rot control programme.

Below are two suggested dry rot control regimes - one for dealing with an active dry rot outbreak and the other an historic outbreak. It is important to remember that each dry rot outbreak is unique and these are only suggested control measures. You should always obtain a professional opinion when dealing with dry rot and the primary and secondary control measures outlined in BRE Digest 299 Dry rot: its recognition and control should always be adhered to. Failure to do so is likely to result in on going decay.



Dealing with an active outbreak


DR 2.1 Identify the source of moisture responsible for the outbreak, rectify it and promote rapid drying in well ventilated conditions.


DR 2.2 Identify the area or areas of risk by visual appraisal and use of a surface moisture meter to plot the distribution of moisture in masonry and any timber wetted as a result of the moisture source responsible for the outbreak.


DR 2.3 If not already infected any timber within the identified area of risk that is attached or adjacent to masonry of increased moisture content should be considered at risk of decay. All timber surfaces within the area or areas identified to be at risk and for 500mm beyond, even if not visibly infected should be exposed and carefully examined for dry rot growth. If present within the risk area and provided it is safe to do so for examination purposes this should include:


a) Removal of architraves and skirting boards

b) Removal of window/door linings and if considered necessary the frames

c) Raising floor boards

d) Removal of masonry to expose concealed timber surfaces of any embedded joist ends, wall plates, trimmers, bressumers, lintels, bonding timbers etc. Provide immediate temporary support to any structural timbers found to be infected and inform others as necessary.


DR 2.4 Within the areas identified to be at risk hack off wall plaster back to the structural masonry. This may mean both sides of a wall. If present remove any wall linings and associated fixing timbers.  Subject to timbers infected, sections of ceilings and decorative cornices may also have to be removed for exposure and repair purposes.


DR 2.5 If required provide any necessary temporary structural support. Cut out and remove all timbers supporting surface or internal dry rot growth to a distance of 350mm beyond last visible signs of growth. For structural reasons it may be necessary to completely remove trimmers, lintels and bressumers. It is important to note that dry rot is able to grow a considerable distance internally within large dimension timbers.

Note: There are occasions when for economic, structural or other reasons dry rot affected timbers are left in position and an 'in situ' treatment regime is undertaken. In such circumstances the work should be supervised and timbers and monitored for a period of time by a suitably experienced person until the risk of ongoing decay has subsided.


DR 2.6 Apply a fungicidal gel to all newly exposed cut ends of in situ timber.


DR 2.7 Around the perimeter of the identified risk areas, introduce into the masonry a toxic box or cordon sanitaire using a fungicidal gel or masonry biocide and within the area contained by the perimeter apply a surface application of masonry fungicide in accordance with the following instructions:


DR 2.7.1 Toxic Box - Cordon Sanitaire - Hole Drilling:

In conventional brickwork at the top and bottom of the risk area now hacked off, use a 10 mm drill bit and drill two rows of holes at 70-mm horizontal centres into two adjacent horizontal mortar beds to within 30mm of the opposite face (28/30 holes per linear metre). At the left and right side of the risk area again drill two rows of holes at 70 mm horizontal and vertical centres which in conventional brickwork is the top and bottom of a vertical mortar joint (perp) and 70 mm to the side of this. For other building materials try adhere to this drilling pattern as best able.


DR 2.7.2 Toxic Box - Cordon Sanitaire - Hole Filling:

Have available on site sufficient r masonry fungicide in its ready to use state. Inject the masonry fungicide into each pre-drilled hole at the manufacturer's prescribed rate under low pressure using a suitable pump and a lance trigger with an injector rod attached


DR 2.7.3 Toxic Box - Cordon Sanitaire - Surface Application:

Using a coarse low pressure spray apply a masonry fungicide to the surface of the exposed masonry contained within the perimeter drilling. The usual recommended application rate for this procedure is 2m² per litre but check with product manufacturer. Always read and adhere to the recommended application rate stipulated on the label of the material being used.


DR 2.8 Flush out any recesses formed following the removal of any built in timbers with a Masonry Fungicide


DR 2.9 Whenever possible and practical to do so replace previously removed decayed timbers with inert materials such as concrete and steel.


DR 2.10 If inadequate sub floor ventilation was considered to be a contributory factor introduce an adequate number of efficient sub floor vents which equates to one 230mm by 150mm vent every 1.2 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 good air movement in the sub floor void is possible by forming sub-floor openings as necessary.


DR 2.11 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 should be thoroughly coated with a fungicidal gel. Isolate any newly introduced timber, including joinery from direct contact with masonry with a physical damp proof course material and/or an air gap.


DR 2.12 Reinstate disturbed wall and ceiling plaster. Apply a fungicidal gel to the reverse side of skirtings/architraves and re-fix once plaster has dried.


DR 2.13 Inform the owners of the property the importance of maintaining their building in a dry weather proof condition at all times and explain the potential consequences for not doing so.



Dealing with an historic Outbreak

It is not unusual to discover an old historic dry rot outbreak whilst undertaking other works within a property. This means that the moisture ingress responsible for the outbreak happened at some time in the past, a dry rot outbreak occurred, the moisture ingress was repaired, the walls dried down and as a consequence the 'undiscovered' dry rot died without ever being seen or identified. The old mycelium strands and the decayed wood remain but no further damage is being caused. Such historic outbreaks are usually discovered during property repairs or improvements such as putting in new windows, rewiring or plumbing. Since the dry rot is dead there is no risk of further deterioration to the fabric so what remedial action is necessary in such circumstances?


DR3.1 To identify the areas of risk, assess the location of the discovered outbreak and try to establish what the source of the moisture ingress was that brought about the outbreak. Remember if for example you are looking at an outbreak on the ground floor the cause may well have been a leaking gutter or rain water down pipe at roof level thus all timbers in contact with the wall beneath that point should be considered at risk of decay having occurred.


DR3.2 Having assessed the likely source of moisture that caused the outbreak and identified the areas of risk carry out opening up and exposure as described in DR2.3 above paying particular attention to structural timbers.


DR3.3 If structural timbers are found to be affected these should be drilled and probed to ascertain whether or not their structural integrity has been compromised. Structurally weakened timbers should be removed and replaced whenever possible with inert material such as concrete or steel but if this is not possible industrially vacuum treated timber should be used. As a precaution against future moisture ingress at what has been proved to be a vulnerable point consider isolating any re-introduced timber with a damp proof material.


DR3.4 If structural timbers are affected but are still sufficiently sound then provided the timber and adjacent masonry is dry there is no reason to replace them. In these circumstances the application of a fungicidal gel may be considered appropriate.


DR3.5 Provided the walls and timbers within the areas considered to be at risk are dry there is no need to remove plaster. However if structural movement is observed (cracking of plaster, cracked brick joints etc.) within or adjacent to the areas of risk extra care should be taken to establish whether or not decayed structural timber is responsible for this. It may be necessary to remove strips of plaster to establish whether or not any concealed embedded timbers are present. If such timbers are present and decayed they could be responsible for structural movement.



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