Identifying and Treating Dry Rot and Wet Rot on Structural Timbers

Author: Danny Neill - Operations Manager at Platinum Chemicals Ltd

How To Treat Dry Rot and Wet Rot

Welcome to our Guide to Identifying and Treating Dry Rot & Wet Rot on Structural Timbers.

Platinum Chemicals supply a wide range of Dry Rot and Wet Rot Remedial Treatments used for the eradication of Wood Rotting Fungi on timber / wood.

Identifying Dry Rot and Wet Rot on Timbers:

Dry Rot and Wet Rot can affect any building at any time, and if there are any signs of decay on timbers it should be identified and treated promptly.

Wood Rotting Fungi obtain their food by breaking down wood cell walls, causing loss of strength. They cannot utilise wood below 20% moisture content and therefore do not occur in internal building timbers unless poor design or building faults have led to water entry or condensation.

Not all fungi found in buildings cause wood rot, however the presence of any fungal growths indicate that damp conditions exist which are also suitable for the development of wood rotting species and some remedial measures may be necessary to locate and remove the source of dampness, otherwise there is a risk that wood rotting fungi could become established.

As already mentioned, there are two main types of Wood Rotting Fungi found in buildings - Dry Rot and Wet Rot.

With Wet Rot, it is not always necessary to distinguish between the many types of wet rot which can be found in building timbers, as the same remedial measures are required for all of them. However, the successful remedial treatment of Dry Rot may require more elaborate and sometimes expensive measures.

It is therefore essential to establish whether Dry Rot or Wet Rot is present.

Dry Rot (Serpula Lacrymans):

Dry Rot Fungus is found mostly on softwoods. A major outbreak of Dry Rot often causes extensive damage to buildings.

The appearance of a Brown Rot typically occurs on wood embedded in, or in contact with wet brickwork.

Dry Rot is sensitive to high temperatures (over 25°C) and drying and is therefore rarely found on exposed timbers or situations where fluctuating conditions are likely (well ventilated sub-floors, roofing timbers etc).

Dry Rot is able to grow through Masonry / Bricks and Mortar (See Masonry Treatments for products to treat Dry Rot on Masonry), although it cannot feed on these substrates as it does with timbers.

Dry Rot Strands can transport moisture from damp areas, allowing spread of fungus to dry wood in non-ventilated conditions.

The appearance of a 'fruiting body' may be the first indication of an outbreak.

Decayed Wood that has been affected by Dry Rot may be a dull brown colour, typically with deep cracks along and across the grain, light in weight and can crumble between fingers.

Fungal Characteristics of Dry Rot:

Mycelium: Silky white sheets or 'cotton-wool like' white cushions with patches of lemon yellow or lilac tinges where exposed to light. In less humid conditions forms thin, felted grey skin. During active growth the advancing hyphal edge forms a silky fringe.

Strands: White to Grey, branching sometimes as thick as a pencil and brittle when dry.

Fruit Body: Usually found on wood / wall joint, rarely found on the exterior of a building. The Fruiting Body of Dry Rot is tough, fleshy which is 'pancake' or 'bracket' shaped. The centre of the fruiting body is yellow-ochre when young which darkens to a rusty red colour when maturing, owing to spore production, covered with shallow pores or folds; margin white or grey.

Spores: Profuse and may settle as a fine layer of red-brown dust on horizontal surfaces.

Remedial Treatment of Dry Rot on Timbers:

Firstly, establish the size and significance of the attack. In particular if structural timbers are affected, carry out or arrange a full structural survey to determine whether structural repairs are necessary and, if they are, take appropriate steps to secure structural integrity.

Locate and eliminate the source(s) of moisture and promote rapid drying of the structure.

If timber has lost its structural strength, cut away the affected timber approximately 300mm - 450mm beyond the last indications of fungus.

Sound timbers can be treated with products such as Lignum Pro D156, Lignum Pro Gel or a Boron Wood Treatment to prevent further outbreaks. Replacement Timbers must also be treated with one of these products.

Prevent further spread of Dry Rot within masonry / brickwork by use of a separate Masonry Treatment. The use of the above mentioned Timber Treatments will have no effect on Masonry.

Bower Beams should be used for joist ends which have been damaged and lost their strength caused by Dry Rot.

BAT Splice Plates are suitable for butt jointing timbers of similar cross section that have been replaced due to fungal decay.

Do not retain Dry Rot infected timber without seeking expert advice. There is always a risk in retaining infected wood which can be minimised by preservative treatment and subsequent re-inspection.