WOOD-ROTTING FUNGI

ISSE

Wood Rotting Fungi

• Decay of timber occurs only when wood is allowed to remain permanently or regularly damp.

• Decay is described as either wet rot or dry rot; although both can occur together in damp wood, what type of rot develops depends on the conditions in the building.

Wood Rotting Fungi

• Can develop on susceptible wood if the moisture content of the wood remains above about 22% regularly and for prolonged periods.

• Develop from minute airborne spores which germinate if they land on a suitable substrate such as damp wood.

• Produce thread-like hyphae, which collectively form a mycelium.

• The hyphae making up the mycelium penetrate the wood, breaking down the wood cell walls and feeding on them.

• Mycelium produces a fruit-body; this releases spores to the atmosphere, completing the life cycle.

Wet Rot

• Wet rot is a general term used to describe the majority of fungal species responsible for timber decay. This rot attacks wood in damp conditions and can cause significant damage over time, if left unidentified and untreated.

• Can be divided into two groups, according to their effect on the wood.

• Brown rots cause the wood to become darker in colour and to crack along and across the grain (Fig 2); when dry, very decayed wood will crumble to dust. Many common wet rots are brown rots; dry rot is also in this group.

• White rots (Fig 3) cause the wood to become lighter in colour and lint-like in texture, without cross cracks. All white rots are wet rots.

Wet Rot

Typical Damage Caused by White Rot

Typical Damage Caused by Brown Rot

IDENTIFICATION

• Distortion, discolouration, softness, cracking and the loss of strength to the timber

• Smell - there may be a damp musty smell.• Visible fungal growth - this may sometimes

occur.• Fruiting Bodies and Spores• Mycelium and strands• Form of decay. White or Brown rot

• The presence of wood-rotting fungi in building timbers indicates a fault in building design, construction or maintenance.

• The primary aim of remedial treatment of all wood rot is, therefore, to identify and eliminate all sources of wetting and to promote drying of the structure

• Where it is particularly difficult or impracticable to dry a structure adequately, greater reliance must be placed on the secondary measures; sometimes it is advisable to replace rotted wood with alternative materials.

Remedial Treatment of Wet Rot

Remedial Treatment of Wet Rot

BRE Digest 345 (1989) Wet rots: recognition and control

Dry Rot

• Decay of timber occurs only when wood is allowed to remain damp over

• long periods. In buildings, the most serious type of decay is dry rot which

• is caused by the fungus Serpula lacrymans.

Appearance of wood affected by dry rot

• Difficult to detect dry rot because it nearly always develops out of sight,

• Some indications of the possible presence of dry rot are softening of the wood in some areas, shrinkage and distortion and a distinctive 'mushroom' odour

• Wood rotted is light in weight, crumbles under the fingers and has a dull brown colour. It no longer has the fresh or resinous smell of sound wood. Often, the wood shrinks and splits into brick-shaped pieces formed by deep longitudinal and cross cracks, similar to the appearance of charred wood but not as darkly coloured.

Similarity in appearance between dry and wet-rotted wood

Dry Rot

Wet Rot

• The fruit-bodies are shaped like pancakes or brackets and are tough and fleshy when fresh, but brittle when dry.

• The margin remains light but the centre becomes rusty red when spore production begins.

• On vertical surfaces the fruit-body may grow as a thick bracket and the pores appear as small stalactites.

• Fruitbodies may form under floors and in other concealed locations. The spores are microscopic, very light and can be spread throughout a building by air currents, often settling to form a thick layer of reddish dust on horizontal surfaces.

Fruiting Bodies

Sheets of dry rot mycelium exposed by removal of timber wall panelling

Ability to spread

• Dry rot fungus can also grow on the surface and within other substrates such as brick and plaster. The fungus cannot however derive nourishment from non-organic materials.

• Fungal growth over these materials is sustained because the dry rot strands are able to transport dissolved nutrients to the advancing hyphal edge from the wood at the source of the outbreak.

• In suitable damp conditions, dry rot growth can extend a distance of several metres from its food source through plaster and masonry.

• • If the original food source becomes exhausted before the fungus

reaches more timber, it will die. But if further timber is encountered this may act as a new food source, allowing further spread of the outbreak.

Remedial Treatment of Dry Rot

• Because the dry rot fungus cannot grow on wood which has a moisture content below about 20%, the primary aim, which all too often receives inadequate attention, should be to identify and eliminate sources of dampness and then dry the structure as rapidly as possible.

• It may take a long time, perhaps a number of years, to re-establish dry conditions and secondary measures, which are temporary but quick acting, will often be required to prevent further damage before drying is completed.

Remedial Treatment of Dry Rot

BRE Digest 299 (1993) Dry rot: its recognition and control

Safety of preservatives

• Wood preservatives are regulated under the Control of Pesticides Regulations 1986. Under these Regulations it is an offence to supply, store, sell or use wood preservatives unless they have appropriate approval. Products approved under the Regulations are listed in Pesticides published annually by HMSO. Approved products are given an HSE number which must appear on the label. Specifications for wood preservative supply or application must specifically require that only products and processes approved under the Regulations shall be used.

All information contained in this presentation has been taken from:

• BRE Digest 345 (1989) Wet rots: recognition and control

• BRE Digest 299 (1993) Dry rot: its recognition and control