Timber Contamination and Dry Rot

Timber contamination

Moisture content

A moisture content of about 22% is required in any timber before fungal decay can occur. The speed of decay will depend on the moisture content. A moisture content of approximately 26% will encourage rapid deterioration. For practical purposes a Surveyor should only assume that timber can remain safe from fungal decay when it has measured moisture content of 20% or below. There is a 2% difference here, this will allow for a margin of error within the metering equipment or the circumstances that exist at the time of the inspection. This allows for some fluctuation in moisture content within timbers that are not fully accessible such as the ends of joists etc.

 

There are a number of commonly found species of timber destroying fungi, which we will come into contact with on a day to day basis. These fungi are spread by microscopic airborne spores; these are around us all of the time in the air. If they enter a property and settle on the surface of damp timber they can germinate and develop fine thread like filaments called hyphae. Hyphae will penetrate to the wood releasing chemicals; these chemicals are known as enzymes. The decay is caused by the process of these chemicals removing the nutrients from the wood. The nutrients are then absorbed by the hyphae, encouraging further growth to occur. If the moisture content of the timber reduces below 22% this process can no longer operate and the decay will stop. Moisture content above 22% will as previously described encourage further growth of the thread like network. When a system is formed throughout a section of timber, this is known as mycelium.

 

The mycelia of different types of fungi will vary in colour and appearance, once you are familiar with the different types of hyphae and mycelium, this will aid your identification when establishing the exact type of fungus responsible for the damage being caused. The actively growing mycelia will spread and the hyphae behind will fuse to form thicker strands; these will conduct food and water. Periodically the fungal cycle will be completed and the spore bearing fruit body or sporaphore will be produced on the surface of the decaying timber. Fruiting bodies take a variety of shapes, sizes and colours , these are also very useful in helping the surveyor to identify the type of fungus responsible for the damage which is being caused.

 

Types of insect species

 

Common furniture Beetle - (Anobium Punctatum)

 

Habitat

Soft woods and European hard woods, especially birch, used in plywood. Sap wood is mostly affected but heart wood can also be affected when wood rot is present. The recently formed sap wood with higher nitrogen content is preferred. The level of larva growth decreases towards the older sap wood where the nitrogen content is less. This insect is frequently found in old furniture or unfinished surfaces such as drawers, backs of cupboards; under tables etc. it is also found around loft accesses and under stairs and areas affected by damp and moisture content, which has an influence on larva development. This insect is probably the most common domestic pest present in this country at this time.

Identification

The common furniture Beetle is between 3-5mm long, it is dark brown with lines of pits (punctuations) along white surfaces. The larva of the common furniture beetle is up to 6mm long it is curved and a pale cream colour. It will remain at this stage for 3-5 years. The emergence holes and tunnels are 1 ½-2mm in diameter. They are of round orientation but mainly along the grain. The fraze or bore dust is gritty to feel between the fingers and pale cream in colour, containing lemon shaped pellets.

Common Furniture Beetle

 

Common Furniture Beetle Larva

 

 

True Dry Rot Fungus - (Surpula Lacrymans)

There is only one true dry rot fungus in this country, the latin name is surpula Lacrymans. It is always necessary to include the latin name quoting the most commonly used terms for these types of rot, as professionally speaking, you should after identifying a particular type of fungi be able to provide the client, customer, or Architectural Surveyor with the correct latin name. Dry rot is a brown rot which will only attack the celluloids within the timber. The most striking result of this will be the large cubiodal cracking which will be visible across the grain. The cuboidal sections are generally approximately 25mm or slightly larger in size, and generally speaking are larger than the cubiodal sections created by damage as a result of other brown rot species. Although called dry rot this fungal species does require a minimum moisture content of about 22% for the decaying process to occur. When dealing with dry rot special consideration is necessary when recommending remedial treatment. Dry rot has a much higher tolerance to alkaline and bearing in mind our traditional building construction in the country, this allows the hyphae and mycelium to spread through inert material such as brick and stonework and on the surface of masonry, generally tends not to spread through the mortar joints, or the fabric of the building itself. Where sand and cement or lime based renders have been used in modern times and the alkalinity of the lime is particularly high due to the newness of the material, dry rot is unlikely to be found.

 

Fruiting bodies frequently appear on infected masonry, in some instances it can be present in areas that appear to be devoid of any decaying timber. Clearly the fungus can derive no nutrition from the masonry itself and subsequently will cause no damage to the masonry. But of course the fungus can spread quite rapidly from its early stages in search of a new food source of timber. Growth rates of dry rot vary somewhat, recorded growth rates can be up to an equivalent of 1 meter per year in modern buildings and in time the fungus will spread several meters directly through masonry. Given the right conditions dry rot can spread from basement level through ground floor, first or second floor levels, to the roof of building quite quickly. Therefore it is essential that an outbreak is dealt with immediately without delay, in order to minimise the extent to which remedial work will be necessary. Dry rot growth can only be sustained when there is a timber food source available and moisture is present, logically it will then follow, However if you remove the food source and moisture supply dry rot cannot exist. However as the wall dries out it is very likely that fruiting bodies will occur in order to distribute fresh spores within the vicinity.

 

Obviously this is extremely dangerous, as the possibility of further outbreaks of dry rot in adjacent areas will be vastly increased in this type of situation and it is the main reason why sterilisation of the masonry is required. Not simply to eradicate the infection at its source but also to prevent further colonisation of other parts of the property. Dry rot has the ability to remain dormant in relatively dry timbers including debris fragments. These might be found in areas such as earth oversights and cellars etc. these can all sustain dry rot growth if the conditions conducive to its development reoccur at some later stage. Dry rot has the particularly clever ability to conduct moisture from within the atmosphere thus allowing the fungus to spread from an initial outbreak into adjacent relatively dry timbers particularly in areas of inadequate ventilation or where a further moisture supply may be obtained. Poorly vented areas such as behind dry lining, badly vented sub floor voids etc. are ideal for the development of dry rot

Identification

The fruiting body is a tough, fleshy, pancaked, bracket shaped fungus the centre is yellow in colour when young and begins to darken to rusty red when mature. The bracket fungus is covered with shallow pores or folds the edges of which will be white or greyish. Spores are porfuse and will settle as a fine layer of reddish brown dust on horizontal surfaces. The damage characteristics to the wood are that the timber will show a dark dull brown colour typically with deep cubiodal cracks across the grain, the crack being approximately 25mm sq. or slightly larger. The timber will appear light in weight and when fully decayed will crumble in fingers. The mycelium consists of silky white sheets or cotton like white cushions with white patches of lemon yellow or lilac tinges where exposed to light. In less humid conditions the mycelia forms a thin, felted, grey skin during the course of advancing growth. The hyphae forms a silky fringe to the edge. Strands are white to grey branching out sometimes as thick as a pencil when very well established. The strands are brittle when dry and will appear so when snapped.

Evidence of a Mature Hyphae in skirting board

Evidence of Dry Rot in timber

Dry Rot evidence in masonry