Dry Rot

Dry rot - otherwise known as “serpula lacrymans” – is the term given to a timber destroying fungus which is caused by unprotected timber becoming damp. It is a very serious timber condition that often does damage to timbers that are hidden from view, behind wall fabric or below floors. The term itself is a misnomer, as all wood decaying fungi need a minimum amount of moisture before decay begins.

Dry rot can be found in many countries across the world and it is extremely common. Dry rot originally affected forest timbers, but now it is commonly known for the ability to destroy timbers in both ships and buildings. The problem of dry rot can be dated back as far as the 18th century. It is still not certain where dry rot migrated from to become active in the UK. It has been speculated that dry rot was transported to British shores by ships from Europe carrying infected timbers.

What Causes Dry Rot?

Dry rot can occur in a property when unprotected or untreated timber has become damp. Timber is at risk of a fungal attack when the moisture content of the timber is above 20%. There are many reasons that can cause the timber to become damp:
Dry Rot Repairs - floor
Even if your damp problem has been solved, Dry Rot that colonised whilst your timber was damp may well still be there and active. Any damp problems that can affect your property can dampen your timber to above 20% moisture content and raise the risk of fungal attack. If you have not noticed a damp problem but think you have Dry Rot, this indicates that there is or has been a concealed damp problem affecting the area. The area of your house affected by the rot will be a useful indicator of the type of damp problem you may be facing.

Dry Rot Identification

It is important to identify whether timber decay has been caused by dry rot or another wood-destroying fungus such as one of the wet rots. This is because dry rot has the ability to travel through building materials other than timber, giving outbreaks the potential to spread quickly through a building. For this reason, additional measures (e.g. masonry sterilisation) may require to be carried out when treating dry rot outbreaks which is over and above those necessary when dealing with outbreaks of other wood-rotting fungi.
Typical indications of dry rot include:
  • White, fluffy ‘cotton wool’ mycelium develops under humid conditions. ‘Teardrops’ may develop on the growth.
  • A silky grey to mushroom coloured skin frequently tinged with patches of lilac and yellow often develops under less humid conditions. This ‘skin’ can be peeled like a mushroom.
  • Strands develop in the mycelium; these are brittle when dry and crack when bent.
  • Fruiting bodies are a soft, fleshy pancake or bracket with an orange-ochre surface. The surface has wide pores.
  • Rust red coloured spore dust frequently seen around fruiting bodies.
  • Active decay produces a musty, damp odour.
The decayed wood affected by dry rot takes on a dark or browner, crumbly appearance, with cubical like cracking or checking, that becomes brittle and can eventually crush the wood into powder.
Dry rot spore

Dry Rot Life Cycle

The dry rot life cycle consists of 4 main stages, each with their own tell-tale signs that can help you identify a dry rot outbreak.

Dry Rot spores
Spores are omnipresent, meaning they exist everywhere. Individually they are invisible to the human eye however when in large numbers they appear as orange and brown dust, this is one of the simplest ways of identifying a dry rot issue. The lifecycle of dry rot begins when spores come in contact with timber in a favourable environment. Once on the timber the spore will germinate and produce Hyphae growth.
Like any life form, dry rot can be stopped by a lack of air, food or water. But what makes dry rot annoyingly unique is that when short of such vital elements, the dry rot produces a self-reproduction organ known as a sporophore. This allows the spore¬ bearing surface of the sporophore to shed orange/red coloured spores into the atmosphere in the hope that that the spores can land once again in the right environment to carry on germinating and extending the growth period of the dry rot.
Sporophore dry rot
Hyphae Dry Rot
Hyphae act as the root of the rot, stringing fine strands to grow through the timber. The hyphae will then feed on the sugars within the timber known as cellulose, hemicellulose and lignin. The dry rot fungi / fungus produces enzymes to split the sugars, reversing the formation of the wood. These enzymes, however, are unable to break down lignin. The subtraction of these sugars results in cross grain cuboidal cracking, reducing the timber to an unsound structural state. Hyphae then multiply and colonise together, generating mycelium growth, a fluffy cotton-wool like substance.
Mycelium can travel great distances to find new sources of food, and it is this ability to spread through various building materials (it can even go through bricks and mortar!) that allows a dry rot outbreak to progressively feed on timbers throughout an entire property. Following the structure of a property drying out, it can lay dormant for anything up to ten years, and has the ability to spring back to life should the environmental conditions become damp.
Mycelium dry rot

Dry Rot Treatment

Dry rot will only affect timber that is damp, typically affecting timber with a moisture content in excess of 20%, typically in the region of 28 – 30%. For this reason, removing the source of moisture should form the core of any dry rot eradication strategy. 
Timber can become moist for a variety of reasons. Among the most common reasons are the following: leaking washing machines, shower trays, baths, and condensation. External sources of dampness can also be a problem; leaking roofs, for example, rising damp, and penetrating damp are all external sources of moisture that can lay the groundwork for dry rot to take hold. Whatever the source of the moisture, if it is combatted and allowed to dry out, the dry rot infestation will eventually come under control. It should be noted that dry rot draws moisture from the colonised timber and can transfer this moisture to adjacent unaffected timber to create the appropriate damp conditions for growth.
Dry rot repairs in progress
It is not, however, always possible to ensure that the timbers will remain dry in the long term. Therefore, it is important that alternative and secondary measures are taken to protect against the return of dry rot. Any and all affected timbers should, where possible, be removed and replaced with pre-treated timber protected from touching the damp masonry. Any remaining timbers at risk of infestation by dry rot should be treated as quickly as possible with an effective fungicide. Where the dry rot has passed through the masonry – something that can happen more often than people realise – it should be isolated using physical containment and/or masonry sterilisation.

Dry Rot Treatment in Stone and Brick

The best product to use to treat and eradicate Dry Rot is a masonry biocide. You can brush the solution onto the affected masonry or spray it depending on the size of the affected area.

Dry Rot Treatment to Timber

There are several different products available for dry rot treatment to timber. There are products that enter the timber itself as well as ones designed for coating the timber surface. Fungicidal gel can be painted onto the timber surface, and comes in different concentrations depending on whether you want to prevent Dry Rot or treat it.

Rods made from highly concentrated borate can be inserted into holes driledl in the timber, and fungicidal paste can be injected into pre-drilled holes in the timber using a skeleton gun. The use of these chemicals should be specified and applied by a property preservation specialist.

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Frequently Asked Questions about Wet Rot

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