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Understanding rot

Rot spores are omnipresent, looking for ideal conditions to germinate and grow. Timber rot needs sufficient food (timber), oxygen and water to germinate and grow. The best way to stop timber rot is to stop the source of water. Good ventilation and airflow normally prevents wood from becoming damp.

Wood cells in contact with moisture, swell and split open, allowing enzymes excreted by rot to digests cellulose. This is called the fibre saturation point . The saturation necessary for Dry rot is between 20% and 30% moisture. Wet rots, which are effectively any wood attacking rot other than dry rot, tend to need more moisture about 28% or more. Damp meters are useful tools for determining the water content of timber.

Identifying rot

Once the source of water has been stopped, the main concern with rot is structural damage. Structural damage depends on species, so we need to identify the key characteristics of three fungal types. Timber rot is classified as Dry rot, other brown rots or a white rot.

Dry rot

Dry rot needs a special mention as it is by far the most virulent and therefore the most expensive rot to treat. Latin name Serpula Lacrymans, it often causes structural damage to sub-floor joists, floorboards, skirting boards and ceiling joists. It is most commonly caused by condensation or leaking downpipes. A common feature is poor sub-floor ventilation. It can grow across masonry and survive for a year without water.

This page has been updated following a fascinating talk at the SPAB by Dr Jagjit Singh, Mycologist, Building Pathologist and founder of Environmental Building Solutions Ltd.

Dr Singh was the first person to link domestic Dry Rot with Dry rot in the wild. He found a description and image of a rot that looked like Dry rot, and traced it back to the Himalayas.

Both the Dutch East India and The East India Trading Company Ltd were founded around 1600. Ships were sometimes built in India from timber felled in the Himalayas. When old ships came to the end of their useful lives, timber was reused in buildings, transferring the deadly Dry rot to European buildings.

The fruiting body only shows when the rot starts to outgrow its food source or it comes under biological attack.

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Dry rot, Serpula Lacrymans – visible water droplets

Rot digests glucose with enzymes; glucose + oxygen → carbon dioxide + water
Water is the bi-product. In poorly ventilated areas these water droplets promote and accelerate cell damage in timber perpetuating rot growth.

Dry rot is hard to find, especially if covered with carpet, laminate etc. If Dry rot is visible, then look for the risk of rot, poor sub-floor ventilation (through flow from front to back), high sub-floor humidity and a source of water.

Dry rot

Dry rot is the most virulent brown rot. It often causes structural damage to sub-floor joists as well skirting boards and ceiling joists. It is most commonly caused by condensation, or leaks to poorly ventilated joists. It can grow across masonry and survive for a year without water.

The tell-tale sign of a Dry rot attack is a dusting of rust coloured spores mainly up to 10 metres. Spores come from a fruiting body, which is often pancake, or bracket shaped.

Looking closely at the fruiting body, spores are released from long jelly like tentacles from within the fruiting body.

Strands, or hyphae act like plant stems for growing across dry material.

The Latin for Dry rot is Serpula Lacrymans, or snakes tears, so named because rot releases water during respiration.

Rot digests glucose with enzymes; glucose + oxygen → carbon dioxide + water

Water is the bi-product. In poorly ventilated areas these water droplets promote and accelerate cell damage in timber perpetuating rot growth.

Looking under timber we see a mixture of colours from white, mauve and yellow mycelium. In most circumstances wood damaged by Dry rot should be replaced. If wood has had limited damage, it can be kept, so long as the damp has ceased, ventilation is improved and the timber is treated with Di-sodium octaborate. Rot will return if humidity returns, so monitor for recurrence of rot and relative humidity to ensure dampness has ceased.

In the above image the dry rot damaged joist has lost all strength, disintegrating like powder.

Perhaps the most damaging place to find dry rot is in and under painted skirting-boards, on any floor. Look for distortions in the wood and a source of water, which is often condensation.

Wet rot

Wet rots are by definition, all timber rot other than Dry rot.

White rot

White rots are wet rots (i.e. not Dry rot) that digest lignin, resulting in cracking along the wood grain. White rots are normally slow growing and rarely cause significant structural damage in residential settings.

Brown rot

By contrast brown rots attack carbohydrates in wood, causing cracking both along and across the grain. This is known as cuboidal cracking, that is cubes of timber, across and with the grain. As a result of the cuboidal cracks, wood loses more strength under a brown rot attack compared to that of white rot.

Dry rot is also a brown rot (but not a wet rot).

Tips for identifying and treating rot

  1. Identify the root cause of damp.
  2. If rot is in the sub-floor, improve sub-floor ventilation, such as additional air-brick and removing obstacle to airflow. Don’t forget that a majority of dampness in properties is caused by condensation, so consider internal ventilation and monitor relative humidity.
  3. Determine the loss of strength from the timber.
  4. Remove damaged timber. It may be necessary to remove whole sections of timber in contact with dry rot, but not necessarily.
  5. Less can be removed if:
    • The risk caused by damage is limited, for example Dry rot is in a corner, with a shallow sub-floor compared to a first floor corridor.
    • Sub-floor humidity is monitored (to ensure root cause of water has been resolved, and there is sufficient ventilation. This to ensure relative humidity comes down to normal levels (below 85%RH). Consider using a blue tooth connected data logger.
    • Timber is observed occasionally such as with a borescope.
  6. Consider applying Sodium Octoborate, the professional treatment for rot.

In the words of a senior PCA lecturer;

“Dry rot guarantees are the best, as Dry rot only grows if there is water.”

Therefore if the owner doesn’t remove the source of water, or allows water to return, it is not the damp proofer’s fault.

Failed treatment – avoid rot irrigation

Damp proofers recommend treatment with irrigation, often referred to as a toxic box. Walls around the Dry rot are drilled into and Disodium octobrorate is injected. These toxic boxes make no difference, in fact they can increase the risk and growth of rot through increased humidity.

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