Wet rot decay and dry rot decay have many similarities. There are many types of wet rot fungi and only one dry rot fungus; however, both require the timber to be quite damp in order to take hold. In both cases, decay is fundamentally caused by fungi that attack wet timber. In the case of wet rot decay, the fibre saturation point is around 28%, with optimum moisture conditions for fungal growth between 30 and 70%.
The dry rot fungus (Serpula lacrymans) can affect timber with moisture content as low as around 22%, but the optimum moisture content for growth is between 30-55%. In other words, the timber has to be quite damp to become affected by decay of any type. Timber is unlikely to be susceptible to decay if the moisture content is below 22%, but for practical purposes timber is considered to be outside a risk of decay if its moisture content is maintained below 20%.
Beam end affected by wet rot decay. This is a common defect where timber is built into damp masonry. |
Wet rot decay affects this floor joist. The surface of the wood has been removed exposing cuboidal shape cracks beneath. |
Wet rot fungi can be subdivided into two broad categories: brown rots and white rots. Brown rot fungi break down cellulose in the timber causing it to darken and to crack across the grain into characteristic cubes. Brown rot fungi often affect floor joists and skirting boards in contact with damp walls. White rot fungi break down both cellulose and lignin in the timber, bleaching the wood, and causing it to crack along the grain into fibrous strands. White rot fungi tend to affect window frames, fascia boards, and other external carpentry.
Strands of the common wet rot fungus Coniophora puteana growing on a wall. |
Skirting board affected by wet rot decay. The timber has cracked across and along the grain, creating cubes characteristic of brown rot decay. |
It is definitely within your best interests to have a wood rot specialist investigate any suspicions you may have. Dry rot is a brown rot fungus and causes damage that appears very similar to some wet rot fungi. However, dry rot requires other conditions to propagate, and it is sensitive to heat and humidity. Dry rot is often characterised by extensive fungal growth and fruit bodies (sporophores) that aid in identification.
The term 'dry rot' is something of a misnomer because the fungus requires a significant moisture source to support growth. However, the real problem lies with dry rot as it has the ability to spread through masonry and other inert materials that are damp. This manoeuvrability means that dry rot is typically more complex to eradicate than wet rot outbreaks.
Here, dry rot mycelium and fruiting bodies (sporophores) are growing on a wall. The spores released by the sporophores have covered the items stored close to the wall in an orangey red dust. |
This dry rot outbreak occurred in the ground floor timbers of a converted Victorian hospital. The fungal growth affected the floor timbers, and a fruiting body appeared above the skirting board. |
This is a close up of the area in front of the hearth and clearly illustrates the destructive effects of dry rot decay. |
Both dry and wet rot fungi are living organisms, which means they require air, water, and a source of food (i.e. timber) to support their growth. While it is not feasible to attempt to take away air, the removal of timber and water is much easier to achieve. Consequently, both wet rot and dry rot remedial treatments should include the following:
• Removing the decaying timber;
• Removing the moisture that allowed the decay to develop, and;
• Replacing the timbers using methods that ensure they are adequately protected from future decay.
Under no circumstances should timbers be left in direct contact with masonry that has been affected by dry rot decay, but, in many cases of fungal decay, the extensive use of preservative chemicals is simply not warranted or necessary. Fungicidal treatments can be targeted at specific problem areas such as masonry that has been affected by dry rot growth or, in the case of wet rot decay, the in-situ treatment of adjacent timbers that need to be left in place.
Britannia Preservation’s surveyors expertly identify the nature of the wood decay, and our repair techniques ensure that the works are carried out in accordance with good building practice while minimising the use of preservative chemicals. Our methods mean that wet rot and dry rot outbreaks are properly checked but costs, disruption, and health risks associated with treatment are kept to a minimum.
© Britannia Preservation 2011