Dry Rot

Dealing with Dry Rot

Dry rot is the term used for identifying timber decay. The "dry" rot was called as such for it was believed that the decay was caused by internal infestation not brought about by an external element such as water.  Dry rot has long been identified as the decay of timber (building, ship's haul and other wooden structures)) caused by fungal infestation. 

The term dry rot is a misnomer for the fungi need water to thrive.  However the name persisted as this form of decay is found in timber that is not damp or wet. Dry rot is not evident on timber that is constantly wet and on timber that is perpetually dry.

What causes dry rot?

Dry rot is caused by two types of fungi: Serpula lacrymans in the UK, Europe and Asia and Meruliporia incrassate in the United States.  The fungi attack both hardwood and softwood timber. They "feed" on the fibre of the wood tuning the wood spongy and soft with a musty-wet smell.  The fungi destruct the fibre of the timber which in turn discolours the wood which would eventually leads to its decay.  The infestation is mostly coloured brown thus in some countries dry rot is synonymous with brown rot. The fungi thrive in moist and humid places and in wood that has not been pre-treated.

Dry rot can attach itself on concrete and stones too. Though the fungi will not be able to "feed" on the moisture content of the stone and concrete, its mycelia (thread-like part that feeds) has the capacity to creep across stretches of stone to source water from a feasible location.  The mycelia help propagate the spread of the specific fungus and affect more wood in the process. If the infestation is not stopped in due time the structural strength of the wood will be compromised and the building itself will eventually be affected. Dry rot is deceiving for it can infest and spread on wood that has been coated with paint.

What are the signs of dry rot?

Dry rot is probably a term feared by most homeowners. Dry rot can very well result in the growth of other more toxic moulds such as the black mould. There had been instances when whole structures were burnt down due to extensive dry rot that resulted in an uncontrolled black mould plague.  What are the telltale signs of dry rot? If you suspect dry rot, consider if the affected wood has some source of moisture as the first "invisible" sign of dry rot is wet-musty smell.

Find the source of the odour. Once you have pinpointed it, inspect the wood and see if there are cracks that are more or less cube-like in shape.  A fairly new infestation will show cotton-wool and off-white mass on timber and sometimes on brick or stone walls. As the fungi takes a stronger hold on the infected surface, strands called mycelia will start its steady spread to infect more wood and at the same time source water for the growth and spread of  dry rot. These mycelia can grow to a finger-thick diameter and are brittle when dry. The "wetness" or "dryness" of these thread-like parts is a good indication to see whether the dry rot is active or not. 

Some infestations will show "teardrops" growth on the cotton-wool mass.  Dry rot damage is often limited to wood but in some instances, fruiting bodies that resemble large mushrooms can grow on stone, concrete, plaster and paint finishes.  These fruiting growths are soft and pancake-like and are dark yellow in colour.  Their edges are dotted with red spores.

How do you treat dry rot?

Dry rot affects timber that has 20% to 30% moisture content. Eliminating the source of moisture is the primary course of action in treating dry rot.

Houses and other structures ideally should be moisture-proof but such is not the case.  Wood within the structure can absorb moisture through leaks in the plumbing system. Other probable sources of leaks are washing machines, bathtubs, shower stalls. Damp could also be caused by the normal condensation that occurs in buildings that could have been caused by leaks in the roofing system, moisture that seeps through walls and in some cases, rising damp.

If the source of moisture has been identified, eliminate the source and allow the timber or wood to dry thoroughly.  If mechanical source is required to dry the timber, then you can outsource the equipment. It would even be better if you let professionals do the procedure.

In some situations, the dry rot could have caused damage to an extent to the timber. If this is the case, replacement of the timber is the best option. If dry rot damage is still minimal and there is no need for replacing the affected wood, then chemical treatment is the best solution.  This option should also be done in timber that is at risk of being infected because they were around wood that has been infected and replaced.

An effective fungicide is borate or boric acid.  If the moisture cannot be controlled or if the dry rot is well rooted, then treating the wood to eliminate the fungus and inhibit its growth is your next best course. Borates are generally applied or sprayed in liquid form. This would hold effective for dry wood. But for wood that is already damp or saturated with water, liquid borate might not prove effective.  In "wet" wood, using fuse borate is your best option for treatment.

To prevent dry rot, make sure that all timber parts in your house are properly treated during construction or during repair works.