Wet Rot, and Dry Rot, what’s the difference?
The two most commonly used terms for decay in wood are ‘wet rot’ and ‘dry rot’. They are often spoken about as if they were two completely different phenomena. Although there are many different species of fungi that can attack wood, the two types of decay are fundamentally the same. The exact biology is highly complex, but hopefully a simplified explanation is useful to boat owners to understand how to prevent and treat decay in their vessels.
Fungal decay begins as microscopic spores which can lay dormant in timber at low humidities for decades. Mostly they cannot be seen, but in high concentrations can appear as an orange dust. Seasoned timber has a natural moisture content of around 14%. When measurements are taken, the presence of paints, varnishes and penetrative treatments can raise the apparent moisture to 16% to 18%. Surveys will often show these figures without comment as they are quite acceptable.
When water soaks into wood and the moisture reaches close to saturation point at 28% to 30% the spores become active and germinate, multiplying rapidly producing more spores. As with all organisms, they need two things to grow, food and water. The water is present, and the food is cellulose which are the wood fibres. The fungus consumes the fibres and the lignin resin which binds the fibres is left weakened and soft. This is apparent to all as ‘wet rot’
What is commonly termed as ‘dry rot’ is most usually seen in buildings, but it was first observed in ship building. Ships hauled from the water for repair were often considered to be perfectly sound. When the wood started to crumble and collapse it was thought that a wholly new and different type of phenomenon was occurring in the dry wood. It was therefore termed ‘dry rot’.
Moisture needs to be at around 28% for the spores to germinate. But once they are active they can continue to consume the fibres with moisture dropping to as low as 20%. So, what was being observed by the shipwrights was just a continuation of a previous wet rot decay. The structure appeared originally to be sound because the moisture was actually binding the decayed wood together. When it dried the damaged wood would shrink, cracking into cuboid like cells, and the cell walls would eventually crumble to dust.
To combat decay requires three stages. First, and most obviously, the source of water ingress must be removed. But this is not enough, the wood must be dried in order to remove the water and stop the germination and thirdly the spores must be killed as otherwise they could continue to consume the cellulose, or at least lay dormant until re-activated.
Common fungicides are borate solutions or ethylene glycol which is most commonly found as blue antifreeze. These chemicals can be injected into the affected wood once it is dried. Once these have done their work and themselves dried, penetrating epoxies can be used to harden the wood. This process is only really practical however, in thin sections and cases of minor damage.
There are commercially available epoxy treatments which can claim to treat and harden damaged timber. But saturating treatments cannot penetrate already saturated wood. Also, even if the wood is dried, the saturation must pass right through the damaged wood and penetrate into the sound wood. Otherwise it cannot be guaranteed that all the active spores can be killed. Also, the epoxy not bind the whole together and reinstate the integrity of the structure. In larger sections and structural timbers, there is no real substitute to replacement of the affected timber.