What is the best way to clean my teak deck?

The simple answer is carefully! Never use a pressure washer or a stiff brush. When pressure washing or scrubbing teak the brown staining to the water that is running off is not dirt, it is the wood itself that is being washed away.

Teak is one of nature’s best natural materials for decking. It has good non-slip properties and is particularly rot resistant due to the natural oils and minerals in its tissues. This has led to the timber being grossly over harvested for both house and boat building materials. For example, the area of natural teak forest in Thailand decreased from 2,324,300 hectares in 1954 to about 150,000 hectares in 2000 (Source: UN Food and Agriculture Organisation).

The felling of naturally grown teak has been banned in all countries and only Forestry Stewardship Council (FSC) forestry grown teak is now legally available. This is fast growth timber developed by advanced tissue culture which allows teak trees to mature in less than 15 years, compared with over 40 years for traditional natural growth teak.

A tree’s growth rings are made up of alternate bands of fast growing springwood and slower growing summerwood. There is no growth in the other seasons. The springwood is a wide band and is light in colour; it is relatively soft and has a thin walled cellular structure. Summerwood is a narrower, darker band and the cell structure is much stronger. It is these alternating coloured bands that create the attractive grain pattern in the face of the timber. In currently available fast growth teak the springwood bands are wider and softer due to the advanced growth rate making it particularly vulnerable.

The use of pressure washers or deck scrubbing has the effect of scouring out the soft springwood leaving the harder summerwood and the caulking as raised ridges. This raised grain will then be rapidly eroded by foot traffic. For this reason a modern manufacture of teak product or decking should never by cleaned using a pressure washer or by aggressive scrubbing along the grain. Modern ‘kit’ decks consist of thin veneers of teak laid over a plywood or an epoxy scrim base. The teak veneers and shallow caulking seams can soon be lost if badly treated.

If sanding off is necessary, it should be very light orbital sanding or across the grain. It can also be cleaned a chemical oxalic acid cleaner. A teak oil should then be applied which will retard the drying out and dying off of the cellular structure. The pale yellow/tan colour seen when the vessel was new is the sign of freshly cut wood. It is natural for the wood to age to a silver/tan colour. To try to maintain the ‘new’ wood look to the teak is to slowly destroy it and the cost to replace a teak deck to a 36ft yacht can be well into five figures.

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.