How to treat wet rot

Wet rot is the term used for many types of common wood-eating fungus. Wet rot grows on and attacks untreated timber with high moisture content. Before waterproofing garden timber, discover how to prevent and treat wet rot with biocidal protection as well as carry out wood rot repair. The differences between wet rot and dry rot are numerous– but most important is that wet rot requires more moisture and is less destructive. 

Wood is one of the oldest construction materials on the planet. The earliest-discovered wood growing plant dates from approximately 400 million years ago. There is evidence that humans have been building wooden structures since the tail end of the Stone Age. Despite this resilience, wet rot can make short work of unprotected wooden structures. 

Wet rot poses the biggest threat to unprotected timber. For sheds, wooden patio decking and trellis panels, its presence can be a death sentence. The best wood protection strategy is always preventative. Find out what causes wet rot and what we can do to treat it below.

Wood-decaying fungi eat at the internal structure of wood to cause cataclysmic damage. They eat the organic matter produced by chlorophyll-rich organisms like trees and other plants. Decaying wood absorbs more moisture, which accelerates the damage. This is the process we commonly call rotting. 

Once wood-decaying fungi have fully grown, they release wet rot spores into the environment. Those spores land on moist timber and the cycle begins again. Wet rot can even spread to damage organic fibres, affecting carpets, plaster, furnishings and wallpaper.

A substantial amount of moisture must be present in wood for wood-decaying fungi to grow. The ideal wood moisture percentage for most wood-decaying fungi is between 35% and 70%. The ideal temperature for growth is usually around 18 – 35℃. This makes summer the optimum time for wood-decaying fungi to colonise sheds, garden furniture and decking. 

Fungi are near-unstoppable. Freezing temperatures will slow fungal growth, but it will not kill the infestation. On the other end of the scale, fungi can tolerate heat up to around 60℃ before thermal death. That’s over twice as hot as the UK’s hottest summer days! 

Wet Rot fungus is incredibly resilient. It’s going to take more than a cold snap or heatwave to eliminate the problem.

All types of rot degrade wood at a cellular level. This means that timber pieces stricken with wet rot will weaken. This can be especially dangerous in structural timber, where failure could potentially threaten human life. Once rot has set in, there is no cure. The only thing to do is to cut the rotten pieces away from the unharmed elements. This is not always possible, depending on the type of structure and extent of the damage.

Wet rot is less destructive than dry rot, but should still be prevented at all costs. There are 3 main types of wet rot caused by the most common families of wood-rotting fungi. Wet rot is a general term that encompasses countless different types of fungi within those families. Fungi that cause wet rot in the UK include Poria Vaillantii and Phellinus Contigus. Most commonly, wet rot in the UK is caused by Coniophora Puteana, also known as cellar fungus.

1. Brown rot, commonly caused by Brown-rot Basidiomycota fungi.

   
   This fungus digests cellulose and sugar in timber, which gives this rot a brown hue. Cellulose is the polymer responsible for giving wood its strength and this is what brown rot feeds on. The most common type of wet rot is caused by the Coniophora puteana, which is a type of Basidiomycota fungi often called cellar fungus.

2. White rot, commonly caused by White-rot Basidiomycota and Ascomycota fungi.

   
   Lignin is one of the main components of wood. It is the polymer responsible for wood’s brown colour, and this is what white rot fungus feeds on. Some white rot fungi also eat cellulose. Without lignin, wet rotted timber can be soft to the touch.

3. Soft rot, commonly caused by Ascomycota and Deuteromycota fungi.

   
   These fungi also digest cellulose.

Wet rot is less destructive than dry rot, but should still be prevented at all costs. Wet rot is a general term that encompasses countless different types of fungus. Common fungi that cause wet rot in the UK include Poria Vaillantii, Phellinus Contigus and Coniophora Puteana.

All types of rot thrive on moisture. Wet rot usually starts to grow when timber reaches 30-50% total moisture content. It will only grow in contact with an active water source. 

Wet rot flourishes in the rainy season. In winter, it will find a home on any wooden structure or piece of furniture exposed to rainfall. 

In the garden, wet rot is a risk throughout the year around untreated ponds and water features. It is common in timber that stands on lawns or soil as well as damaged timber. Look out for overgrown shrubbery that can transfer moisture to contacting timber. Standing water caused by a blocked drain system is also something to look out for.

Signs of wet rot:

1. Softer, moister sections of wood
2. Cracked and flaking timber
3. Discoloured areas
4. A damp, musty smell
5. The presence of fungal growths
6. Flaking and damp painted timber surfaces

If wet rot has already set in, take action as soon as possible. Wood with wet rot is substantially weaker than healthy wood.

Once wet rot is detected, any furniture or buildings must not be used in case of breakage.

Once you’re sure that wet rot has set in:

1. Find the moisture source

   
   Wet Rot is a combination of fungi and moisture. Without a steady stream of moisture, wet rot cannot set in. Has the affected timber been outside during the rainy season? Is it propped up against a leaky pipe? Is it in an area which sees regular spillage, such as near a flower bed that is regularly watered? Excess condensation can also lead to wet rot growth. Any timber with a wet rot infection needs to be thoroughly dried out.

2. Cut out and fill rotten timber

   
   Once affected timber has dried out, all soft, loose and visibly rotten timber must be cut out. Any timber that crumbles or is spongy to the touch must be cut back. You can start by using a screwdriver to check the structural integrity of all potentially affected timber. Soft, weak areas of wood can be easily picked away and discarded. Visibly rotten areas must be cut away or discarded wholly with a suitable sharp knife or saw.

3. Apply biocidal treatment

   
   Once the timber has dried out and all visibly rotten and soft elements are removed, a biocidal treatment must be applied to the entire timber object. Remember, the spores that cause wood rot are invisible to the human eye. Just because a piece of timber looks untouched by mould does not mean it is. To ensure you are using an effective wood rot treatment, look for British Standard EN 113. This will ensure preventative measures against white and brown rot.

Hollow timber can be filled with a wood filler. For best results, look for a wood filler that is:

• High strength and durable
• Fast Curing
• Styrene Free
• Excellent adhesion and modelling properties
• Easy sanding and accepts most paints and solvent-based wood stains
• Suitable for internal and external applications

Find out more

Soluguard Woodworm & Rot Treatment is a comprehensive treatment against wood-boring beetles, wet and dry rot. Its water-based formulation protects timber against white and brown rot while destroying all stages of the woodworm lifecycle.

It is certified to 6 BN standards for wood-boring beetles and woodworm protection. For maximum coverage over large areas, a coarse sprayer can be used.

But wet rot isn’t the only biological threat to garden timber. 

Outdoor timber is just as vulnerable to mould growth as the inside of the home is. Many types of mould love the UK’s temperate climate. This includes the same type of black mould that likes to grow in bathrooms. Airborne mould spores are always present in the atmosphere and form new mould colonies in:

• the right temperature conditions
• in contact with moisture 
• given the right nutrients

Timber is nutrient-rich and the UK has the right temperature conditions for most of the year. Moisture is introduced into the garden from ponds or water features or via rain. With all this moisture, mould growth is inevitable.

When combined with a wood waterproofing treatment, a biocidal wood preserver offers total protection against mould growth on outdoor timber.

Dry Rot

A Dry Rot Fruiting Body

Dry rot is usually found indoors and can be much more immediately destructive than wet rot.

For everything you need to know about dry rot, check out our Dry Rot page.

Algae

Algae is a very general term covering a wide group of organic growth. Algae thrive in environments rich in moisture, carbon dioxide, minerals and sun. They thrive in water but often grow on rendered walls and roofs. Algae often find themselves growing in and around ponds, pools and other water features.

Lichen

Lichen is a complex symbiotic combination of fungi and algae. It grows most commonly on trees and patios but can be found outside almost anywhere. Lichens favour humid conditions and fresh air. Although they do not damage walls, floors and patios in the same way that fungus does, they are unsightly and can cause surfaces to become slippery.

Mould

Mould is another very wide umbrella for fungi. A common mould that affects outdoor wood is Blue Stain Mould (commonly caused by Ophiostomatales, Alternaria alternate or Cladosporium sphaerospermum). These types of fungus cause unsightly dark blue, black or grey stains.

Biocides are chemical additives that target harmful living organisms. Disinfectants used to clean bathrooms, kitchens and even wounds are all types of biocide. Just as we aim to remove harmful bacteria from our bodies, we must protect wood in the same way. Biocidal wood preservatives protect against both fungal and insect attack by penetrating deeply into wood. 

Roxil Coloured Preserver

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Roxil Wood Protection Cream

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Roxil Coloured Wood Preserver is designed as an undercoat for Roxil Wood Protection Cream. It is a biocidal treatment with an odourless formula for comfortable DIY use.

Wood is one of the oldest construction materials on the planet. Let’s make it last.