What is Wood Rot?

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Rotted wood is more than just a nuisance. Advanced decay can cause structural failure. We’ve talked about wood rot in previous posts, and about wood rot repair. But what is wood rot in its various guises?

We thought it’d be useful to take a look at the nature of wood fungus in general, dry rot, wet rot and other types of rot that can make your wood look less than its best and ultimately destroy it.

About wood decay fungus on dead and live wood

Wood decay fungus comes in various varieties but they all digest damp wood. Take brown rot, the major contributor to decay in dead wood, or the Honey Fungus which colonises and attacks living trees. Some fungi actually grow on the wood and consume it, effectively destroying their own home. Others affect the carbohydrates contained in wood and others go for the lignin, the organic polymer that keeps the cells in plants rigid.

How is wood rot classified?

The commonest types of wood decay fungus are soft rot, brown rot and white rot, and because they each contain their own unique destructive enzymes they can take hold on a wide variety of trees.

About brown rot – A common wood killer

Brown rot fungi break down cellulose using hydrogen peroxide, a substance that comes from broken-down hemi-cellulose. A tiny molecule, it slips into the wood and spreads fast. The wood shrinks, goes a nasty brown colour and ultimately cracks into separate cubes, all weak and crumbly.

There are many different types of brown rot fungus, but those causing the most damage to the wood humans use are Serpula lacrymans, what experts call ‘true’ dry rot, and Coniophora puteana, a cellular fungus which attacks timber in buildings.

About dry rot…which doesn’t actually exist!

Experts used to classify wood rot as dry and wet, but this was misleading since all rotten wood is wet or has been wet at one time or another. Because brown rot makes wood dry and easy to crumble it’s often called ‘dry rot’, but this isn’t strictly accurate since wood has to be damp in the first place or it won’t decay. So the term dry rot tends to be an umbrella term for brown fungi in general.

About soft rot – Loves harsh conditions, rare indoors

The fungi responsible for soft rot do their dirty deeds by secreting the enzyme cellulase which destroys cellulose, generating tiny holes inside the wood which eventually discolour and crack, very much like brown rot.

Soft rot fungi in general have the ability to thrive where it’s too hot, wet or cold for their brown or wet rot relatives. They can even cause damage to the bark, which often contains so much tannin it’s notoriously difficult to decompose. Common variants include Chaetomium and Ceratocystis, but they’re not the worst offenders. The top chart position for mayhem and chaos is taken by white rot fungi, one of the most aggresive and successful decomposers of all.

About white rot – Commonest in hardwoods

Some white rot fungi destroy both lignin and cellulose, leaving wood in a horrible moist, soft, sponge-like or stringy state with a sickly white or yellowy colour. Others only attack the lignin, using particularly powerful enzymes like laccase. White rot can involve all sorts of enzymes, some of which are even strong enough to oxidise lignin. Take the honey mushroom which, despite its pretty name, attacks live trees and causes untold damage.

Other white rot fungus variations include the turkey tail and artist’s conch, both fascinating and oddly beautiful. And some are even edible, like the famously delicious Shiitake mushroom, prized the world over for its delicate flavour.

Is rot the same as mould?

No. Mould and mildew are also fungal and love damp wood, but they only cause discolouration, not decay. On the other hand because they can make wood more absorbent, they can leave it vulnerable to rot.

How fungus gets into wood

Dry wood doesn’t tend to rot. It lasts for centuries – all you need to do is visit one of Britain’s spectacular cathedrals or minsters to see ancient wood in action, doing a brilliant job many generations after it was put in place.

How wet does wood need to be to start rotting? A good general rule is ‘when it goes past the Fibre Saturation Point’, which means  about 30% or more water content. If it’s soggy to the touch, it’s time to identify where the damp is coming from and fix it, then treat the timber for fungi.

How does fungus get into wood? Tiny wood decaying spores are blown onto the wood by the wind and settle on the surface before germinating once the temperature is high enough. The resulting mini-fungi ‘plants’ penetrate the wood and secrete their enzymes, softening it and making it easier to digest. Under the right conditions they multiply like crazy and before you know it, they’ve smothered the wood.

Common causes of rotten wood

Wood can rot for all sorts of reasons but it will never rot if it’s dry and has always been dry. The most common causes of timber rot are:

  • Wood that comes in contact with the soil – you need to leave at least six clear inches between the wood and the earth to stop damp wicking up through capillary action
  • Badly-placed lawn sprinklers and badly-fitting drainpipes that keep wood wet
  • Too much greenery against the building, which holds moisture
  • Leaky plumbing and bad drainage
  • Not enough ventilation, a common issue in new-build homes with their excellent insulation
  • Water sitting under the building
  • Roof damage which lets water in
  • Neglected wooden decking and sad, saggy garden sheds

Can I buy rot resistant wood?

Yes, and it comes in many guises from fairly resistant to extremely. Here’s a list.

Reasonably rot resistant woods:

  • Cypress
  • Redwood
  • Old-growth pine

Rot resistant woods:

  • Pressure treated pine (a process using harsh chemicals)
  • Old-growth cypress and redwood
  • Cedar
  • White oak

Very rot resistant woods:

  • Mahogany
  • Spanish cedar
  • Teak
  • Ipe
  • Accoya

Wood rot treatment and wood preservative

Wood can rot all over your home. In structural timber, door trim, eaves, exterior trims, anywhere there’s damp. The best approach is to remove all rotten timber and replace with new where possible. As a guide, when removing old rotten wood, you should also remove 2 to 3 inches of good sound wood around the rot just to be certain that you have removed all of the affected timber. This will ensure that no trace rot has migrated on to as yet unaffected timber that can potentially start the process again. If this isn’t an option then there are a range of wood repair and dry rot treatment solutions.

Recommended products for dry rot treatment

Brown rot epoxy treatments work by filling gaps in the damaged wood, killing the rot and restoring the timber’s structural integrity. Oddly enough, commercial antifreeze also works beautifully to prevent brown rot in the first place, as well as killing the fungus itself.

Some compounds of copper, including copper naphthenate, come in solution form and are often used to fix brown rot damage. When you remove the rotten bits then drench the surface with the compound, it helps protect against further damage.

Brown rot can spread way beyond its original site and even travel through and along walls, hiding behind plaster and render. This is serious, and you’ll need to get down and dirty stripping the plaster and wall coverings right back, as far as a metre beyond where the fungus has reached, then treat the lot. But you need to exercise common sense – since brown rot only attacks wet wood, there’s no need to remove plaster and so on where there’s no timber present at all or the timber is completely dry.

It’s also important to find out where the water is coming from and stop it making your wood wet again. Fungi need moisture so when the affected wood dries, the rot also dies.

Recommended wood rot treatment

Can we recommend a really good anti-fungal product for decking treatment, indoor wood rot and other fungal nightmares?

Yes. We highly recommend Barrettine Mould and Mildew Cleaner, a powerful fungicidal wash perfect for removing nasties like fungi, mould, mildew and algae from all manner of different surfaces – inside and out – including stone, wood and even UPVC. It gets rid of the horrid black mouldy stuff wood fungi creates and it barely smells at all, something our customers appreciate! Once you’ve treated your wood with it, you can beautify the wood again using a good wood preserver or wood oil.

Any questions?

As ever, we’ll be delighted to put our technical hats on and provide expert advice. Simply call us or send us an email.

6 COMMENTS

  1. Hey, great post,

    I stupidly left a pumpkin on my oak table, not noticing it started to rot and not has left a small dark patch. Where 2 parts of the table join up. Just wondering if there’s anything I can do to help this? Starting to panic ? Thankyou

    • Good Morning Helena,

      You are not the first! Can you tell me what the current finish is on the table ? If oiled it will be easier to patch repair, simply sand back the effected area with a 120 grit paper and then with the same oil re apply and blend with existing finish.

      If the finish is varnish then this could be harder to deal with. If you would like to send a photo of the effected area and some further details of the finish and I may be able to advice further. You can email me directly at wfd.advice@wood-finishes-direct.com

      Kind regards Samantha.

  2. Hi I found some lovely old wooden trestle tables in our scout hut shed. The shed roof had obviously been damaged for a while and the tables have been wet. They have dried out fine but the feet of the legs, and the edges of the tables look a little crumbly. There may be a little insect damage. Is there anything I can treat them with to rescue them?
    One set of legs has a small amount of white fungus on it. What do I do with that?
    If it’s worth rescuing them I would like to clean them off and re-cover them. What do you recommend – varnish-wise or similar?

  3. Hello,

    Many thanks for this article. Found much of the information to be very helpful.

    A specific question concerning wood rot. My husband bought a plant and kept it on our wooden counter, using a bamboo plant to catch excess water. We have since discovered that the seepage has damaged a patch, leaving what looks to be white rot. Any advice as to how to address?

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