Dry rot

Dry Rot is a misnomer - because all wood decaying fungi need a minimum amount of moisture before decay begins. The decayed wood takes on a dark or browner crumbly appearance, with cubical like cracking or checking It is brittle and if handled, the wood will just crumble into powder.

The damp industry love Dry Rot – it is one of their favourite 'scarey' things that is used to sell countless millions of pounds worth of useless chemical treatments.  For the most part it is over-hyped by the damp industry - if found, drying the area out will stop it dead in its tracks.  All you need do is carefully vacuum and clear out any rotted timber, and make sure the area is well ventilated.

The most common place to find this is under floors, and in cellars. Often when a floor has no ventilation – it's been blocked – and there are cracked surface drains supplying moisture to the sub floor. Unfortunately this is an all too often occurrence – frequently mis-diagnosed by the damp wallies as rising damp because they can't be bothered to get in under the floor, or admit that the problem will go away if ventilation is improved.

Signs of dry rot

Often the first sign of dry rot is a ‘fruit body’, which looks like a fleshy yellow ochre pancake. Over time it matures to a rusty red colour. The fruit body generates a profuse quantity of spores, which settle as a reddish brown dusty layer.

Mycelium will appear as silky white sheets and or a cotton wool like appearance with yellow tinges. Strands of white or grey ‘branching’ may be evident; these can reach the thickness of a pencil.

Another common place to find it is in roofs, and associated rainwater hardware - gutters, downpipes, anything running off them that is lead covered - like valleys, valley boards, under parapets, and so on. Where rain is able to get through coverings like lead flashings it can cause high humidity, and help start dry rot.  Leaky plumbing can be a problem.  Sometimes you will see it in skirting boards - the damp wallies will tell you it's rising damp - but high humidity in a building, together with inappropriate wall coverings like gypsum (often previous 'damp proofing' coatings) will trap condensation into the wall and create damp areas which the fungus can feed on.  

I once did a survey on a huge country house with an 18 inch wide spine wall built of brick - down the middle of the building, which held the roof and all the floor timbers above. It originated deep within the cellars. After a day of fruitless searching, I finally found the door to the cellar hidden behind an organ in the hallway. I dragged this to one side, and opened the door to peer into Santa's Grotto. Blinding white sheets of fungus draped the stairwell, droplets of water shimmering on the skeins of fungus. Armed with a broom, I got into the cellar, and found to my horror that a HUGE oak beam, 2 foot diameter, ran the length of the cellar – and the brick spine wall was built on top of this timber. I scrunched through the remains of old cider barrels and rotted timber – the huge beam was crumbling – I put my fingers straight through it. Every entrance to the cellar was blocked – landscaped walls covered the air vents – so moisture had built up and created ideal conditions for dry rot. Fortunately the buyer and seller took a pragmatic view to the damage, and we specified a big steel beam to be installed in place of the rotted timber – less than £10,000 later the spine wall was secure, the cellar dry, dry rot gone – and everyone happy. No chemicals were used – just sensible removal of the fungus covered timber and a jolly good vacuuming out of the entire area. It's fine now – a lovely well ventilated wine cellar.

The term 'dry rot' is used in reference to damage inflicted by either Serpula lacrymans predominantly in the United Kingdom and northern Europe; and/or Meruliporia incrassata in North America. Both species of fungi cause brown rot decay, preferentially removing cellulose and hemicellulose from the timber leaving a brittle matrix of modified lignin.

The term dry rot is somewhat misleading, as both species of fungi Serpula lacrymans and Meruliporia incrassata require high moisture content to initiate an attack on timber (28–30%). Once established, the fungi can remain active in timber with a moisture content of more than 20%. At relative humidities below 86 percent, growth of Serpula lacrymans is inhibited, but it can stay dormant at relative humidities down to 76 percent. These relative humidities correspond to equilibrium moisture contents of wood of 19 and 15 percent, respectively (the sort of measurements which are taken using a 'damp meter' – which is actually designed for this sort of measurement. 

The process of wood decay itself produces water but in this respect dry rot is no different from any other wood-rotting fungus and, likewise, its ability to produce moisture in this manner can be negated by ventilation.  Decay stops when this moisture content is below about 20% moisture.

Saturation of wood with water inhibits dry rot, as does perpetual dryness.

Dry rot is a paradoxical term seemingly indicating decay of a substance by a fungus without the presence of water. Its historical usage dates back to the distinction between decay of cured wood in construction, i.e. dry wood, versus decay of wood in living or newly felled trees, i.e. wet wood.

Treatment of Dry Rot

The damp industry will tell you chemicals are essential.  I for one, do not want my house, or my kids, exposed to toxic chemicals.  The best way to start off with treatment of dry rot, or any other wood decay for that matter, is to properly diagnose the cause.  Stop the cause - dry the air out.  Make sure there is good ventilation.  Remove any rotted timber.  Vacuum the area to remove dust and spores. Make sure that any damp masonry is dried out.  There is no point in re-instating plaster on a damp wall - give it time - let it dry properly and ventilate well.  When timber is replaced, make sure it is not in contact with any damp masonry.  

The damp industry will also tell you it can remain active for years, leads to structural collapse and so on. Yes, of course if you leave ANY form of fungal attack for long enough, the timbers will fall to bits.  Once found, it's easy enough to dry the area out and replace any damaged timber.

Some people will tell you to treat with insecticides if death watch beetle is expected - but again, death watch beetle does NOT attack dry timber - it likes it's food to be moist - above about 17% total moisture - so dry timber yet again, is not at risk.

The bottom line with any treatment - before you go flooding the place with toxic chemistry - let it dry out.  Properly diagnose the cause.  Stop the root cause of the outbreak.  Dry the walls and floors - reduce humidity, increase ventilation. 

The damp wallies will tell you that affected masonry should be treated.  This is rubbish, and only serves to sell yet more of their toxic chemicals.  Masonry does not contain the things fungus needs to grow on - it feeds on cellulose - and there ain't much cellulose in brickwork.  

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