Dry Rot & Wet Rot

In the UK the term ‘Dry Rot’ refers to outbreaks of wood decay by the fungus Serpular Lacrymans. Unlike wet rot for which relatively simple procedures are adequate for control, the successful remedial treatment of dry rot may require elaborate and sometime very expensive control measures.

It is important therefore in the first instance that an outbreak is identified correctly. There are a number of diagnostic features but presence of several of them may be required for positive identification.

APPEARANCE OF WOOD AFFECTED BY DRY ROT

All types of wood rot can be difficult to detect in early stages, this is particularly true of dry rot because it nearly always develops out of sight, often spreading behind paneling and plaster or beneath floor boards. Some indications of the possible presence of dry rot are softening of the wood in some areas, shrinkage and distortion and a distinctive mushroom odour.

Wood thoroughly rotted by Serpula Lacrymans is light weight, crumbles under fingers and has a dull brown colour. Often the wood shrinks and splits in to brick shape pieces similar to the appearance of charred wood but not as darkly coloured. However the appearance of damaged wood cannot in itself be used in recognition, because dry rot infected timber can resemble very closely timber that has been damaged by wet rot fungi.

MOISTURE REQUIREMENTS

Serpula lacrymans require hard or soft wood or other cellulosic materials as a food source. The fungus grows most rapidly on wood that has a moisture content above 30% though not saturated. It is unable to colonise wood which has a moisture content below 20% but the fungus has the ability to cause localised wetting of previously dry timber.

RECOGNITION

Usual effect on the wood
Wood becomes light in weight, crumbles under fingers and has a dull, brown colour. It shrinks and splits into cube like pieces.

Strands
Strands grey or white 2-8mm thick, become brittle when dried

Mycelium
In damp, dark places, soft white cushions or silky growths. In drier places thick silver-grey sheets or skins usually show patches of lemon yellow and tinges of lilac.

Fruit-bodies
Fruit-bodies fleshy, soft, but rather tough. Shaped like pancakes or brackets, spore bearing surfaces rusty red with shallow pores or ridges and furrows, margin white.

Spores
Often settle on horizontal surfaces as a layer of rust colour dust.

TREATMENT OF DRY ROT

Depending on the circumstances RTM Services would establish the size and significance of the attack. In particular if structural timbers are affected.

Arrange for a full structural survey to determine whether structural repairs are necessary and if they are take the appropriate steps to secure structure integrity.

  1. Locate and eliminate sources of moisture
  2. Promote rapid drying of the structure
  3. Remove all rotted wood cutting away timber 1 meter beyond the last indications of fungus
  4. Remove all timber that has lost its structural strength decide addition support if considered necessary treat timber
  5. Prevent further spread of fungus within brick work and plaster with treatment
  6. Treat replacement timbers
  7. Treat remaining timbers

Introduce preventative measures such as ventilation pathways between sound timber and wet brick work or where ventilation it not possible providing a barrier such as damp proof membrane.

Replace all infect wood with sound treated wood.

RECOGNITION OF WET ROTS

Ciniophora Putenana & Coniophora Marmorata (brown rots) coniophora puetnana is commonly called cellar fungus, and c. maraorata together are the commonest cause of rot in timbers which have become soaked by water leakage.

Both fungus produce thin fruit-bodies which lie flat on the substrate c. puetnana fruit-body is olive green to olive brown with cream margin paler when young, its surface is covered with small irregular lumps c. marmorata fruit-body is pinkish-brown and has a smooth to lumpy surface.

Both fungi produce strands which cream when young but mature to characteristic brown to blackish colour. They are commonly seen on the surface of decayed wood and spread across wall surfaces.

Mycelium

The normally cream mycelium is produced only under conditions of high humidity. It may remain almost white under impervious floor coverings and may then be mistaken for dry rot.

Damage caused is typical of brown rots, the wood darkens and cracks both along and across the grain but usually less deeply than dry rot.

Cellar fungus moisture content prefers 30-50% lethal 80%, temperature prefers 3c lethal 65c and survives at 15-20% MC dormant but can revive after several years.

White Pore Fungus or Mine Fungus

Poria fungi brown rots, the fungi loosely referred to as Poria species are grouped together because they are difficult to distinguish in buildings unless fruit-bodies are present.

The most common species in the group are fibroporia vaillanti, often referred to as white pore or mine fungus. Fungus of this group are a common cause of rot of damp woodwork in buildings, frequently in areas of higher temperature.

The fruit- bodies of all these fungus are irregular lumpy sheets or plates covered with pores.

Fibroporia vaillanti is white to cream when fresh remains soft and usually has strands running from the edge. The pores are quiet large 0.4-1.0mm and angular in contrast.

Amyloporia xantha is a pale to sulphar yellow, smells of lemons when fresh and has small pores, 4-5 pores per mm.

Poria placenta is white to straw colour, sometimes with pink patches, is leathery when fresh and becomes brittle when dry, pore size 2-3per mm.

All species produce strands white to cream although they are well developed only in the fibroporia vaillanti. They can be distinguished from dry rot strands since they seldom thicker than twine and remain flexible when dry

The mycelium of all species consists of white or cream sheets or fern-like growths and hanging tassels. It does not have the same texture as dry rot mycelium. It may discolour brown on contact with iron.

Poria fungi produce typical brown rot damage, the decayed wood is usually lighter in colour than dry rot and although the wood breaks up into cuboidal pieces the same way with dry rot the crack are rarely as deep.

Moisture content prefers 35-55% lethal 80% and temperature prefers 28c lethal 80c

TREATMENT OF WET ROT

Open up the affected area cut out and discard structurally unsound timbers as for dry rot.

Replace remove timbers preferably with timbers pre-treated 2=or with timbers treated on site with a wood preservative approved for the purpose ensuring that any cut ends are retreated and there is adequate isolation from damp walls.

Thoroughly clean the sound timbers in the vicinity and treat with a fungicidal wood preservative.

We can issue guarantees on rot treatments more information with be provided with the report and estimate.

Contact us today for a free survey and estimate on 0800 955 8995 or info@rtmservices.co.uk