Rot and infestation management

Wet rot does not promote the same kind of fearful reaction as dry rot, yet it causes the most damage inside and outside buildings. It is usually considered that timber with a moisture content of 20% or more might succumb to the fungus. But beware: thicker skirtings or framings registering only marginal damp meter readings could actually be much damper – and even rotting unseen behind. Take wet rot seriously - the wet rot case study shows just how much havoc the fungus can wreak.

The most common wet rot fungus is coniophora puteana – the cellar fungus. It causes brown rot that consumes the cellulose in wood. Unfortunately the fungus breaks down timber, producing cracking both along and across the grain, so the damaged can be confused with the cuboidal cracking of timber typical of dry rot. However, there are other clues to help us.

Fruiting bodies of this particular wet rot fungus are, according to the BRE, rarely seen, whereas the fruiting bodies of dry rot are more common. But beware: Timber decay in buildings by B. Rideout informs us that 'the fruit is relatively common in buildings and when it is found it is invariably called dry rot'. Wet rot fruiting bodies are olivey green, and have white edges, and they do look a little like dry rot sporophores.

Dry rot tends to damage timber to produce cubes, with the timber damaged for its full thickness, whereas wet rot-damaged timber so often has a veneer of seemingly sound timber. This wet rot fungus produces brown branching surface mycelium. The timber darkens as it rots.

Wet rot outbreaks tend to be more localised than those of dry rot. Perhaps the cause is faulty plumbing. Maybe a very damp wall or floor produces joinery damage from wet rot.

The BRE Guide gives yet more useful pointers to successful identification, including a comprehensive key for identifying fungal growths in buildings. Bear in mind that it is probably wisest to spend your valuable time working out why the building is damp enough for rot. Devise improved detailing for a drier building, instead of exactly identifying a particular rot species and homing in on its obscure Latin name. In survey reports, however, you must not get the name of a rot or a wood damaging insect wrong. Until you have confirmed the insect or fungus responsible for damage, simply label the cause as a 'rot' or an 'insect attack'.

Dry rot conductor strands are often quite thick, white or greyish – often spreading behind wall linings – and are brittle when dry. Do not confuse them with the thickish feathery conductor strands of the wet rot fibroporia vaillanti (mine fungus).

The remedy for wet rot is usually to cure the leak or damp, strip out and replace at least the structurally damaged timber, and making sure good detailing helps keep the new timber dry. Good detailing could use barriers to keep timber away from any potentially damp substrate, ventilation to help keep timber dry, and measures that help drain any collecting water away from vulnerable timber. New timber is best pre-treated – if only to deter woodworm, which can damage timber with a moisture content even below 20%. Try applying fungicidal paste preservatives for timber during the drying-out phase, after repairs have been executed, to stop water ingress.

In old properties, 'built-in timber' is a wet rot risk – when the masonry around the timber can be damp. New detailing might be designed to hold timber away from the damp masonry – for example on a metal support.

Most wet rot damage is by brown rots, such as coniphora puteana or fibroporia vaillanti, but can also be from white rots such as phellinus contiguous – more common on external joinery – where timber is degraded, bleached and stringy and the fruiting body brown and porous, a little like a hard, encrusted moss.