Rot and infestation management

Dry rot

Key issues

Look for as many field marks as you can to correctly identify the fungus causing timber degradation, in the same way you might identify a tree by leaves or flowers. Less experienced surveyors have been known to state in reports that cobwebs or even plant roots are conducting strands – i.e. evidence of fungal decay. This would happen far less if surveying students had been shown examples of timber decay, either on a site visit or in the classroom.

To confuse matters, some wet rots also produce quite thick conductor strands (e.g. the rather feathery rhizomes of the white pore or mine fungus, fibroporia vaillantii). These strands are whiter than dry rot strands, and remain flexible when dry. So it could be foolhardy for anybody but a mycologist to identify a rot from rhizomes alone. However, although general house surveyors may not be fully competent in identifying a rot or beetle infestation, they can have the advantage of an open mind concerning remediation – and may be more inclined than a remedial treatment surveyor to specify traditional building repairs.

So exactly what is 'dry rot'?

It is said that dry rot accounts for £400m worth of damage annually. In essence, dry rot is a fungus. It has a life cycle that requires a spore to germinate. It is a brown rot. It destroys wood's cellulose, leaving the lignin.

Fungus life cycle

The spores are more or less everywhere. Although the life cycle is commonly described as fruiting body-then-spore, you could rightly ask how this tiny spore came into existence without having been released from the fruiting body. It is very much a chicken-and-egg life cycle. Although you would find it hard to single out a spore sized at 1/100mm, collections of dry rot spores form a reddish dust and confirm the presence of a fruiting body.

The spore needs to be deposited on timber in the right conditions for germination. The spore germinates, producing a small tube-like thread (the hypha), which resembles a fine root. Ultimately a mass of hyphae forms and is then called mycelium. As hyphae multiply, timber gradually decomposes and loses its strength. The fungus can produce conductor strands to support or spread growth, and sporophores or fruiting bodies to produce spores to further spread the fungus. The fruiting bodies are a rusty brown colour with white margins. They seem to be produced particularly when the organism is threatened or suffers change in its environmental living conditions, which it prefers to be stable.

Key requirements for fungus development:

  • a viable spore;
  • damp or wet wood – for germination, moisture content close to fibre saturation, 28–30%, RH 95–98%; for growth, optimum 20–55%;
  • suitable temperature - optimum for growth: 15–22°C; and
  • oxygen – in confined spaces the fungus can use up oxygen in the air – making it dangerous for humans to enter.

Predictions vary about the rate of spread of dry rot, from 2.25mm/day to 5mm/day to 11mm/day. Similarly, decay rates quoted may differ.

For basic requirements and biology, see Chapter 7 in Timber Decay in Buildings.

Dry rot seems to thrive where calcium is present in addition to the basic timber food source. This could be why dry rot is so commonly found in timber close to or embedded in lime mortar based brickwork or lime plaster ceilings using timber laths for support. It is thought that dry rot uses the calcium in lime to 'control acid-mediated degradation'. Dry rot is also said to need iron.

Dry rot treatment and timber repairs – issues of remedy selection

The source of moisture causing dry rot must be stopped. Doing this will eventually kill it off. The remedial treatment industry has long-standing experience in researching and eradicating woodworm and rots – although it has a bias towards chemical treatments. The guidance offered in the chemical treatment manufacturer's handbook Dry rot and its control stresses the need to first stem the entry of any water: this alone may be sufficient to 'eventually control and eliminate the activity '.

However, in practical terms this approach on its own is not always an option. Few surveyors would be confident to just starve the fungus of moisture, and few house surveyors would have the monitoring skills or the equipment to know for sure whether the building was actually drying out after primary repairs and maintenance. The biggest problem is the sheer time it takes for masonry and the associated components and finishes to dry back down to a safe moisture equilibrium, after perhaps years of water ingress. However, the often-cited drying time of '1 month for an inch thickness of masonry' does not take account of the fast drying times that can now be achieved using modern drying regimes and equipment.

Those who support 'environmental control' of dry rot are not just theorists or academics. Their aim is not to bring down the chemical treatment industry, but to simply find the best way to deal with dry rot from understanding it. Their approach is that of avoiding a misuse of chemicals, rather than never using them. Misuse means using too much chemical, applying it incorrectly or where it presents a risk to the building or its inhabitants or users, including animals, pets and plants.

Unfortunately chemical treatment is too often a substitute for careful detailing of construction elements, components and materials. More 'natural' approaches to dry rot eradication require closer supervision of site works and a greater understanding of how to control the built environment to deny dry rot the niche it seeks to grow and thrive.

Close supervision of construction work means regular site visits which create fees, perhaps not within the client's budget. More often than not, supervision of refurbishment by surveyors is sadly little more than contract administration – in other words a paper chase. So surveyors may choose to rely on a quite heavy-handed chemical treatment of the dry rot and the legal security guarantees provided. The costs of the dry rot treatment will usually be but a small proportion of the total refurbishment outlay, so will be lost among much larger figures.

A more conservative approach to dry rot treatment is more likely when minimum intervention is a prime consideration (e.g. when there is a need to avoid damaging or disturbing as little building fabric or finishes as possible, such as in a listed or historic building). The BRE advises 'rapid drying' of the structure. This would involve specialist drying techniques and monitoring, but may be a useful strategy.

Whichever approach or combination of approaches is used, it is important not only to starve the relevant zone of the building of water ingress but also to help as much moisture as possible already within the fabric to escape out. The less water you apply to the fabric during dry rot treatment, the better. The longer you delay replastering, the more time you give the brickwork to dry out.

Consider how you might ventilate the structure to prevent a build-up of moisture in the future and help any lingering water find a way out by evaporation or drainage. Consider using less dense internal plaster finishes that might help the wall breathe. Although more costly, consider traditional lime plasters to help maintain a breathing wall. Certainly application of hard and dense cementitious renders internally can only trap moisture within the wall's thickness. Allowing masonry to remain too damp or wet for too long could be providing the much-needed moisture for any lingering dry rot to re-establish.

Dry rot treatment used to be more chemical-based, with large areas of masonry flooded with fungicides. The timber remedial treatment industry uses chemicals more conservatively these days.

Many of the more dangerous chemicals have now effectively been banned for use as in situ remedial treatments, e.g. PCP (pentachlorophenol), TBTO (tributyl tin oxide), dieldrin and lindane. Active ingredients in common use for remedial treatment of timber today are permethrin and borates – chemicals considered safe enough to use in sensitive wildlife sites where nesting and roosting barn owls or bats might be at risk. Chemicals used for remedial timber treatment must be approved by the Health and Safety Executive (HSE) in accordance with the Control of Pesticides Regulations 1986. Products displaying an HSE number will be listed as safe.

It was once common practice to blow-lamp masonry in the vicinity of an outbreak. Ridout makes the point that using a blow lamp for treating dry rot could start a fire, so it is not altogether surprising that this practice has pretty well ceased. Ridout also points out, however, that a blow lamp is only likely to have an effect on the surface.

Further information

Ridout B, Timber decay in buildings, E & FN Spon, 2000