of the dry rot
fungus Serpula lacrymans
affecting floor joists
materials are decayed by the effects of adverse environmental conditions
and the extent of damage depends on both the materials and the conditions.
Among the most vulnerable materials are timber, paint, textiles and
paper. Timber remains one of the most useful in a world of diminishing
resources and is a major component in most historic buildings. It
has many positive structural and aesthetic properties as well as being
an energy-efficient and renewable resource. However, timber provides
specialised ecological niches and many organisms have evolved to use
it as a food. The most common and destructive to timber are dry rot,
wet rot, common furniture beetle, and death watch beetle.
remedial treatments often entail the loss of irreplaceable decorative
finishes, floors and ceilings. Furthermore, treatment of the infestations
with insecticidal fungicidal chemicals is not only expensive, inconvenient,
hazardous to the operatives and occupants but also environmentally
unacceptable and usually unnecessary. Environmental control and preventative
maintenance provide an alternative, less destructive solution, and
remain the most widely used methods for preventing biological decay.
Biodeterioration of materials was defined by Hueck in 1968 as 'any
undesirable change in the properties of material of economic importance
brought about by the activities of living organisms'. A wide
range of materials are subject to microbiological deterioration, which
are caused by a broad spectrum of micro-organisms.
responsible for the decay of timber principally include woodworm (Anobium
punctatum), death watch beetle (Xestobium rufovillosum), powder
post beetle (Lyctus spp), and house longhorn beetle (Hylotrapes
bajulus). It is their larvae which cause most damage as they bore
through the wood, feeding off it and causing damage to the structure and
strength of the timber.
fungi are capable of enzymatically degrading complex cellulosic materials,
such as wood, into simple digestible products. The decay of wood cells
by these fungi results in the loss of weight and strength of the wood.
There are two main types of wood-rotting fungi found in buildings;
wet rot and dry rot (see Table 2).
principal environmental factors favouring the biodeterioration of
building materials are temperature, humidity and a lack of ventilation.
Moisture may be contributed by penetrating or rising damp; condensation;
building defects and disasters such as leaks; and from construction
moisture introduced in mortar, concrete and plaster for example.
CONTROL - THE GREENER APPROACH
Environmental control relies on controlling the cause of the problem
by controlling the environment. It is designed to ensure the future
health of the building and its occupants by avoiding the unnecessary
use of potentially hazardous and environmentally damaging chemical
pesticides where possible and their consequential legal and management
complications. Eradication of dry rot spores or insect pests in an
historic building and its contents is in practice, impossible. The
volumes of chemicals necessary and the toxicity required would be
damaging both for the buildings and all its occupants. Where chemical
treatment cannot be avoided materials and techniques should be used
which have minimum adverse environmental effect.
reducing the need to expose and cut out infected material, environmental
control also reduces damage to the fabric and the finishes of a building.
Where an historic building is concerned, this is particularly important,
and the specification should ensure maximum conservation of existing
materials to maintain the historic integrity of the fabric, as well
as to avoid unnecessary expenditure. Its success depends on a thorough
investigation of cause and effect. Through a methodical approach such
as this, it is possible to decrease the cost of remedial timber works
significantly or in some cases eliminate it altogether.
the building should be thoroughly inspected using non-destructive
techniques to locate and identify all the significant decay organisms
within it. In cases of actual or suspected problems of woodrot or
wood boring insects in buildings, investigation should be by an independent
specialist consultant, architect or surveyor to establish the cause
and extent of the damp and timber decay, including the potential risk
to the health of occupants before specification or remedial work.
Correct identification of the fungi and insect material is important
as not all fungi are equally destructive. Some rots are present in
timber when it is cut or are acquired in storage. Fungal material
may also be dead or dormant, the product of conditions now past.
identified the nature of decay, the environmental conditions which
are required to support it should be considered (see Table 1). Only
then will it be possible to devise a scheme to deal with the problem.
aim of remedial building works is to control the timber decay, to
prevent further decay and to correct any significant building defects
resulting in conditions of high moisture content or poor ventilation
of timber. In particular, it is important to reduce sub-surface moisture
content of all timber to below 16-18 per cent. Timber should be isolated
from damp masonry by air space or damp proof membrane, and free air
movement should be allowed around timber in walls, roofs and suspended
floors. All other sources of water should also be eliminated, such
as overflowing gutters, leaking plumbing, condensation and rising
or penetrating damp. Humidity in voids should not exceed an average
relative humidity of 65 per cent. In addition, all active fungal material
should be removed together with all rotten wood, and the structural
strength of the remaining timber and fabric construction should be
assessed to determine whether reinforcement or renewal is required.
In the case of insect infestation, measures should also be introduced
to avoid recontamination. Dirt, dust and builders' rubbish provide
a haven for insects and fungi. Voids and cavities should be cleared
and the areas cleaned with a vacuum cleaner to remove dust. A programme
of building maintenance and monitoring may then be instigated to prevent
any future problems.
1: CAUSES OF DECAY
(dry rot, wet rot, moulds and others) bacteria; actinomycetes;
lichens, mosses and algae wood-boring insect larvae (woodworm,
death watch beetle and others) carpet beetle, moths, book
lice and silverfish termites
alkalis and solvents
abrasion, general handling and others, decomposition by physical
agents such as prolonged heating, fire and moisture
use, visitor wear
sunlight, heating, fire, damp
2: FUNGI WHICH INFEST BUILDING MATERIALS
moisture content in timbers of about 20 per cent
Optimum growth occurs at 30-40 per cent
Spore germination requires wood moisture content of 30 per cent
optimum temperature for dry rot growth in buildings is about
23°C, the maximum temperature for continued growth is about
25°C and the fungus is rapidly killed above 40°C
rot fungi usually occur in persistently damp conditions needing
an optimum moisture content of 50-60 per cent
fungi differ in their optimum temperature but for most the range
is between 20-30°C
Dry rot: Serpula lacrymans
rot fungus (Coniophora puteana); Poria fungi,
(eg Amyloporia Xantha; Fibroporia vaillantii and
Poria placenta); Phellinus continguus; Donkioporia expansa;
Oyster fungus (Pleurotus ostreatus); Asterostroma
spp; Paxillus panuoides; Lentinus lepideus; Dacrymyces stillatus;
rot: Chaetomium globosum
Cladosporium spp; Penicillium spp; Aspergillus spp; Trichoderma
spp; Alternaria spp; Aureobasidium spp
fungi: Coprinus spp; Peziza spp; Pyronema spp
Stain fungi: Cladosporium spp; Aureobasidium spp
Remote monitoring systems can be very useful in increasing the efficiency
and decreasing the cost of maintenance programmes. They can be especially
useful for checking the moisture content of inaccessible timbers in
roof spaces, behind decorative finishes and in walls.
can be placed at all critical points after the investigation or after
remedial building works. Areas can then be closed up and finishes
reapplied; for example sensors may be placed in lintels, joist ends,
valley gutter soles or in damp walls to monitor drying. It is important
to use enough sensors and to place them with an understanding of the
moisture distribution processes, because conditions can vary even
in a small area. It is these local variations in conditions that produce
the environmental niches which decay organisms exploit.
more than 30 sensors are deployed, taking the readings can become
onerous and this may result in human error or negligence. In these
situations automatic monitoring systems become desirable and a number
of specialised systems have been developed. With larger systems the
wiring of sensors can also become a problem. For systems requiring
100 or more sensors, a computerised unit is used, working via a single
four-core mains cable connecting up any number of nodes, each supporting
four sensors. This system can be programmed to record and log data
at regular intervals with alarm limits for each sensor. The data is
then transmitted to a remote computer via a modem connected to a telephone
line. Data from the system can then be analysed using CAD and programs
for statistical interpretation.
MOISTURE MONITORING SENSORS
water content of building materials can be determined through a range
of direct and indirect methods. Direct methods involve removal of
a sample of the material to be tested which is weighed and then dried
to determine its water content. This has the disadvantage of being
destructive and it cannot be used for remote monitoring.
methods are based on measurement of characteristics related to the
moisture level in the testing material. These involve thermal conductance,
electrical capacitance and resistivity. Measurement of a surrogate
material in equilibrium with the first material is another method.
The use of electrical resistance moisture meters provides a quick
and relatively accurate method of determining the moisture content
of wood if a knowledge of their limitations is taken into account.
Moisture meters measure the changes in resistance, due to changes
in moisture content, between two electrodes placed in the timber.
Increasing moisture content results in a reduction in electrical resistance.
sensors are fabricated from hygroscopic material which has been calibrated
to match the moisture content changes in timber. They are encased
in a protective shell. The sensor is then inserted into a previously
drilled hole to the required depth and the hole sealed. In most instances
the sensor cable seals the hole to the outside. The sensor will fairly
rapidly come into equilibrium with the atmosphere within the hole.
Due to their small size the sensors can be inserted into the centre
or ends of large dimension timbers allowing the best chance of detecting
MOISTURE MONITORING SYSTEMS
for use with masonry can be based on the direct measurement of the
material's moisture content or by the use of a surrogate material
which changes in moisture content in a similar way to the host material.
This material may be of any hygroscopic type which, providing it has
been calibrated correctly, can be used as the basis of a remote sensing
sensors are placed in the material to be tested at the required depth,
or in an array and the hole sealed to the external atmosphere. Sensors
will come to equilibrium with the relative humidity within the cavity
or drilled hole and hence with the surrounding material. Single sensors
can be placed at varying depths but must be sealed within the area
to be measured. A series of sensors individually sealed within the
drilled hole can provide a profile of readings across the material.
These can either be wired up and resistance measured remotely or can
be removed, weighed and oven dried to calculate their water contents.
Changes in the water content of masonry can be rapid when wetted so
that it could provide an early warning of building defects leading
to water penetration. However, drying down can take many weeks or
The Building Conservation Directory, 1996 (where it appeared as 'Environmental Monitoring and Control')
SINGH BSc MSc PhD CBiol MIBiol AIWSc FIRTS is currently Associate
Director with Oscar Faber Consulting Engineers (St Albans) and
is the head of Oscar Faber Heritage Conservation. He has over
15 years' experience in both academic research and industry. He
specialises in the study of the biodeterioration of building materials
and associated environmental health problems.
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