The detection and mapping of dry
rot in buildings using colour visualisation technology
Abstract
A greater understanding of the fundamental processes
involved in wood decay has led to the development
of new technology for detecting and ascertaining
the spread of dry rot in buildings. This technology
can save many thousands of pounds of exposure
work usually associated with finding dry rot.
In addition, the early detection of dry rot before
any wood decay takes place offers increased opportunities
for conservation and preserving important historic
and cultural building features. The new dry rot
sensing technology, without the usual disruption
to the building fabric and features, tells when
dry rot and conditions for it are present, if
it is dead or active and how far it has spread
and whether any treatment for it has been effective.
Dry rot, the decay of timber by the fungus Serpula
lacrimans, is the most serious timber decay problem
in buildings in the UK and other temperate regions.
When environmental conditions permit (usually
resulting from a failure in building maintenance)
the fungus decays timber components and, without
remedial action, considerable structural damage
can ensue. Damage may be serious and extend far
beyond structural elements to valuable artefacts
such as ornamental plasterwork and timber panelling.
The unique ability of the dry rot fungus to penetrate
the non-timber elements of buildings, such as
masonry and plaster, in the form of mycelial strands,
and to transport water through those strands,
allows the fungus to spread considerable distances
from its point of origin.
The conditions for growth and development of the
fungus are usually present in locations within
the building that are not immediately accessible.
The detection of rot is therefore difficult and
assessment of the extent to which it has developed
usually involves disruptive investigation. Such
disruptive investigation can involve damage to
valuable features.
The current procedures in common use for the detection
of timber rot in buildings are broadly those described
in the technical literature like publications
from the Building Research Establishment (1993)
and Bricknell (1976, 1991). These procedures typically
involve visual inspection and probing, sometimes
supplemented by moisture measurement usually using
an electronic meter. Further investigation typically
involves the uplifting of floors and removal of
plasterwork, panelling or other building components
in an attempt to see how far the condition has
spread.
More recently, the availability of more sophisticated
instrumentation such as fibre optic cameras and
endoscopes has enabled some visual observation
of concealed areas to be made. The detection of
volatile metabolites by using sniffer dogs has
been used in the UK and Denmark (Lloyd and Singh,
1994; Bech-Andersen, 1995). The detection of volatile
metabolites by electronic equipment has also received
attention in recent years (Esser and Tas, 1992).
It seems, however, that the complexity of the
range of compounds together with the broadly similar
volatile profiles emitted by decay organisms has
necessitated the use of elaborate and expensive
equipment. Such equipment, at least at present,
is too complex and expensive for routine survey
work.
The development of low-cost and reliable detecting
technology has proved elusive until a recent focus
on the detection of biochemical metabolites. The
detection of biochemical metabolites using colorimetric
methods has proved to be very useful as a result
of its being specific and reliable together with
being inexpensive. Although the metabolites produced
by the dry rot fungus Serpula lacrimans are similar
in many ways to those produced by most wood-destroying
fungi, the conditions under which they are produced
within a building are very different, which allows
the specific detection of dry rot. The specificity
of this technology in respect of dry rot is largely
due to two main factors.
The unique and narrow range of moisture conditions
that support dry rot development in buildings.
The metabolites being detected are only produced
by the fungus in the presence of wood; they are
substrate induced.
This is important because the metabolites being
detected are being produced within the sensor,
as the fungus begins to attack the sensor. They
are not residual, and are not present as a result
of earlier fungal growth.
It has been known for many years that the dry
rot fungus brings about changes in its immediate
environment as it grows and develops. It is only
comparatively recently, however, that the significance
of some of these has become well enough understood
to enable them to be exploited commercially. In
particular, the biochemistry of wood decomposition,
while still not fully understood, is understood
well enough to enable the detection of certain
biochemicals to be used to map out the spread
of fungal growth in buildings, especially those
biochemicals that are produced at the very early
stages of wood decay. The detection of such compounds
offers the prospect of being able to detect the
presence of dry rot in a building before serious
wood decomposition takes place, thereby increasing
the opportunity to implement a conservation-oriented
approach to dry rot repair. There are clearly
enormous benefits in adopting such an approach.
It offers not only the chance to save on the cost
of repair but also the ability to retain features
of the building that are of historic and cultural
importance.
In all cases, timber rot including dry rot arises
as a result of water ingress. Water ingress and
the resulting spread of dampness through the fabric
are the principal means by which timber rot conditions
are initiated and sustained. It is therefore essential
that an accurate spread of moisture throughout
the building fabric can be ascertained. The assessment
of moisture content in buildings is complicated
by the differing water-holding capacity of the
components of the building fabric. Different materials
have a different natural water-holding capacity.
The author’s own investigations have demonstrated
that surface moisture readings recorded with an
electronic meter do not correlate well with readings
recorded directly from within the fabric of a
lath and plaster wall. It is of fundamental importance
to measure the free moisture (or moisture available
to initiate and sustain fungal colonisation and
growth) not simply the total moisture which might
include natural water such as water of crystallisation,
or water absorbed into the fabric as a result
of natural or contaminant-induced hygroscopicity.
Problems of structural dampness or moisture ingress
through flues or rising dampness can lead to contamination
of the building fabric and to an increase in the
natural water-holding capacity of a substrate
by the presence of hygroscopic salts. Such salts
enhance electrical conductivity and lead to erroneous
readings when electronic moisture meters are used.
Sensors will absorb only the free moisture within
the location in which they are installed. The
free moisture is the moisture available to the
dry rot fungus to break down the wood.
The development of dry rot within a building is
wholly dependent on the conditions for fungal
growth and development being present. On a practical
level, moisture is the most important growth requirement
for the development of dry rot in buildings. When
moisture conditions are present within the narrow
range that supports dry rot the sensor will detect
biochemical metabolites produced by the fungus
in the first stages of cellulose decomposition
and before wood decay takes place. Dry rot therefore
can be detected early enough to permit beams that
are affected, but not yet decayed, to be conserved.
PROCEDURE
There are five stages in the procedure as described
below. Sensors are installed in small, pre-drilled
holes within the areas of wood masonry or plaster
identified as being at risk. They are monitored
usually for about two weeks during which time
they reach equilibrium with the building substrates
and react to the presence of the dry rot fungus.
Sensors installed beyond the ‘at risk’
areas will confirm that there is no dry rot activity.
There is no need to uplift floors or to remove
plasterwork. The extent of the spread of dry rot
can be determined from examination of the sensors.
Fit sensors
Allow to equilibrate
Examine sensors
Record affected areas and ‘at risk’
areas
Monitor ‘at risk’ areas for a further
period.
This new technology has been developed as a result
of a greater understanding of the fundamental
nature of the growth and development of the fungus
and the way in which it breaks down wood. It is
based on the biochemical principles that underpin
the mechanism of action of the decomposition of
wood.
HOW THE TECHNOLOGY WORKS
Organic acids, in particular oxalic acid, is considered
to form part of a non-enzymic process of cellulose
decomposition that contributes to the complex
wood decay process. Oxalic acid is produced in
large quantities by the dry rot fungus and some
other wood decay fungi. The precise nature of
the process of cellulose decomposition remains
unclear but it is considered likely that the oxalic
acid functions as a catalyst to enable a hydrolysis
reaction to take place within the crystalline
lattice of the cellulose component of the wood
cell wall. The oxalic acid seems to facilitate
hydrolysis of the glycosidic bonds of cellulose
chains as a first stage in a multi-component cellulase
system. This first stage enables the multi-component
cellulase enzyme systems to gain access to the
cellulose polymer chains. The enzymic components
of the cellulase complex are too large to be able
to penetrate the capillary structure of the cell
wall. It is clear that brown rot and many white
rot fungi produce organic acids, especially oxalic
acid, and that this plays a part in the microbial
decomposition of the cellulose cell wall. The
cellulose component of wood cell walls represents
around 30 per cent of the structure by weight
of most softwoods. The detection of oxalic acid
produced by Serpula lacrimans in parts of a building
where there is a sufficiency of free moisture
has proved to be a reliable and reproducible aid
in the detection of dry rot in those parts of
the structure concealed from view.
The early detection of wood decay by examining
colour change has been noted previously. Rypacek
(1966) and Willeitner and Peek (1979) were able
to detect fungal growth in wood before there was
any visible evidence of decay or any substantial
(greater than 1 per cent by weight) weight loss.
It was further reported by Peek et al. (1980)
who recorded a colour change reaction with 22
species of fungi responsible for brown rot (all
the species under test) and 15 out of 25 species
of fungi causing white rot.
The difficulty in using this principle in practice
as an on-site survey procedure is partly because
there is great variability in the natural acidity
of wood in service. In addition, wood contains
a large number of extraneous substances that interfere
with both moisture absorption and desorption,
and the colour change that enables visualisation.
These difficulties have been overcome, however,
by using a pre-conditioned, vacuum-impregnated,
wood sensor manufactured from a carefully selected
type of wood. A period of pre-conditioning is
required to remove the extraneous substances that
interfere with the colour change.
After pre-conditioning and vacuum-impregnation
with the colour reagent the absorption and desorption
of moisture, and the colour change reaction, can
take place reliably and reproducibly in the presence
of Serpula lacrimans. Independent testing of the
sensors has been carried out at The University
of Leeds. All the tests have verified that the
sensors function reliably and reproducibly, and
that clear visual colour change can be observed
a very short time after the sensors are exposed
to active colonies of the dry rot fungus. The
time taken to change colour is usually between
four and ten days.
BENEFITS
The benefits conferred by this new technology
are too many to number, but following are a
few obvious ones.
No need for disruptive surveys
Cornices can be saved
Ornamental woodwork can be saved
‘At risk’ areas can be tested
The full extent of the spread of dry rot can
be mapped out
Growth can be monitored
Activity or non-activity of dry rot can be confirmed
Environmental control methods can be implemented
and evaluated
The success of the remedial work can be verified.
HOW TO USE THE SENSORS
The procedure involves placing special sensors
in areas that are affected by dry rot or are at
risk of becoming affected by dry rot and examining
them from time to time to see if they have undergone
a colour change. It is really that simple; in
fact, it is simple, reliable, neat and inexpensive.
The technology in action is illustrated in Figures
1–3.
Figure 1: The commercial product.
Figure 2: Sensor that has given a positive indication
of dry rot
Figure 3: Sensors mapping out the spread of dry
rot in a room adjacent to an established outbreak
of dry rot
The sensor is especially useful if a surveyor
is suspicious of a location within a building
and feels that more information is needed. For
example, this can happen in a situation where
there is a stain in plasterwork that seems to
have arisen from an ingress of moisture. Sensors
placed adjacent to structural timbers that might
be at risk from dry rot can detect if decay has
started or is about to start. Appropriate intervention
then can be implemented. Similarly, in the event
of a typical domestic flood from a washing machine
or toilet overflow, the affected area can be tested
to ensure that dry rot does not develop as the
area dries out.
Sensors are also appropriate when it is necessary
to collect detailed information about the spread
of moisture within a building and to map out the
areas affected by dry rot or that might become
affected by dry rot. Such information is required
when carrying out a detailed survey of a building
or when an outbreak of dry rot has been discovered
and it is necessary to map out the full extent
of the spread of dry rot without the normal disruption
of lifting floors and removing plasterwork.
WHO CAN USE THE SENSORS?
An important advantage in the application of dry
rot sensors is that they can be fitted and examined
by anyone. No specialised equipment is necessary.
The sensors are simply fitted then removed for
examination. The sensors can be used in conjunction
with an electrical moisture meter for determining
the spread of moisture when they are removed for
examination.
After examination, sensors that have not reacted
can be re-fitted for future examination. Sensors
that have reacted may be replaced for future examination.
If desired, this can form part of a long-term
monitoring procedure, for example, in a historic
building where it is desirable to deal with the
dry rot using environmental control technology.
It is useful to renew the sensors annually to
ensure there is no loss of sensitivity.
A FEW EXAMPLES OF THE SENSORS IN ACTION
First, a surveyor carrying out a routine inspection
notices a damp, stained area that he thinks might
give rise to dry rot. He advises his client that
he wants to check it out and does so by fitting
sensors. He gets either a ‘yes, there is
dry rot’ or ‘no, there is not’
response.
Secondly, a surveyor finds dry rot in a roof void
but cannot see how far it has spread into the
room below. He can fit some single-purpose sensors
and will get a ‘yes, there is dry rot’
indication until the point where the dry rot stops.
Alternatively, he can map out the spread of moisture
that will tell him the whole ‘at risk’
area, then examine the sensors and establish how
far the dry rot has spread within that area. He
has then established how far he has to go with
his treatment in order to stop the dry rot from
spreading further.
Thirdly, an area previously treated for dry rot
becomes wet as a result of a flood from a pipe.
Dual-purpose sensors can be fitted in the affected
area allowing the moisture to be monitored until
it dries out fully. As the area dries out the
sensors can be checked to ensure that there is
no dry rot redevelopment. Once dry there is no
longer a risk and the sensors can be removed.
Finally, an outbreak of dry rot has spread behind
some ornamental panelling in a historic building.
To treat the outbreak using conventional methods
would involve damage to the panelling. Sensors
can be fitted within and around the affected area
and the activity of the fungus monitored. Environmental
control principles then can be applied and the
sensors used to monitor the dying back of the
fungus as the control takes effect. Subsequently,
the sensors may be used as a regular or even permanent
way of monitoring the area, all without damage
to the historical artefacts.
8 Pack
References
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