Dry Rot and its Treatment in Historic Buildings
Janice Carey and Colin Grant
'Dry rot' is the decay of wood caused by the fungus Serpula lacrymans, the effects of which in buildings can be truly dramatic. Diagnosis of dry rot in a building, like the diagnosis of cancer in a patient, has the ability to strike terror into the heart of the recipient of this dire news. However, all wood-rotting fungi require both food (wood or other cellulosic material) and water, and the dry rot fungus is no exception; deprived of either, it cannot survive.
Much
of the mythology surrounding dry rot is founded on the ability of its
strands to penetrate through non-wood building materials, to transport
water to otherwise dry areas and for the fungus to 'manufacture' its own
water. In reality, the delicate hyphae are the primary colonisers and
the ability to conduct water is limited and can be negated by good ventilation.
The process of wood decay itself produces water but in this respect dry
rot is no different from any other wood-rotting fungus and, likewise,
its ability to produce moisture in this manner can be negated by ventilation.
Decay will cease if the moisture content of the wood is reduced to below
about 20 per cent, and many extinct outbreaks of dry rot are discovered
in buildings where the fungus has died out as a result of this happening,
probably following maintenance which has eliminated a water source.
CONTROL STRATEGIES
Because of the total dependency of dry rot on moisture, the primary control
strategy must be based on environmental considerations aiming to restore
and maintain dry conditions. However, in many cases drying will take a
long time, often measured in years, especially where some types of historic
buildings are affected. Therefore, secondary measures will often be required
to prevent further damage by the fungus before it is effectively arrested
by the drying.
Primary Control Measures
A detailed survey should be carried out to identify and locate sources
of moisture ingress. Particular attention should be paid to roofs and
rainwater systems with emphasis on gutters and downpipes, parapet roofs
and roof coverings. Rain penetration can also be through renderings and
flashings or around windows and doors. Rising dampness through missing,
bridged or otherwise defective damp-proof courses must be rectified. Any
plumbing should also be inspected for leaks.
Rapid drying should
be encouraged through the provision of heating and ventilation which may
also require specific building work to prevent moisture ingress and transfer,
and to encourage aeration. Dehumidifiers can remove moisture from the
air but their effectiveness in aiding drying of walls depends on the rate
of evaporation from the wall surfaces.
Other Measures
Assessing the outbreak
It is necessary to determine how far the dry rot has spread.
All woodwork in the vicinity of any outbreaks should be inspected carefully
to assess the extent of decay and the current moisture content of the
timber. Extensive removal of plaster is necessary only if it is suspected
that timber is embedded in the walls and is at risk.
Removing
affected timber
Removal of all timber affected by dry rot is destructive but necessary
in principle. Retaining affected timber presents problems for the structural
integrity of the building and falling debris can be a hazard to occupants
and others if decay continues. Timber already below 20 per cent moisture
content presents little risk of further decay but, at higher moisture
contents, the level of risk depends upon the speed with which drying can
be induced and the ease of monitoring the reducing moisture content. Higher
risks may be acceptable where timbers are of historic value or where their
removal cannot be achieved without damage to important historic fabric
- for example, where they support a fine plaster ceiling. In such cases
the retention of some timbers may be essential or at least highly desirable.
If the wood can be
removed, it can be sterilised in a kiln. The temperature throughout the
wood must be maintained at just over 40°C for 15 minutes. Care is needed
to prevent splitting and distortion and this method provides no protection
to the wood after reinstatement.
Special building measures
are necessary if timber is to be retained, including isolation from damp
masonry.
Wood
preservative treatments
If timber infected
with dry rot has to be retained for special reasons and decay cannot be
arrested in the short term by drying, preservative treatments that penetrate
throughout the affected part of the timber can be used. For example:
- application
of a preservative paste
- repeated
addition of liquid preservative to sloping holes drilled into the wood
or by pressure injection
- insertion
of borate rods or tablets (these are only effective if the wood is wet).
Treatment of hardwoods must include an insecticide if there is a risk
of infestation by death watch beetle.
All new timber used
in repairs should be pre-treated with a wood preservative. Detailed guidance
on the treatment required for various timber components is given in the
British Standard BS 5268: Part 5, which deals specifically with structural
timber, and BS 5589 which covers a wider range of uses. Guidance is also
given on the use of naturally durable timbers (see also Digest 429 published
by the Building Research Establishment Ltd).
Masonry Treatments
Although strands can grow through and across masonry, the dry
rot fungus derives no nourishment from it. The concept of killing the
fungus within masonry by wide-spread irrigation with a fungicide traditionally
has provided a 'comfort factor', but it has to be questioned in each case
whether this procedure can be justified. First, it is usually difficult
to achieve a thorough treatment and, secondly, the treatments introduce
large quantities of water which then need to be removed, increasing the
risk of salt efflorescence and damage to the masonry, as well as prolonging
the time it takes to dry the structure.
The most important
role of chemical treatments of the masonry is to prevent the fungus from
obtaining access to a fresh food supply in the form of timber in adjacent
areas, or replacement timbers being introduced into the area. For this
purpose, localised chemical treatments of the masonry can create a useful
barrier between the fungus in the wall and the wood. Examples of such
treatments are:
- surface
application of fungicidal fluid (which also helps prevent fruit-body
formation during the drying phase)
- use
of fungicidal renderings
- insertion
of preservative plugs or pastes
- localised
irrigation treatments.
Whilst these localised treatments play a role in the overall control strategy, they must not be regarded as a substitute for getting the building dry.
Heat
sterilisation of masonry walls and timber in situ
In the past, the use of heat to sterilise walls was condemned because
it was too difficult to apply effectively and provided no residual protection.
In the search for chemical-free control measures, sterilisation with hot
air is now increasingly being used, particularly in Denmark. However,
the process needs to be carefully controlled so as to prevent damage to
the building as well as to ensure that the necessary temperature has been
achieved deep in the affected area.
MONITORING
The importance of monitoring the conditions in buildings cannot be over-emphasised.
Dry rot develops very slowly, so early detection and curing of moisture
ingress will prevent decay occurring in the longer term. Routine monitoring
can be as simple as regular visual inspection to check the integrity of
the building fabric against ingress of moisture, and taking measurements
of moisture content of vulnerable timbers with a hand-held probe. However,
sophisticated permanent monitoring systems are now increasingly used involving
computer-based equipment linked to probes permanently installed in timbers
or other parts of the building fabric. Specific sensors can also be installed
in rainwater goods to indicate overflows.
Dry
rot is potentially a cause of seriously damaging decay for timber in historic
buildings, but it does not have to be devastating or outrageously expensive
to cure if caught in time. A careful diagnostic approach is required to
identify and cure dampness, to treat in a very selective and targeted
way and to re-instate with appropriately pre-treated or naturally durable
replacement timber. Installation of monitoring systems to facilitate routine
maintenance checks can enable massive economies compared with destructive
re-build approaches and will provide greater assurance for the future.
Recommended Reading
IP 19/88
House inspection for dampness
British
Standards Institution
BS 5268 Code of practice for the structural use of timber Part 5:
1997 Preservative treatments for constructional timber
BS 5589 Code of practice for preservation of timber, 1989
BRE
Digests
299
Dry rot: its recognition and control
304
Preventing decay in external joinery
307
Identifying damage by wood-boring insects
321
Timber for joinery
327
Insecticidal treatments against wood-boring insects
345
Wet rots: recognition and control
364
Design of timber floors to prevent decay
371
Remedial wood preservatives: use them safely
378
Wood preservatives: application methods
429
Timbers: their natural durability and resistance to preservative
treatment
BRE
Timberlab Papers
Dry Rot
- Causes and Remedies, TP No: 44-1971 (Reprinted 1981)
Books
For
further information contact BRE, Garston, Watford WD2 7JR
Tel 01923 664000 Fax 01923 664010 E-mail GrantC@bre.co.uk
