BCD SPECIAL REPORT ON
HISTORIC CHURCHES
21ST ANNUAL EDITION
33
good appreciation of the importance of collecting and discharging
surface water clear of the building. (It could be argued that the
principle of effective building design and informed material sourcing
is a large part of what makes any building historic in the first place.)
Consider the rubble stone construction found in many historic
churches, which has a central core containing smaller stones and
fragments of mortar. Originally the building would have been internally
plastered in lime and its solid floor would probably have been covered
in flagstones. Where rising or penetrating damp is suspected, the
investigating surveyor should expect to find plaster separating from
the masonry and/or possibly a visible tide mark. (If the wall coverings
were structurally stable with no obvious tide marks or signs of water
ingress at the wall/floor junction and the damage limited to disruption
of the decorative surface, condensation would be the likely cause.)
Determining the nature of the wall covering is important. Usually a
hard cementitious plaster is quite distinct from one which is lime-based,
although this is not always the case. A very simple method to distinguish
the two is to drop the sample into a weak acid with around six per cent
acidity – household vinegar would suffice. Both will react, although the
reaction of the lime with the acid will be much stronger. Typically, if
there is a problem, the internal plaster is Victorian or later and usually
cementitious. Establishing plaster type will be very informative and in
some situations, such as the wall shown on page 32 (bottom right), it is
quite clear where the cementitious ends and the lime plaster begins.
Any central core will have a high proportion of air voids which
do not produce effective capillary networks. Within the masonry
on either side of the core there may be a coherent capillary network
for perhaps two or three courses of stonework, depending on
the type of stone and mortar used and the extent of the water
problem externally. Plaster applied to the internal face of the
masonry provides a much more effective capillary network.
Lime plasters are both highly porous and vapour permeable.
As such, any moisture from wet masonry is transported by
capillary action laterally (and efficiently) to the evaporation zone
(usually the internal environment). After a relatively long period
the plaster will separate from the wall due to crystallisation of salts
and will need replacing. This could pose a problem for medieval
wall paintings, but generally the plaster at the base of the wall will
have been replaced many times in this zone over the centuries.
However from the 19th century onwards, use of cement-based
plasters became the norm. The pore structure and chemistry is very
different to that of lime-based plasters. As a result the wall will either
begin to dry preferentially to the external environment or respond in
a similar way to that shown in the photograph on page 32 (bottom
right), using the interface between the plaster and the internal face
of the wall to rise by capillary action higher than it would normally
be able to. Various processes, most commonly the formation of salt
crystals, are likely to eventually cause cracking and separation of large
portions of cementitious plaster from the damp or wet masonry.
SOLUTIONS
The base of a church wall is in equilibrium with the ground on which it is
sited. If the ground is wet it may be possible to improve the situation with
effective drainage. However, this will be a slow process, if successful at all.
To achieve acceptable internal wall finishes within a church
which has damp or wet wall bases, the approach must be to allow
any system to transmit water out of the masonry as efficiently as
possible – managing rather than blocking any movement of water.
A very wet wall base will elevate the relative humidity of the
internal environment. Care should therefore be taken to avoid
placing timber objects directly in contact with damp walls and
delicate organic materials such as papers and fabrics may be at risk
of damage. The problem can be exacerbated if there is a change
in the heating regime, as improvements in heating are usually
associated with increased moisture movement by convection.
As with most tasks in building conservation, the control of
rising damp is one where expectations must be managed.
TIM FLOYD
) is a conservation scientist and
chartered surveyor who specialises in the assessment of all types of
building, especially those considered historic. His main fields of expertise are
building physics, materials science, bio-deterioration and historic timber.
Contact Cathedral Communications to
discuss the possibilities
Tel 01747 871717
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