BCD SPECIAL REPORT ON
HISTORIC CHURCHES
21ST ANNUAL EDITION
31
DAMP MOVEMENT
IN RUBBLE WALLS
Tim Floyd
T
HIS SUBJECT
is one of many in the
field of building science that tends to
get oversimplified. It could be argued
that this is perfectly reasonable when the
subject matter is wet walls as opposed to
rocket science. However, the last 30 years
has seen the publication of a large volume
of research on the properties of building
materials and systems. Given the tendency
to oversimplification on the one hand and
the mass of scientific data available on the
other, it is important to get to grips with the
relevant factors at play in a specific case.
Any discussion about water in wall bases
must include a comment on terminology, above
all the term ‘rising damp’. Some commentators
see the term as legitimate (water is rising up
the wall, what else would you call it?). However,
the problem in this case is that the term (like
many others in the field of building science)
has come to be used as a generic catch-all and
has developed a (very unhelpful) life of its own.
Furthermore, misdiagnosis is common: damp
at the base of a building is far more likely to
come from leaks at roof level (for example,
due to failed roof coverings and flashings,
overflowing gutters or deteriorating chimneys)
and from rain penetrating through walls above.
A general rule of thumb is that
water enters masonry in four ways:
• through its base as a result of its
relationship or equilibrium with the ground
on which it stands
• from above via vulnerable wall-heads and
defective roof drainage
• laterally from the exterior as a result of
severe rain exposure
• internally from condensation (often as a
consequence of one or all of the above).
This article focusses on the first process
– specifically with regard to those
solid and rubble stone walls generally
associated with churches – although all
four processes are often interrelated.
WATER SOURCES
Before determining whether groundwater is
contributing to a problem, first consider the
geology and location. The geological formations
required to produce a direct connection between
any foundation and the watercourse are relatively
rare. In most cases sub-soil drains moisture away
from the surface, although some may drain more
slowly than others. Churches sited on clay soils
will be more susceptible to problems caused by
broken drains or badly controlled surface water
than those sited on gravel, for example. As a
result a church in Norfolk (on chalk) close to land
drains would have to be looked at in a different
light to one in London (on predominantly clay).
This is not to say that fluctuation in
groundwater levels can be totally discounted.
Conditions vary with annual rainfall,
and the British Geological Survey has
produced a groundwater timeline which
demonstrates the variability in water table
levels in various parts of the country over
the last 44 years (see
).
One diagnostic tool routinely used over the
last few years to distinguish surface water from
groundwater (or subsurface water) is outlined in
the Building Research Establishment’s Digest 245
Rising Damp in Walls: Diagnosis and Treatment.
Very briefly, its premise is that water which has
moved through the ground (unlike surface water)
will have had the opportunity to take various soil
minerals into solution – something which can be
established by a relatively simple test. However,
it is only in exceptional circumstances that the
movement of sub-surface water up through wall
foundations and bases is likely to be the principal
problem, so such tests are rarely required.
So if water is managing to rise up through
the wall bases, it is generally water which
has collected on the surface and found its
way into the wall bases. In this case surface
water is taken to include that collected by the
rainwater goods and, if they are functioning
properly, discharged away from the building.
HISTORICAL FACTORS
To understand why there seems to have been
a relatively recent increase in incidences of
excessive damp in the bases of church walls and
their related floor junctions, one must look to
the congregation, or rather their unfortunate
migration from the church maintenance
team to the surrounding graveyard.
Churches are not necessarily complex
structures, but they are usually big (and
sometimes extremely big), making access
to and maintenance of concealed parapets,
valleys and hoppers perilous and maintenance
of drainage runs physically arduous.
The phenomenon of graveyard rise is
widespread and well-known. Combined with
periodic maintenance work to dig out drainage
channels around the outside of the church
and any natural settlement of the building,
the increase in levels of the surrounding
graveyard can effectively place the church in a
depression (see page 32) that collects water.
Generally, the older the church and the
more built up its surroundings, the greater
the problem which can result from an
increase in adjacent ground levels, particularly
where impervious road and pavement
surfacing expose the fabric to run-off.
Typical damp at the base of a church wall, resulting in crystallisation damage to the ashlar masonry
