Damp Movement in Rubble Walls
||Typical damp at the base of a church wall, resulting in crystallisation damage to the ashlar masonry
This 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.
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 since 1970.
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.
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 (illustrated below) 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.
|Above left: water confined in a capillary forms
a concave upper surface or ‘meniscus’. The water
travels up the capillary because the adhesion of the
water molecules to the capillary walls is stronger than
the cohesion between them. Similarly, water droplets cling to a leaf because the adhesion of the water molecules to the surface of the leaf is
stronger than the cohesive forces between the water molecules.
To consider the movement of water in
masonry it is necessary to briefly touch on
the following scientific principles (each is a
subject area in its own right so only brief,
simplified definitions are given here):
POROUS BUILDING MATERIALS
The porosity of a material is defined by the ratio
of area occupied by its pores to its total crosssectional
area. All building materials are porous
to some extent, with pores forming either
continuous networks or closed systems. The
principle of pore formation in building materials
is similar whether they are man-made or
naturally occurring. When water is trapped and
then driven out, usually by heat, voids are left
behind (brick production is a good example).
Plasters and mortars have well-developed
pore structures by default, as high volumes of
water (which will eventually evaporate) must
be added to make them sufficiently workable.
Pore space geometry and characteristics
Porosity in natural building stone is a little
more complex, although a rule of thumb is that
rocks generally become less porous with age
and depth of burial. Consolidation of all rock
types (by heat and/or pressure) can produce
complex dual porosities with open and closed
systems. Stones with well-sorted grains tend to produce very coherent pore networks
which enable efficient hydraulic transport.
A material with a high porosity ratio
isn’t necessarily going to be conducive to
effective hydraulic transport. A brick fired
at high temperature will have large pores
within the clay body, but the formation
of a fireskin effectively closes off many
of the pore openings at the surface.
The heterogeneous nature of naturally
occurring building stones makes them
more difficult to categorise as the pores
often contain crystalline deposits which
affect their porosity. This phenomenon is
also seen in mortars, with complex crystal
growth within the pore structures of both
lime and cement-based compounds.
Pore networks constitute effective capillary
The process which ultimately defines how water
is able to move against gravity is capillary action.
Although this is not the most complex scientific
field, it is interesting to note that capillarity
was the subject of Albert Einstein’s first paper,
submitted to Annalen der Physik in 1901.
Capillary movement of water (illustrated above) occurs in a narrow tube because the adhesion
of the water molecules to many solid objects
is stronger than the cohesive forces between
them (this is why the water droplets in the
photograph above are sticking to the leaves).
Surface tension is a product of inter-molecular
cohesion. Water confined in a small tube or
capillary results in the formation of a concave
upper surface or ‘meniscus’ (from the Greek
for crescent). The smaller the tube diameter,
the greater the ratio of circumference to area
and the more efficient the capillary action.
The height to which water is able to
move in a capillary is therefore limited
by its surface tension, the size of the
capillary and the effects of gravity.
Understanding how and why water
molecules behave in this way, is fundamental to
an appreciation of how moisture behaves both
inside and on the surface of building materials.
There are many other factors which
affect the movement of water (in both liquid
and gaseous phases) in capillaries, such as
moisture storage characteristics, vapour
and capillary transport coefficients, vapour
diffusion and effusion in micro pores,
sorption and adsorption isotherms. These
and other factors are of specialist interest
only and so are not expanded upon here.
Some churches have exposed stone walls
internally and some of these buildings have
problems associated with low-level damp
and/or the crystallisation of mineral salts
(which may be the reason why the plaster
was originally removed). Most churches,
however, are plastered internally so the
discussion below deals exclusively with them.
||The rising ground level in this church graveyard, perhaps in combination with
other factors, has created a depression along the foot of the exterior walls
encouraging water to collect in this vulnerable area.
The scientific principles touched on above
would generally have been appreciated (in
a practical rather than an academic sense)
by the surveyors and masons originally
responsible for the construction of a
church. They did not have the benefit of
hindsight with regard to the use of some
stones that have proved especially prone to
weathering, particularly those damaged by
the products of the industrial revolution.
In general, however, they demonstrated a 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
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 below, 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.
|A church wall which has been partly cement-rendered, forcing moisture to rise by
capillary action at the interface
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 to the left, 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.
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.