Page 26 - Historic Churches 2013

26 BCD Special Report on Historic Churches 20th annual edition CATHEDRALC O M M U N I C A T I O N S
RAINWATER
Jonathan Taylor
T he root cause of
most deterioration in
historic buildings and
their fittings and furnishings
is moisture. Whether
trickling down from a
blocked gutter, penetrating by
wind pressure and capillary
action, or condensing out
of warm air onto cold
surfaces, the original source
is usually rainwater.
Most materials are
susceptible to decay
in damp conditions.
Mechanisms include:
biological decay of organic
material, such as the decay
of timber by woodworm or
dry rot, and the decay of
textiles by carpet beetle;
physical damage of rigid
materials such as iron, stone
and glass as a result of the
expansion of ice crystals,
salt crystals or rust; and
chemical damage such as the
corrosion of iron exposed to
air and water, or the reaction
of materials with atmospheric
pollutants in acid rain.
If left unchecked,
penetrating rainwater can
cause extensive damage to
the structure and the fabric
of a building, particularly
if dry rot takes hold. Once
established and in the right
conditions, deterioration
can be rapid, costing more
to put right the longer it is left unchecked.
Damp in the exterior fabric of a building
also reduces its insulation value, conducting
heat out of the building and cooling the
interior through evaporation. High relative
humidity inside the building can also make
it feel cooler. The result adds up to higher
fuel bills and carbon dioxide emissions.
The visual effects of damp should not be
ignored either. Finding the funds necessary
for conservation can be difficult enough
for an enthusiastic congregation, but it can
be far more difficult where the interior has
been disfigured by patches of damp, mildew
and the accumulation of past repairs.
For all these reasons, it is vital to keep
the rain out of a building and to react
promptly at the first signs of damp.
SOURCES OF DAMP
Rainwater ingress can be direct, such
as through a leaking roof or parapet
gutter, or through gaps in walls and
eaves; or ingress may be indirect, such as
ground water accumulating as a result of
inadequate drainage or faulty drains.
Damp can move from one source to affect
fabric far from the original source, either by
gravity or through cycles of evaporation and
condensation. The rubble-filled core of a solid
wall, for example, often forms the path of least
resistance for water to trickle down from a
blocked gutter or slipped roof tile, or from
wind-driven rain soaking the exterior of a
west-facing wall far above. Travelling unseen
through the masonry core, it descends, picking
up soluble minerals along the way, until it
reaches the base of the wall
or is forced to the surface
by an obstruction such as
an impervious lintel. Damp
patches at the base of a wall or
column, often accompanied
by efflorescence (the light-
coloured bloom of soluble
minerals re-crystallising), are
often assumed to be caused
by rising damp, when the
source is not even close.
Nevertheless, damp
at the base of a wall can
be the product of high
water levels in the ground
caused by blocked drains
or heavy rainfall, or higher
ground levels externally.
A functioning system of
downpipes, ground-level
drainage channels and below-
ground drains is essential.
Water pressure
and capillary action are
insufficient to cause damp
to rise much above external
ground level. Vapour,
however, is more mobile, and
can be carried by convection
currents and dissipation. The
warmer the air, the more
moisture it can carry, so
heating a large church space
with high moisture reservoirs
just before the service begins
can be disastrous. Moisture
evaporating from the base of
the walls can be carried by
the warmed air to condense
on those parts of the structure that have not
yet warmed up. In the worst cases, cold damp
walls can run with condensing moisture,
reducing their insulation value further in a
self-perpetuating spiral through the cooler
months, so that the fabric never dries out.
Condensation also occurs out of sight
within structures and porous fabric, wherever
the temperature of moist air falls below a
certain temperature (the ‘dew point’). In a
large building there are many elements that
are particularly vulnerable to this type of
condensation, such as the metal fixings in
the roof structure, and porous insulation
such as glass fibre or sheep’s wool.
A particular problem is condensation
on the underside of lead-sheet roofing,
as this can lead to lead corrosion.