Page 12 - Historic Churches 2013

12 BCD Special Report on Historic Churches 20th annual edition CATHEDRALC O M M U N I C A T I O N S
THE PROBLEM
Wind-driven rain afflicts most churches on the
western side of the UK. In the south west the
rainfall levels are nearly three times that of the
annual average for England and for exposed
towers at high altitudes these can exceed
2,000mm. Most of the towers were built with
hard, impermeable stones which means that
the mortar joints and core deals with all the
moisture movement. The quality of the core
construction can also vary considerably. Many
medieval towers were poorly built, containing
varying degrees of voiding throughout the
wall core. Some cores were simply an earth/
stone mix, occasionally blended with lime.
Subsequent repairs may not have helped, for
example the removal of protective renders or
lime slurries and the widespread use of cement.
Other significant problems include
determining what works have previously been
carried out, the current state of the core, and
how water is getting in. As church records are
very inconsistent it is often extremely difficult
to see what materials and techniques have been
used. There are a great many non-destructive
surveying techniques available today, but
although many were tried in the research, none
was able to provide reliable information on
the extent or position of voids within a thick
church wall. It is often presumed that water
ingress will concentrate at areas where there is
cracked or missing pointing, but the laboratory
research showed clearly that capillary draw is
more significant than the effect of wind pressure
in water transport. In particular, most of the
water was drawn in through perpendicular
joints that appeared to be in sound condition.
WIND-DRIVEN RAIN AND
WATERMOVEMENT
A great deal of research has been carried out by
others on driving rain and water and moisture
transport, but as little of this concentrated on
mortar joints or solid wall buildings, specific
tests were devised for laboratory testing
at Sheffield Hallam University. The tests
showed that a well-built wall with no voids
will not let liquid water pass through to the
inside even when fully saturated. However,
as soon as the wall contains voids, water
will transmit to the inside fairly quickly.
Walls wet up quickly but dry out slowly. To
dry effectively there needs to be moving air at
the surface (wind) that has a relative humidity
of less than 100 per cent so that it can remove
water molecules from the surface. A flow of
water to the surface is therefore needed. Water
can flow but unfortunately vapour travels much
more slowly, so only a percentage of rain will
dry out quickly from a wall. In a temperate
climate like the UK’s all walls will contain
moisture, but that is not a problem so long as
most moisture does not flow to the inside but is
allowed to evaporate away to the external air.
GROUTING
Grouting was devised as a technique for filling
voids in the late 18th century and was used
mainly on bridges and tunnels. It was employed
more widely over the next two centuries on other
buildings including churches, particularly in
the 20th century. However, most of the grouts
were based on liquid cement mixes and, more
worryingly, there was no research or evidence on
how the grouting was performing. Investigating
would have meant invasive work that would only
have been justified if there were obvious faults.
In the early 1990s English Heritage ran
courses on grouting at its training centre at Fort
Brockhurst. The practical work was carried
out on purpose-made masonry ruins so, after
grouting, the wall faces were taken down so that
different grout mixes and application techniques
could be evaluated. This allowed some measure
of performance testing and showed that
mixes based on hydraulic* lime, PFA* and
bentonite* were capable of filling all voids and
minimising shrinkage when injected using
low-pressure pumps. However, the performance
and strength of grouts varied. Developing an
effective technique was also pursued further
at Sheffield. This included how best to test for
voids, how to ensure wetting would adequately
remove loose debris in the core and minimise
surface tension, and then how to determine the
correct ratio of grout to water to ensure both
an adequate flow and comprehensive filling.
After careful research to refine
these techniques, the results at Sheffield
showed that grouting could be the answer,
as it could prevent driving rain from
conducting water to the inside face.
The next stage of the research was to
monitor a church tower which was to be grouted,
both during the work and again afterwards to
assess its effectiveness and the rate at which the
masonry joints and core of the tower dried out.
* Words followed by an asterisk are defined in the
glossary on page 14
Figure 2 St John the Baptist Church, Stowford in 2007 showing (top) a very damp south elevation after a short
storm. The tower is exposed to the prevailing weather and driving rain and has a long history of water ingress
to the inside (illustrated top right).