Page 11 - Historic Churches 2013

BCD Special Report on Historic Churches 20th annual edition 11 CATHEDRALC O M M U N I C A T I O N S
GROUTING SOLID
MASONRY WALLS
Chris Wood and Colin Burns
Solidmasonry walls, particularly
those of composite construction
(figure 1), have performed extremely
well over the past millennium. Most churches
are built with two skins of stonework and a
rubble core, and their towers are often very
exposed. Despite enduring extremes of weather
they still continue to function as originally
intended. Over the centuries though, voids
have developed within the core of the walls of
many church towers which has resulted in the
rapid conduction of water into the church after
driving rain. One solution is to grout the core.
Recently English Heritage completed
the Damp Towers research project which
set out to assess the relative merits of
grouting in such cases, and to establish a
best practice method for carrying it out,
even though the irreversible nature of the
technique means it can be controversial.
This article introduces the findings.
BACKGROUND
Driving rain problems are most acute in areas
of high rainfall and exposure, so the research
concentrated on church towers in the south
west of England. A number of on-site repairs
were studied, including grouting the whole
tower at St John the Baptist, Stowford in
Devon, which followed a series of laboratory
tests. The research was prompted by the fact
that problems of water ingress into some
churches in the region were actually worse after
repairs, and considerable sums of money were
apparently being wasted. Investigations revealed
that in most cases the repairs had focussed on
repointing, carried out in a variety of mortar
mixes, although some did include deep tamping
and selective grouting where large voids were
suspected. It was clear that repointing alone
was not effective, regardless of the mix, if the
wall still contained voids. This observation
was corroborated by subsequent laboratory
testing of trial walls in a driving rain chamber.
The Damp Towers research concentrated on
the traditional repair techniques of rendering,
pointing, grouting and internal plastering, using
materials bound by lime. In practice, some stone
replacement and reconstruction often took
place, particularly where a core was substantially
voided or the outer face had moved away,
but this type of repair was not studied.
A considerable amount of research has
been carried out on pointing mixes and much
important and useful literature on renders and
plasters is now available. But there is precious
little written about grouting, despite this being
a potentially invaluable technique that offers
considerable benefits. In particular, grouting
can obviate the need for substantial masonry
reconstruction or the alternative treatment
of rendering. Although there is considerable
evidence that most towers were rendered,
certainly before the 19th century, there is a
general antipathy among parishioners to the
visual loss of familiar masonry elevations.
So research was carried out into
grouting, both in the laboratory and on
site. Working closely with the architect
and contractor to develop the specification
and site practice for grouting the tower at
St John the Baptist, Stowford also provided
an opportunity to gather data on the effects
of the work by monitoring moisture levels
in the walls both during and afterwards.
proving hole sealed
with cotton wool
swabs and
wooden plugs
cotton wool swab
inserted to stop
localised leakage
of grout
open joints repointed
prior to grouting
grout injected through
entry port until it runs
from proving holes
open joints
repointed
prior to
grouting
Grout delivery nozzle
with stopcock
Figure 1 Most churches have composite walls built with two faces of stone, while the core is filled with rubble
and mortar. Over time, voids develop in the core and pathways form which can direct water to the inside face.
The basic grouting method for filling the voids is illustrated here. (Image: Iain McCaig, reproduced from English
Heritage’s Practical Building Conservation: Stone, 2012)
Grouting voids in a composite wall
3rd and
subsequent
lifts
2nd
lift
1st
lift