22
BCD Special Report on
Historic Churches
17th annual edition
The Limits of Lime
Repointing a damp church tower in the Hebrides
Paul Harding
I
t is
generally held that if a solid stone
wall is pointed with a permeable lime
mortar, the high rate of evaporation from
its surface will limit rain penetration to just a
shallow surface layer. It would certainly never be
expected to reach the inner face. However, there
have been a few recent cases on exposed sites
which have shown that this assumption cannot
be relied on. One of these is the category B
listed Victorian church of Our Lady, Star of
the Sea on the Hebridean island of Barra.
Our Lady, Star of the Sea
Barra is the most southerly inhabited island
of the Western Isles, lying west of the
Scottish mainland directly in the path of
frequent Atlantic gales. It has some of the
most extreme weather in the British Isles,
with very high average wind speeds and an
annual rainfall of about 1,100mm, almost twice
as much as London. Te fact that the Gulf
Stream ensures that air temperature rarely
drops below freezing is little consolation.
Te church faces the prevailing south-
westerly wind from an elevated site above the
village. Designed by George Woulfe Brenan, an
engineer and architect based in Oban, Our Lady,
Star of the Sea was completed in 1888, and has
squared walling of the local Lewissian gneiss with
ashlar dressings of imported Elgin sandstone.
Lewissian gneiss is a very hard metamorphic
rock which is difcult to work but has an
attractive range of colours from reddish-brown
through grey to black. Tis stone appealed to the
mainland Victorian architects and engineers who
used it widely on the island for constructing new
larger churches, schools and other buildings. Te
choice was infuenced by the Gothic Revival and
the tracts of Pugin and Ruskin. Very few of these
designers followed the traditional Scots practice
of coating walls with harling, the protective
mortar harled or hurled onto the wall which is
ideally suited to severe environmental conditions.
Damp problems have beset Our Lady,
Star of the Sea since it was frst constructed.
Te church consists of an arcaded aisled nave
with a square clocktower over the entrance
porch, where the heavy bell is supported
of walls built in three stages, thicker at the
porch than at belfry level, and buttressed
at the external corners. Only the tower
has this stepped section, and this is where
most of its damp problems originate.
Te building had undergone a series of
earlier repairs. In the 1980s a major dry rot
outbreak resulted in most of the original
timber safe lintels (the innermost lintel over
an opening) being replaced together with
the boarded linings. Te walls were then
repointed with cement-based mortar, and
coated with silane water repellent. How long
this coating lasted can only be guessed, but
it had no apparent efect. Te tower walls
continued to leak so much that a system of
internal gutters and downpipes was installed
to try to intercept water inside the building:
these ran for several days after rain. Lime
leaching from the tower core created stalactite-
like deposits in sheltered areas inside and
out. Te porch walls were very wet, and its
boarded linings were black with mould.
A programme of repairs was undertaken
on behalf of the RC Diocese of Argyll and the
Isles in 2004–2005. Te approved project
included re-slating, new leadwork and
rainwater goods, as well as repointing the
whole building using lime-based mortar. Te
scheme was grant-aided under the Listed
Places of Worship (LPW) programme, which
in Scotland is administered by the Heritage
Lottery Fund and Historic Scotland. Te new
scheme provided the opportunity to remedy
the long-standing penetrating dampness
in the tower walls, but as the diocese was
informed, there could be no guarantee that the
penetrating damp would disappear completely.
Tere was, however, an expectation that the
problem would not be more acute afterwards.
Assessment and specification
A thorough survey of the structure found the
stonework to be in very good condition, and
no indenting work was needed. Petrographic
analysis by the British Geological Survey
confrmed that the gneiss was extremely
impervious, eliminating the possibility
that moisture was getting in through the
stone. Te focus of the repair work to the
tower walls was therefore on ensuring the
integrity of the mortar joints, and that
rainwater was properly shed from the roof.
Most pointing on the building was found
to be mechanically sound, and only at the
west gable and on the tower walling was it
cracked or not properly bonded. Because of
this, and since removing well-bonded cement
risked damaging the adjacent stonework,
especially the fne sandstone, Historic Scotland
agreed that sound areas should be left alone.
Repointing was therefore largely confned to
the tower and west gable. In the tower joints
the residue of the original bedding mortar was
found to consist mainly of wet sandy granular
material, the result of lime leaching, under
20mm or so of cement mortar. Raking out
this material and tamping new mortar into
joints to an average depth of 150mm was a
requirement. Te particular problems of the
tower, including the likelihood of water being
retained in intramural voids, had been reviewed
Our Lady, Star of the Sea stands high above the sea at Castlebay on the Hebridean island of Barra, and its tower
takes the brunt of the wind and rain.