24
BCD Special Report on
Historic Churches
17th annual edition
Preliminary conclusions
Te tower walls’ construction can be
summarised as follows: it is largely made of
impervious stone; in the lower stages between
the two masonry skins there is a relatively
wide zone of backflled random rubble where
the original bedding mortar has degraded
to sand, leaving occasional voids; there are
impermeable bonding stones tying the two
skins together, including those of the diagonal
buttresses; and the outer walling is now
pointed with a well tamped hydraulic lime
mortar to almost the full depth of the joints.
Since the walls continue to admit moisture
despite the porosity and depth of the new
mortar, the assumption made in authoritative
technical papers is not correct: in extreme
environments the rate of moisture evaporation
from the surface of a lime mortar joint is not
sufcient to keep the structure dry. Tis may
be because there are signifcant air pressure
diferences between the exterior and interior
over much of the year, and between diferent
sides of the tower. Rainwater pushed through
the porous joints by strong wind remains in the
core, either in voids or in the degraded mortar,
transforming it into a sponge, and which air
pressure diferences then pump into the interior.
Rainwater is also able to percolate out along the
repointed external joints much more than it
could with the previous cement pointing, and it
is leaching out lime, disfguring the masonry.
In an environment like this where a
masonry wall is constructed of an impervious
stone and is pointed with a lime mortar, one
solution would be to introduce a lime-based
overcoat (harling or render), as this would
dramatically increase the area available
for surface evaporation. But the church’s
appearance could not be altered: its listing
means that its character is protected by law.
Another option would have been to grout
the core of the wall, and it may have been
premature to dismiss this option. However,
the engineer’s assumption of failure was
based on his experience of similar masonry
structures in similar environmental conditions,
and there are other well known examples
where grouting has been tried and failed,
as in the case of some Norfolk fint towers.
Furthermore, grouting is not reversible, and
this alteration would not have been supported
by Historic Scotland without further research.
For a project like this, where the
scale of the work is very limited, it is
not possible to undertake case-centred
research prior to embarking on the scheme,
and so the true nature of the problem
may not be understood before work is
irreversibly committed or complete.
With Historic Scotland’s approval a line has
been drawn under the project. Nevertheless,
with the evident failure of the pointing to keep
wind-driven rain out of the interior, if more
efective remedial action is to be taken, then
more (possibly expensive) work will be needed.
Finding a successful approach will require a
careful review of realistic options, and a frm
recommendation based on evidence. Any
alteration to this listed building will need to
be justifed both to the client and to Historic
Scotland, as will further fnancial outlay.
The way forward
Penetrating dampness in historic masonry
structures of all kinds is not uncommon, and
from the evidence of several other Scottish
churches, it seems that chronic penetrating
damp often persists even after repointing
with lime mortar. Published guidance for
professionals dealing with such situations
does not adequately cover the problem
and may need to be reviewed. Research is
long overdue into the precise causes, the
mechanisms involved, and the alternative
specifcation options – particularly where it is
not possible to introduce an external render
or harling of lime mortar. Until then, the
difculties and disappointments are likely to
continue. However, help may be at hand.
Over the past seven years English Heritage’s
Damp Towers Project has undertaken the only
signifcant research in this area, working with
Oxford and Shefeld Hallam Universities.
Te research takes forward the 1980s study
into rain penetration in West Country church
towers by examining a group of exposed church
towers in Devon and Cornwall that sufer from
penetrating damp, and by assessing the remedial
methods adopted in similar grant-aided repair
schemes. Research is nearing completion,
lab work has fnished and English Heritage is
currently reviewing case studies and inspecting
towers that were repaired many years ago. Tis
work is expected to be completed by April
2011. When it is published it should go some
way to provide the research data we need.
From preliminary information there
seems to be evidence to support grouting in
cases where render or harling is not an option,
provided the wall construction is suitable.
A third option subsequently considered
for Our Lady, Star of the Sea is to provide
a continuous damp proof course through
the tower walls using the same construction
method as underpinning. Tis would ensure
that water percolating down through the core
of the tower wall could at least be collected
and drained. However, this option presents
considerable technical challenges, and
since the masonry would remain saturated,
it would not provide a suitable solution
for buildings exposed to severe frosts.
Inevitably there are technical and cost
issues to be resolved with all such proposals,
and as no two buildings are the same, there
is a strong case for some building-centred
research to investigate the best option.
In the interim, until a defnitive answer
can be agreed, the diocese has specifed minor
work including a ventilated strapped lining
for the aisle wall and removing the remaining
built-in timber in the tower wall. Tis will
help to alleviate or avoid more problems,
but the damp walls will remain until an
efective treatment can be found. Stripping
the paint (which is non-original) from the
afected stone to allow evaporation will also
help, although there will be some minor
implications for comfort and heating costs.
Repair projects such as this are inevitably
of great local importance. Te church building
is the focus and symbolic heart of many rural
communities which often have few resources
except enthusiasm. Te eforts made to
organise the project and raise funds to allow
it to proceed cannot easily be repeated, but
somehow funds for further work will need
to be found. Our Lady, Star of the Sea could
provide an example of how to resolve a difcult
problem afecting many similar buildings.
Paul Harding
RIBA ARIAS FRSA (paul@
benjamintindallarchitects.co.uk) is a partner with
Benjamin Tindall Architects based in Edinburgh. He
has over 30 years of experience working with historic
buildings, many in the Western Isles and the Orkney
Islands. For 15 years he was technical advisor to the
Edinburgh Old Town Renewal Committee, and the
Edinburgh World Heritage Trust.
Te stonework repair project at the Church
of Our Lady, Star of the Sea, Castlebay was
funded by the Listed Places of Worship
programme, which is administered by the
Heritage Lottery Fund and Historic Scotland.
References
1 Preparation and Use of Lime Mortars:
Technical Advice Note 1, Historic Scotland,
1995
2 Rain Penetration in West Country Church
Towers, English Heritage Research
Technical and Advisory Services, 1989
3 ibid, pp 3, 26
4 ibid, Part 4 Conclusions and
Recommendations, para 4.7
Te tower from the south showing the entrance to the
porch at the base of the tower and (below) a detail of one
of the buttresses: Less than fve years after repointing,
lime is leaching out of the mortar once again.