Page 23 - Historic Churches 2013

BCD Special Report on Historic Churches 20th annual edition 23 CATHEDRALC O M M U N I C A T I O N S
and fibrous plaster. The chancel pendant arch,
which was constructed wholly from oak, was
found to be in particularly good condition.
Plasterwork
A specialist plasterwork survey was
commissioned to assist in the specification of
repairs. The plaster nibs which key plaster to
laths were generally intact throughout most
areas of coving and flat ceilings, although it
was noted that they were set a little too close
together in places. The major problem was the
extent of disintegrated lath and the consequent
lack of support of up to 75 per cent in some
areas. A system of lightweight supports to
reintroduce structural integrity was developed.
Drainage
A CCTV survey was carried out to assess the
drains and soak-aways. This showed that the
below ground drainage was in poor condition
with various fractures, misaligned joints and
build-ups of debris. One soak-away was full
of water and another was in poor condition.
WORKS CARRIED OUT
The aim of any repair to a historic building is
to conserve its historic fabric with minimum
disturbance. The extent of intervention has to
be carefully considered in each case, assessing
each element of structure or fabric both
individually and as part of the whole building.
At Shobdon the repair was essential to
the structural integrity of the building, and
the decay to the chancel roof at wall-head
level necessitated much greater structural
intervention than initially envisaged to ensure
that the loads from the roof were distributed
more evenly onto the walls. While the distinct
character of the building is more evident
internally, the castellated parapet is a typical
feature of the Gothic style and the extent of
reconstruction necessary was a concern.
Parapets and wall-plates
Various methods for stabilising the parapets
were reviewed. One option that would have
minimised loss of fabric was to inject grout
into the voids formed by the rotten timbers
and stitch the masonry with vertical stainless
The inner leaf of the south transept parapet was
rebuilt in brick and hydraulic lime. A new oak
wall-plate was fitted and the ends of the rafters were
cut back to prevent them being built into the parapet.
New timber framework and stainless steel lath trays
were added behind the existing cornices. Suspension
wires were fixed to steel flats above and plaster of
Paris was poured through the mesh to bond with the
existing plaster nibs.
steel rods and resin anchors. However,
as the injection would be done ‘blind’, its
effectiveness could not be assured. Another
option was to open up internal finishes, cut
out defective timbers and replace them with
masonry, but this would not address the
parapet’s tilt and would cause considerable
damage to the decorative plasterwork.
A third option was to dismantle the
parapet, repair the wall-plates and reconstruct
the parapet using as much original stone
as possible. However, the parapet had been
constructed from a local siltstone which was
susceptible to expansion in its horizontal
beds, and the more weathered stones were
already exhibiting horizontal delamination.
Dismantling the parapet would release pressure
from the stones, exacerbating delamination and
increasing the amount of replacement. However,
this approach offered full access to the wall-
plate timbers with minimal disturbance to the
internal plaster and was consequently adopted.
Opening up the structure revealed double
wall-plates supporting the chancel roof, with
the principal rafters rebated into both the
inner and outer plates. The timbers were in a
far worse state than anticipated and required
complete replacement. Reconfiguration of the
wall-head using a single wall-plate ensured
that the timber would have air all around it and
that the parapet above could be built off the
solid masonry of the outer half of the wall.
The configuration of the roof and the
parapet suggested that the roof had originally
been intended to have overhanging eaves, and
that the parapet and parapet gutter had been
an afterthought as the 1756 reconstruction
proceeded. This could explain why the
truncated rafters terminated on a roof-plate
along the top of the ashlar pieces above the
inner wall-plate, and why the outer timber
wall-plate was built into the parapet.
The replacement of the wall-plates in
the chancel and transepts required the roof
structure to be supported while new timber
was inserted and the wall-heads consolidated.
Initially it had been hoped that all temporary
support of the timber roof structure would
be from above to avoid interference with
the historic plasterwork. This approach was
A detail of the foot of the chancel roof following
removal of the parapet, cornice and dentil course,
which shows extensive decay to the outer section
of the double wall-plate (bottom), and the poor
connection between the principal rafter leg and the
wall-plates
The same detail after repair, showing the new single
wall-plate with a stainless steel bracket to the
principal rafter leg to ensure a positive structural
connection: new ashlar pieces (the vertical studs which
rise from the wall plate to support the roof plate) can
be seen to the right of the principal rafter leg
feasible over the flat-roofed transepts, which
have a lattice work plaster ceiling, as a steel
beam could be propped off the transept
wall-heads with temporary steel hangers
attached to each ceiling joist and rafter.
However, replicating this approach on the
pitched chancel roof was unrealistic. Here the
main trusses had to be supported on scaffold
props from below. This meant that the ceiling
had to be punctured, but only through flat
plaster, and on three sides of the chancel the
coving needed to be replaced in any case.
Replacement of the chancel wall-plates
was carried out in sections to ensure that the
roof structure was always connected to the
walls along at least 66 per cent of its length.
As a precautionary measure the decorative
plaster frieze, pendentive cornices and ceilings
were fully supported from an internal bird-
cage scaffold for the duration of the works.
In rebuilding the parapets the opportunity
was taken to replace the delaminated and
eroded dentil course and various cornice