Page 22 - Historic Churches 2013

22 BCD Special Report on Historic Churches 20th annual edition CATHEDRALC O M M U N I C A T I O N S
INITIAL SURVEYS
A measured survey was carried out of the
chancel, transepts and crossing at roof level to
provide detailed plan and section information.
The parapet elevations were photographically
recorded and rectified images were produced
to provide a detailed record of the structure
and to assist with specifying the repairs.
Bat survey
An ecologist was appointed because there
was a high likelihood of bats being present
and their habitat would be disturbed by the
proposed works. Ironically, it transpired
that the roof space had been free from bats
but while the roof was left open during
inspection work (beneath a temporary roof)
three lesser horseshoe bats moved in. As a
result, a European Protected Species licence
was required and a temporary screen had to
be erected to provide a bat sanctuary for the
duration of the work. A bat access slot also
had to be provided to compensate for the loss
of access once the slate roof was reinstated.
Parapet
The structural engineer’s initial investigation
of the parapet included dismantling part of
the transept wall down to cornice level. The
investigation established that the 250mm wide x
75mm deep oak wall-plates to the roof timbers
had been built into the wall below the parapet
level and were heavily decayed. A second
decayed timber bulk was found built into the
wall at the level of the ceiling joists and internal
cornice. The parapet was also partially built off
poor fill between outer and inner wall faces.
These conditions were causing the tilt
in the wall. Measuring the verticality of the
wall showed that, in most cases, the centre
of gravity was close to or extended outside
the middle third of the wall. The structure
was consequently deemed unstable. The
structural engineer was also concerned
that the wall-plate would continue to decay
and the wall would continue to move.
Roof timbers and ceiling support
A survey of insect damage to the accessible
roof timbers was commissioned to devise
a management plan for current beetle
activity. When wood-boring insect flight
holes are discovered in structural timber
it is often assumed that the remainder of
the timber will succumb to attack. Without
chemical modification of the timber by
fungal decay, however, only the sapwood
is susceptible to insect attack at moisture
contents over approximately 16 per cent.
The heartwood remains impervious to
attack due to the various chemicals it
contains and that confer its durability.
The visible damage in the roof timbers was
confined to the sapwood edges of the historic
oak. The structural significance of the damage
therefore depended on the proportion of
sapwood in the timber section. There was clearly
a history of perceived problems with insect
infestations as many already eroded sapwood
edges had been treated with insecticidal paste.
Historic powderpost beetle attack
(which tends to occur in the first 25 years
after construction) was recorded in the
sapwood of a significant amount of timber.
The insects produce large quantities of bore
dust that is not compressed into frass pellets,
and the resulting increase in volume causes
the surface of the timber to rupture. The
damage is often mistaken for furniture beetle
infestation because both produce small round
emergence holes, but powderpost damage is
quite distinctive, especially when the absence
of pellets is demonstrated with a hand lens.
The survey for beetles strongly suggested
that a widespread deathwatch population no
longer existed although there could be residual
populations in the lower wall-plates where
moisture levels were higher. The roof was
generally dry and where timbers remain dry
deathwatch beetle populations will not become
established. Replacing the damp wall-plates
(using oak with the lowest possible proportion
of sapwood) would therefore remove the main
beetle populations while natural predators such
as bats and spiders would control any remaining
beetles. It was decided that chemical treatment
of the timbers was not appropriate or necessary.
Thermal imaging was used to survey
the pendant arch and beam above the south
transept opening where cracks and deformation
in the plasterwork were evident. The survey
was reasonably successful in determining the
lay-out of a timber ‘goalpost’ structure with
two intermediate hanging posts and associated
framing to create the pendants. However,
localised opening up of the plain plaster was
still necessary to check beam ends where built
into damp masonry, and the junctions between
the hanging structures and the main crossbar.
Micro-drilling of the main oak timber
beam ends over the south transept arch
established that the cross-sectional area of
heartwood was sufficient for bearing and
shear, despite the considerable degree of
beetle attack to the outer sapwood. The
proposed strengthening of the beam ends
with steel splices was therefore unnecessary.
The south transept pendant arch framing
was found to be a mixture of oak, lime,
sweet chestnut, beech and poplar of such
variable quality that some elements had to
be replaced due to beetle attack. Iron fixing
cramps to the goalpost structure had pulled
out of the wall and the hanging framework
forming the pendants had mortice and tenon
joints to the goalpost crossbar with defective
pegs. To ensure transference of loads, these
joints required mechanical fixings, as did the
goalposts to the external walls. More extensive
opening up to fully inspect all timbers and
joints within the hanging pendant arches
was considered. It was a difficult decision to
make because of the intrinsic importance of
the decorative plaster elements and because
some of the lath was weakened. Lath and
plaster can provide significant strength and
stiffness to a structure and it was decided,
based on observations during the limited
opening up, that more extensive opening
up might do more harm than good.
The findings in the south transept were
provisionally used as a guide to the condition of
the other two pendant arches. However, once
work had commenced on site it was discovered
that various parts of the south pendant arch and
the beam above had been previously opened
up and repaired, and that the north pendant
arch had been completely reconstructed,
possibly 40 years ago, in steel angle frame
The north transept roof prior to commencement of
work: note the parapet’s pronounced inward lean
The finished work viewed from the tower showing the reconstructed parapets around the nave, transepts and
chancel, new leadwork to the transept roofs with improved ventilation, and a fall protection system fixed around
the roof to aid maintenance