Historic Churches 2017

BCD SPECIAL REPORT ON

HISTORIC CHURCHES

TH ANNUAL EDITION

In some recent cases, bell-cotes or

pinnacles struck by lightning have even

fallen to the ground or through a roof. In

addition to the sudden and catastrophic

damage caused by storms, a constant and

challenging condition is dealing with the

consequences of increased rainfall, with

water penetration through stonework

and the strain on capacity of rainwater

disposal systems.

While the conditions in the west of

Scotland may be among the most severe

in Britain, the solutions and techniques

developed here will be of interest to

churches throughout the UK now

experiencing more frequent extreme

weather events.

As with all conservation work, the

starting point for church roof repairs is

a thorough survey, noting historic and

traditional details, and developing an

understanding of any weak points in the

envelope and its system of rainwater

disposal. When assessing the extent of

repairs required, regular issues are often

found that need not just repair, but more

robust details and solutions to withstand

the changing weather conditions. These

have to be carefully detailed using

traditional materials to be sympathetic

with historic fabric and to ensure long life

expectancy. Here the practical advice and

design details provided by the Lead Sheet

Association are invaluable, as is the advice

given on a rolling programme by Historic

Environment Scotland. The latter’s recent

publication

Short Guide 11 Climate

Change Adaptation for Traditional

Buildings

sets out the correct approach

to take when looking for a more robust

defence against severe weather.

LIGHTNING PROTECTION

It is surprising to find that many of the

churches do not have lightning conductors,

and a specialist assessment for a protection

system is now a routine ‘improvement’

to be considered. Lightning has become

more frequent in the west of Scotland

due to the warmer and wetter conditions.

The location and route of the conductor

cables can be discreetly positioned,

tucked into buttress internal corners,

run along the blind side of a ridge or

inside skews and then down to ground

terminals located appropriately away

from any graves in the church grounds.

FLASHINGS AND MORTAR FILLETS

Major problems often occur on the gable

walls where the up-stand skews are highly

exposed to wind and driven rain. The copes

that perform best have a projecting drip,

but many are eroded or were designed

without the drip. Wind-driven rain can

also penetrate the mortar bedding joints

and this may be improved by re-bedding

the cope stones on to a lead damp-proof

course and securely fixing them through

the lead to the wall-head. Skew abutment

flashings in particular have been failing

under the weather conditions with water

penetration causing damage to church

interiors. Aged leadwork which has had

to be dressed back repeatedly following

storms becomes malleable and brittle

with cracks developing, making it more

vulnerable to wind-lift.

On removing slates and skew flashings

it is common to find that the sarking

boards abutting the skew are rotten,

with a strip of decay up to one metre

wide. When water penetration has been

prolonged, the first rafter may also be

rotten if it is touching or close to the skew

stonework. Replacement sarking should

be formed with treated softwood timber

boards of the same thickness as the

original and preferably butt-jointed. Ply

sheet is not a suitable substitute because

it does not allow the same level of air flow

across the roof and it is difficult to nail

slates to it securely.

If stonework remains saturated

following prolonged periods of rain, damp

is likely to spread, penetrating the core of

the walls and damaging interior plaster.

A traditional detail found where skew

copes have shallow up-stands is a mortar

‘parging’ fillet, which can be badly affected

by cycles of saturation and wind-drying

that produce cracking. If it is not possible

to replace the fillet with a lead flashing, a

more robust alternative is to form a new

fillet over a stainless steel mesh that is

wedged into the skew bed.

Erosion of stone underneath lead

flashings can occur when the stone has

been under saturation conditions and

indents are often required to ensure a

sound base for new leadwork. The risk of

saturation may be reduced by increasing

the roof-cover width of the leadwork to

form a ‘watergate’ (a lead-lined channel)

with the upstand on one side and a

lead roll to the slate edge on the other.

This detail can greatly improve the safe

disposal of water and reduce seepage

and leaks into the sarking. An increase

to the up-stand of the lead flashing

and deeper cover flashing depth may

be considered if there has been stone

repair, but it may be more appropriate

to dress back into the original chase

to avoid weakening or damaging the

skew stonework. Traditionally, the

leadwork would be pointed in with a

lime mortar. However, with increased

Erosion of skew masonry beneath defective flashings,

with flush cope stones above

Rotten sarking caused by a failing skew abutment

A new ‘watergate’ with a lead skew flashing and a

storm roll to prevent storm water spreading into

the sarking