Page 78 - The Building Conservation Directory 2013

7 8 t h e b u i l d i n g c o n s e r vat i o n d i r e c t o r y 2 0 1 3 T w e n t i e t h a N N i v e r s a r y e d i t i o n
3.1
Structure & Fabric : Roofing
exposed locations, although one example is
known from North Wales which may be the
last remaining example here. It can be used
with or without mortar bedding. Each of
these systems has distinctive ways of dealing
with intersections such as valleys.
Before lead was adopted as the solution
to intersections on roofs, slaters used their
skill and ingenuity to devise watertight
details using the slates or stones. The effort
and effectiveness was proportional to the
exposure of the detail so valleys, which carry
more water, were more complex and effective
than ridges and hips, which carry little.
Forms to help with recording are
included in the English Heritage Technical
Advice Note Stone Slate Roofing (see
Recommended Reading) but making written
notes is not easy on a roof ladder and the
best option is voice recording. A methodical
approach is essential. When recording head
and side laps, lath or batten gauges and
specific details about perhaps 50 courses it is
very easy to get lost. A consistent format such
as ‘Head lap three inches, course six over
four’ will avoid errors. Every course should be
photographed, ideally showing three or four
courses raked back. If the recording does go
wrong the photographs will usually save the
day. Besides the arrangement of the slates the
roof pitch should be measured for each slope
and the constructional details at the eaves,
valleys, ridge, hips and abutments examined.
Finally, the type of mortar and the way
in which it has been used should be checked.
Externally, this can be as spot-bedding or
tail-bedding with or without the mortar
carried up the perpendicular joints and
internally as head-bedding or torching
(Diagram 1). It is important to distinguish
between original tail-bedding and pointing
carried out later, often to prevent drafts, and
to check whether there is continuity between
exterior and interior mortar which will
have drawn moisture to the laths. External
grouting and slurrying is almost always a
repair to try to save a failing roof.
Vernacular details
There are two aspects to the vernacular
detailing of roofs: the lapping system
employed and the way intersections are
constructed. Although these are regional
in character they can be similar to roofs in
distant places and may reveal a continuity
of craft skill over hundreds of years and
thousands of miles.
Broadly there are three slate-lapping
systems encountered in Britain: single,
double and triple lap (Figure 6), with some
variations. There are also variations for
single size (tally) and random size slates and
between slates and stone slates. Double lap
tally slating is the most familiar system and
accounts for the majority of roofs and needs
little explanation. All the manufacturers can
supply details for this system.
Single lap exists in the far north of
Scotland and close to quarries elsewhere. It
uses very large slates which wouldn’t have
survived transport over long distances. It has
links with techniques seen in Norway and
Sweden and was also ‘invented’ by Charles
Rawlinson of Lostwithiel, patented in 1772,
promoted by James Wyatt and adopted
for many prominent buildings (including
the Reform Club, London, Charles Barry,
1841, and Penoyre House, Brecon, Anthony
Salvin, 1848) and most notably as a modular
system on cast-iron frame buildings such
as the Church of St Michael in the Hamlet,
Aigburth, Liverpool. Single lapping is also
used for Horsham stone-slating but with
smaller slates.
Triple lap is best known from Devon and
Cornwall where it is principally used in very
Figure 6 In the UK there are three basic lapping
systems and some minor variations. Double lap is
the most common but single and triple are important
vernacular methods and should not be substituted by
double lap when a roof is renewed.
Figure 8 Wrestler ridge removed from a roof in
Pembrokeshire. In this region the top of the slates is
filled with mortar
Figure 9 Crow steps set into a gable wall as a damp
course and to throw water onto the roof away from
the vulnerable slate abutment. This example is in
Mid Wales
Figure 10 Slating which is only carried half-way
onto the gable wall will have always leaked so it is
difficult to understand why it is so common. This
crude attempt to seal the junction is in Mid-Wales.
It is speculated that the slating has replaced a
thatch roof where the raised abutment would have
secured the thatch as in Figure 13. Figure 7 A ‘cussome’ eaves detail
Recording technical details
The survey should record all the materials
used and the constructional details. For
a modestly sized building this can be
accomplished in a day provided there is
not too much variation in the roof slopes.
Access with roof ladders from a parapet,
horizontal valley or a scaffold tower is
normally adequate and it is wise to have a
roofer in attendance to help with handling
slates and to reinstate work without delaying
the survey. It is possible to use a cherry
picker but work is much slower and the lack
of space on the platform can be a problem.
Stone beam filling
Cussome
slate set
under the
last lath
Figure 5 Mortar is used in traditional slating to
improve wind resistance, to help slates to sit well
together without gaps and to prevent draughts. KEY
A, spot bedding; B & C, tail bedding without and
with the perpendicular joints included; D, head
bedding; E, full torching; F, half torching.