Page 79 - The Building Conservation Directory 2013

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 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 7 9
3.1
Structure & Fabric : Roofing
Figure 11 Carrying the slates over the outer face of the
gable and bedding on a stone cope secures the slates
but also avoids leaks.
Figure 12 A thatched verge abutment at St Fagans
National History Museum, Cardiff
Figure 14 The Welsh valley is common throughout
Mid Wales and the Marches in metamorphic slates
and stones. It is lined with courses of the same
material which may be open or more rarely close
mitred as in Figure 15.
There is little variety in eaves
construction. The main variation being how
far they overhang the wall, this being larger
where there are no gutters and water needs to
be cast well away from the wall. With heavy
materials such as Cotswold stone slates this
has necessitated the ‘cussome’ detail (Figure 7)
in which the eaves slates are set under the first
lath to prevent them tilting over.
Ridges were often constructed using
slates rather than ridge tiles or stone. They
either simply overlap from the windward side
onto the leeward side or are interlocked and
known as ‘wrestlers’ (Figure 8). On older hips
the slates are simply mitred together and
bedded in clay, mortar or mastic.
At abutments with walls the technique is
to try to direct water away from the vulnerable
junction. One solution is to build the wall
with stones projecting out over the junction,
either neatly following the line of the roof
or as ‘crow steps’ which cast water onto the
main slope (Figure 9), and by tilting up the
slating by raising the last rafter. The junction
would then be covered with mortar often with
a further line of slates or ‘listings’ bedded
on. Interestingly, on the Pembrokeshire
roof shown in Figure 1 the abutment was
found to have an open channel formed with
slates under the mortar. Abutments with
verge copings apply similar methods. But it
is surprising that some verges were formed
with the outer leaf of the wall carried up
above the slating introducing a vulnerable
joint (Figure 10). Protecting the slates from
wind-lift is obviously a factor but it could
be provided by bedding copings onto slating
carried through to the outer face of the wall
(Figure 11). This may be a detail unaltered
from a previous thatch roof (Figure 12).
Valleys have two basic forms: the slates
or stones are carried around the junction
in their courses, or the slates form a lining
similar to a lead valley (Figures 13 to 20). The
latter can be open- or close-mitred. These are
very prominent on a roof and should always
be re-instated when re-slating.
Vernacular materials
The materials that should be conserved
when repairing roofs include laths, fixings
(pegs or nails), mortar and the slates or
stones slates. Only where there is a good
technical reason should a different material
be adopted. For example, for durability
copper nails are preferred to iron, but there
is no technical reason to replace riven laths
with sawn battens. For health and safety
reasons, 50x25mm battens are accepted as a
safe working platform but riven laths can be
acceptable if sensible precautions are taken.
For a roof that has performed satisfactorily
in the past without an underlay there is
probably no reason to include one when the
roof is reinstated. In particular it should be
remembered that a roof without an underlay
has a better vapour permeability than one
with a vapour permeable membrane.
The greatest difficulty in roof
conservation is obtaining authentic slates or
stone slates. This topic has been discussed
extensively elsewhere and is far too wide-
Figure 13 Slaters used great ingenuity to devise
effective valleys without using lead. This mitred
example in Denbighshire which uses slate ‘soakers’
was described as a common method by Hughes and
North (The Old Cottages of Snowdonia, 1908) but
they have now almost completely disappeared.
Figure 15 A close-mitred Welsh valley in
Denbighshire. The main slates have been removed
in the upper courses to expose the lining slates. This
is thought to be a rare detail but as it can only be
confirmed by opening the valley there may be more
examples yet to be found.
ranging to address here. Suffice it to say,
many slates and stones are still available
(Hughes, 2008; Emerton, 2000) and there
is a protocol for deciding what to use as
a substitute where the original is out of
production (Hughes, 2009). When trying
to source roofing slates or stones, claims of
unavailability made by importers and roofers
should always be checked with the source
quarry. For anything out of the ordinary it is
essential to draw up a product specification
with the quarry. Failure to do so may result