SLATE ROOFING A NATIONAL TREASURE IN CRISES. SUSAN MACDONALD
slates have been used in Britain since Roman times and are found wherever
rock could be split to form a reasonably thin slab for roofing, and examples
exist in almost every geological period and rock type.
slates are known in different parts of the country as grey slates, flags,
flagstones, thackstones, slats, flatstones, stonetiles and tilestones.
Geologists prefer the term tilestone as these limestone and sandstone products
are not geologically slates. That is they are sedimentary rather than metamorphic,
and consequently split along a bedding rather than a cleavage plane. Stone
slate is the most commonly used term and has been adopted in our work at
principle areas of the country which sustain stone slate roofing are shown
in the map in the English Heritage Stone Slate Technical Advice Note and
is a reminder of how extensively it was one used. The geology of Britain
is rich and complex and this is reflected in the great variety of building
stones used for roofing across the country. It is possible to very broadly
classify the roofing stones as:
and sandy limestones which are found in Dorset, Oxford, Somerset, Wiltshire,
Gloucestershire, Northamptonshire and North Yorkshire (Brandsby)
and limey sandstones which are found in Sussex, Kent, Northamptonshire,
Cumbria, Northumberland, South Wales and Bristol, Gloucestershire, Cheshire,
Derbyshire, Yorkshire and County Durham, Lancashire, the Welsh Marches,
Shropshire, Herefordshire and Worcestershire.
slating is thus a highly regionalised roofing form, arising from the distinctive
local geology and fundamental to the distinctive local character of vernacular
buildings in many parts of the country.
slates have been discovered on Roman sites at a number of locations around
the country, and have been in use ever since. The major building booms
often talked about in the 16th and 17th century seems to have been an important
period for the expansion of stone slating, with the threat of fire making
it much safer and more durable than thatch and thus it began to be used
more widely for all buildings, not just the large manor houses of the wealthy.
Stone slating continued in many parts of the country as an important local
industry until the late 19th century when we begin to see a steady decline
in its use, as noted by Morris. The fact that stone slating has always
been a modest and local activity, generally utilising materials from the
immediate area means that its use is very different in different parts
of the country, and an important contributor to the development of the
vernacular traditions in each area.
local character and distinctiveness are under threat and sandstone and
limestone roofing is becoming increasingly rare and in some areas is now
defunct. As locally produced products come to the end of their natural
lives, because of declining supplies, imported or artificial substitutes
are taking their place These have little to do with the local architectural
traditions, and are resulting in a gradual erosion of local distinctiveness.
Conservation measures are thus needed if we are to ensure that the special
architectural and historic interest and appearance of our towns, villages
and the surrounding countryside is to be protected against the ugly and
the fake. The strengthening of our own policies is a result of work
we have been doing which aims to revive and enhance the stone slate industry.
catalyst of this cycle of substitution and loss of local character which
began in the 19th century was the development of transport systems which
permitted the importation of cheaper, alien products from other regions
of Britain. The extensive transport network which was in place through
the canal and railway systems in the mid 19th century facilitated the widespread
use of Welsh slate on a national basis. Today we can see the same process
on an international scale. The consequence has been that the production
of stone slates in some areas ceased years ago and the few examples that
are still available are in a precarious situation - largely dependent on
indirect support through the grant aiding heritage bodies for their continued
forces responsible for the continuous decline of the industry as noted
by Morris in the 19th century, which is resulting in the loss of our stone
slate roofing traditions, are closely intertwined. I would like to run
through them now as an introduction to the workshop sessions where they
will discussed in more detail.
of the important factors which affected the character of the stone slates
was the manner in which they were extracted, which was in turn a result
of the local topography. They were either mined or quarried, but in both
cases the quarries tended to be small in scale and often consisted of a
small hole in the ground next to the building they were to roof. In Sussex
for instance many of the duck ponds adjacent to old farm houses were the
quarries for the Horsham stones found on the roof. Quarries produced a
mix of products used in historic buildings work: flagging; walling; masonry
as well as roofing.
slates which were mined, such as Collywestons from Northamptonshire and
Stonesfields from the Cotswolds, constitute only a small percentage of
stone slate roofing. Here the rock was quarried through galleries to extract
the suitable stone - known as pendle in the Cotswolds or log in Collyweston.
This was then either stored underground or taken to the surface where
it was wetted and covered with earth until the frosts came. These served
to swell the beds of natural moisture within the stone and assisted in
splitting it into its fissile layers. Frost split slates which use unweathered
rock, tend to be thinner and therefore lighter than the quarried versions.
Most stone slates were quarried rather than mined. The consists of simply
removing the overburden and extracting the weathered fissile rock.
one of the difficulties we have is that the quarrying industry is culturally
split between those producing very high volumes of aggregate which involves
crushing the rock and those carefully extracting stone for architectural
products. The aggregate producers see it as a hassle to bother with the
more careful and hand crafted requirements of stone slate. Those
producing architectural products on the other hand are fixated on the use
of the saw which again is not what we want for our stone slates. Thus in
modern production there is little room for including stone slates within
these peoples lists of products.
as we have seen, stone slates were originally only one of the local stone
products which make up the building stock of an area. Thus there is a mixture
of stone products which will be required for the maintenance and repair
of that area's buildings. Walling stone, kerbing, paving and stone slates
are likely to all be required and it seems unfortunate that many quarriers
are unprepared to provide a more comprehensive range of products.
with these difficulties, environmental pressures were discouraging the
reopening of existing or new quarries. This conflicted with the planning
policies which included conservation measures to protect stone slate roofs
but which required replacement materials to match the originals. In fact
as we have seen, the demand for stone slates is small, and can be satisfied
from existing or former quarries, and when the issue was discussed specifically
with mineral planners they agreed that there was a need and a general acceptance
for the idea to reopen stone slate quarries to fulfil the historic building
requirements within the region. So there is clearly a need to improve the
planning framework that controls the production of stone slates and this
is a subject we will be discussing throughout the day.
the rock had been obtained either by mining or quarrying it was then split
into layers of a suitable thickness for roofing. Recently products have
begun to be produced by sawing the slates to thickness. These slates may
be technically unacceptable, as it cannot be guaranteed that they are
sawn along the bedding plane. If the bedding cuts obliquely through the
slate thickness weathering will occur along the bedding and the slate will
splitting to the required thickness the stone was dressed to provide a
stone slate. The fixing or peg hole was then made for the timber pegs which
were most commonly used to hold the slate in place on the roof.
as nailing is the usual fixing method the holes may be drilled, however
attempts at the mechanisation of the trimming and dressing should be avoided
as it produces a square edge to the slates which is aesthetically unacceptable.
In addition, in some areas the slates were dressed at an angle which
meant that they sat neatly over, rather that butted each other.
production of stone slates is very much a hand process and it is difficult
to introduce mechanisation and reduce production costs much more without
compromising the product.
many parts of the country the production of stone slates has declined to
the point where they are simply no longer available. The availability
of cheaper mass produced products such as Welsh slates and more recently
imported slates, pantiles, plain tiles and since the 1950s concrete and
asbestos roof coverings has provided fierce competition for the small scale
hand driven stone slate producers.
growth of the salvage industry to service the heritage market is an additional
problem. The reuse of existing historic materials is something to be commended,
however the result has been that the salvage market is able to provide
the material far cheaper than the producers of new stone slates, which
has subsequently encouraged a sometimes unscrupulous salvage industry which
in some cases has resulted in theft of stone slates from roofs all over
market for new stone slates which has always been small and localised has
thus been further reduced by the availability of the salvaged product often
unnecessarily removed from buildings and thus contributing in any case
to this loss of local character.
increasing number and cheapness of the substitute product, particularly
with the increased competition from imported versions is another threat
to buildings where the roof is unprotected by listing. Recently imported
stone slates have begun to be available in England at lower prices than
the local alternatives - a recent example being considered for importation
for Derbyshire from India is able to be produced for 4p a slate a price
we cannot possibly compete with here. Some of these are in themselves good
materials such as the French limestone slates used at Woodchester Mansion,
but their use is extremely difficult to legislate against, and again serves
to reduce the market for local products.
stone roof is one of the most expensive to produce and install. However,
it is important that we begin to put these initial costs in perspective.
Stone slates are a high quality, long lasting material. When examined over
the potential lifespan of the roof, and considering that even when the
building is reroofed a hundred or more years later, and even then the stone
slates can probably be reused, they prove to be an economic choice. There
is also the sustainability argument which is now integral with most local
development plans. Stone slates are the most sustainable of all roof materials
- they require low energy to produce and install, are long lasting and
potential for reducing costs by increasing and retaining the market for
new stone slates to a sustainable level is clearly a key issue.
are however other production issues which relate to the local variety of
the vernacular material and the way in which they were fashioned into slates
which require discussion and for which producers need a clear lead. In
the South Pennines for instance where we initiated our study - we identified
a number of different stone types which varied markedly in character. All
are needed to achieve conservation objectives. At Langley Chapel in Shropshire
there are two different materials on the roof, both from the same quarry-
one a sandstone - identifiable by the green lichen, and the other a calcareous
sandstone which supports the white lichen growth.
was the roofer’s skill and style of working that added the final dimension
which created the local and regional character of stone roofs. Distinctiveness
derived from the colour and texture of the rock; the size, thickness, flatness
and surface texture of the slates and the treatment of hips and valleys,
ridges, dormers, eaves and abutments; all intended primarily to resist
the weather but allowing the expression of the craftsperson’s artistry
and made possible by the particular characteristics of the rock. In Yorkshire,
for example, the large, gritstone or sandstone slates result in simple
roof shapes and detailing, whilst on the typical Cotswolds building smaller
limestone slates are used which can be fashioned into swept valleys, and
dormers so characteristic of this part of the country.
size and thickness of the of the material along with the local climatic
conditions also determined the pitch of the roof. The large Pennines slabs
can be used on lower pitches about - 30-35 degrees, whilst smaller stones
require steeper pitches (up to 60 degrees).
is thus a close relationship between the local topography and geology and
the resulting building forms. So if we are to retain this cherished local
distinctiveness it is imperative that adequate mineral and conservation
planning policies are in place to ensure that the local materials and details
are retained. It is important for example that new stone slates should
match the existing as closely as possible in terms of geological type,
colour, texture, and thickness. Existing features of the roof should be
retained, such as the mix of stone slate sizes, the verge, ridge, eaves
and valley details and so on. Of course in many instances the choice will
be limited by the supply problems, but if we are serious about this issue
we need to help the few entrepreneurial people who have responded to our
pleas for local products by ensuring that policies encourage rather than
discourage a market for new stone slates. In any event one should always
be wary of introducing a foreign product into and area which may have very
different climatic conditions and may not be compatible with existing local
gradual decline in available products, and thus in stone slate roofing
has had another knock on effect - that is that there has been a loss
of skills and understanding of the traditional roofing techniques over
the last century and in particularly during the second half. It is not
just the roofers who are responsible for this drop in standards as there
is also a very poor understanding of traditional roofing by specifiers.
Usually there is no reason why a stone slate roof should not last at least
a hundred years, and most early failures are a result of poor execution
rather than a failure of the material itself.
is great confusion between traditional and modern practice and often the
adaptation of traditional practices to meet modern requirements is unsuccessful
and causing early failures. The use of cement mortar to point roofs is
a misuse of what may be a traditional technique but in the wrong material.
Examples exist were bedding and pointing have been used together in an
attempt to recreate what was mistakenly thought to be the historic method,
but which drew moisture into the roof.
for both roofers and specifiers is abysmal - there is none available. There
is little information on stone slate roofing, and this lack of knowledge
and experience is causing a crisis for our historic roofs. There is very
little written guidance available on stone slate roofing. Derbyshire County
Council have now revised their guidance leaflets, which address stone slate
roofing in the South Pennines. The document includes an outline specification
for stone slating in that region. We are also launching at this event a
general guidance leaflet. However, as the methods of roofing vary according
to the available stone types, stone slate roofing is a highly regionalised
activity. We are thus hoping to encourage other local authorities, with
our help, to produce their own local guidance to sit alongside our more
general one. Mike Hill in the Cotswolds is already preparing one and our
colleagues in the Collyweston area are also considering one which covers
local practice in that area.
fact that the decline has gone on for so long and the pressures are so
great, means that funding is now needed to help re-establish the industry
and help counter some of the forces at work. At present funding is limited
to grant aid support from the heritage bodies that helps individual roofs
but is ineffective at a strategic level. So we are in effect starting at
the wrong end with no ability to help the industry or affect improvements
in the specification and workmanship of stone slate roofing.
is needed to revive, protect, and enhance stone slate roofing is a multi-faceted
approach. Firstly we need a consortium of people with the skills such as
architects and surveyors, producers, and roofers with the interest to pursue
the issues. Our own policies and those of the local planning authorities
in terms of mineral planning, environmental policies and conservation legislation
and grant aid need at the same time to support the industry and protect
the buildings. As it is a regional issue it really must have direct and
proactive involvement at a local level. This is essential to assist the
market to reach a sustainable level, and to survive against the competition.
Increasing knowledge through training, and guidance is essential. Awareness
raising is important but success is dependent on all of those involved.