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Clay-tiled Roofs
Stephen Boniface and Tony Redman
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| Pantiles on a Georgian coaching inn in Wiltshire |
There is plenty of evidence that the
Romans used clay tiles extensively on their
properties. Although the use of clay tiles
diminished somewhat during the Saxon period,
by the 12th century there are records of clay
tile use being encouraged particularly in place
of thatch for fire safety. The size of tile (10½" x
6½" x ½") was standardised in 1477.
In the early years the use of clay tiles, like
many other building materials, was limited by
cost. Nonetheless, for those who could afford it,
clay tile was often the material of choice.
Another limiting factor was transport. Prior
to the advent of mass transportation systems it
was rare for clay tiles (or any other materials) to
be transported any significant distance, typically
not more than a day’s cart journey. Exceptions
were made for the roofing of churches and the
homes of the very rich, who often had access
to clay fields and kilns further afield, and
employed the labour, which made the costs
much cheaper.
As a result the pattern of clay tile usage
correlates closely with the areas in which clay
and ‘brick earth’ are found, and it is perhaps not
surprising to find that the manufacture of clay
tile from the later medieval period was closely
aligned to that of brick-making.
By the late medieval period a more stable
social, economic and political climate resulted
in an increase in wealth, generally enabling
more people to afford materials such as brick, glass and
indeed clay tiles.
From the 17th century clay tile became the
ubiquitous roofing material for large parts of
the country where the raw material was close
at hand – mainly the southeast and east of
England and the Midlands.
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Modern nib tiles (bright red) and peg tiles hung on
nails are visible in the top picture (although quite a few
have no fixings). Below, the nails used to fix the peg tiles
can be seen more clearly. |
Greater wealth in the 19th century, improved
transportation and the introduction of taxation
on fired building products such as tiles and bricks
to fund the Napoleonic wars led to a reduction
in the use of clay tiles and the increasing use
of other roofing materials, particularly slate.
However, it was the advent of the railway more
than anything else that caused the roof map of
England to change from red to grey. During the
19th century slate tended to be cheaper and thus
it overtook clay tiles as the roof material of choice for the
rapidly developing urban landscape.
During the 20th century mass-production
of machine-made clay tiles resulted in a
resurgence of clay-tiled roofs, particularly
during the inter-war period. However, increase
in competition from man-made tiles such as
concrete tiles and man-made slate resulted once
again in a downturn in the use of natural clay
tiles. In more recent years homeowners have
rediscovered the beauty of the material and
there has been something of a resurgence in the
use of handmade clay tiles.
The tile typically found throughout this
period is the double-lap tile (one where the
overlap between courses of tiles is greater than
the length of a tile) but one should not forget
the single-lap tile where the tiles interlock
at edges only. Although today we are used to
seeing the single lap tile in the form of concrete
roofing materials, the history of single lap tiling
goes back many centuries. The most common
form is what we generically refer to as ‘pantiles’.
These should not be confused with genuine
Roman tiling, which in fact has not reappeared
in any significant manner in this country since
the 4th century AD.
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| Peg tiles cover the roof slopes of this Arts & Crafts house
in North London and hanging tiles cover its gables. |
The use of pantiles is not as widespread as
clay tiling generally and it tended to focus on the
eastern side of the country. Records indicate
that pantiles arrived somewhere around the 17th century, with
home-produced pantiles appearing from
about 1700. Because the tiles were originally
imported, their distribution tends to focus on
the ports of the eastern seaboard. The exception is Bridgewater in Somerset, where pantiles were
certainly established by the late 1750s and
where a prolific pantile-making industry later
emerged, supplying tiles throughout Somerset
and the neighbouring counties.
MANUFACTURE
The manufacture of clay tiles is relatively
straightforward. Traditional handmade tiles are
a mixture of clay with aggregates rolled out and
cut or moulded to simple rectangles (sometimes
shaped) with two holes at one end for fixing.
These are then fired in a kiln. Sometimes the
ends were extended at right angles to form a
nib, but the majority of clay roofing for many
centuries was simply a baked clay rectangle.
Due to the firing, flat tiles would come out
slightly convex and this added to their character.
Uneven temperatures in the kiln and the nature
of the hand-making process also contributed
to variations in shape and form, and the
quality of the clay resulted in rough and
therefore textured surfaces. The colour would
be determined partly by the clay and the mix
of aggregates but also by the temperature and
length of firing in the kiln.
Sometimes shaped tiles were produced
and occasionally glazed tiles and pantiles can
be seen. During the Victorian period there was
much experimentation and occasionally one
comes across multi-coloured examples.
With modern machine-made tiles, dyes are
added to bring greater consistency of colour.
FIXING
Plain clay tiles are laid in regular courses
with each tile lapping two others, leaving
approximately four inches exposed. The precise
method of fixing depends on the nature of the
tile itself. In the case of the more basic form of
tile, simple tapered wooden pegs were pushed
through the two holes at the top of the tile so
that the tile could be hung over battens fixed
horizontally across the tops of the roof rafters.
The tops of the pegs would be trimmed flush to
the surface of the tiles so that the next course
would lie flat.
Lime mortar, sometimes with straw and
other aggregates, would often be applied to
the internal face of the tiles to fill the gaps and
help improve the general fixing of the tile. This
mortar fillet is often referred to as ‘torching’.
On many roofs the pegs would be limited to
only one per tile. Indeed, roofs can often be
seen with no pegs at all, or at least pegs only in
occasional courses. Although this can be due to
the pegs rotting away, sometimes tiles were laid
bedded in lime mortar with no pegs. In such
situations the fixing of the tile relied as much
on friction and the weight of tiles above as on
any torching or mortar bed.
If a tile had been made with nibs these
would be used to hang the tile over the batten,
and pegs would not be required. With modern
tiling the nibs themselves have holes to enable
nail fixing to the battens, although not every
course is nailed in place.
As the use of slate increased, the need for
nails to fix them also increased. The consequent
increase in the production of nails resulted
in their increasing use to fix clay tiles as well:
nailing was quicker and avoided the need to
trim the timber peg before laying the next
course.
Today we find a wide variety of tiles
available to us, including traditional peg tiles
but also handmade tiles with nibs to facilitate
fixing.
TYPICAL DEFECTS AND REPAIRS
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| Mortar fillets are commonly used at the junction
between peg tiles and a chimney (top), but lead
flashings (above) are much more reliable. |
It is often said that clay tiles have a limited
life of up to 60 years or thereabouts. However,
walking around the countryside you will often
come across peg-tiled roofs that are several
hundreds of years old, so this is clearly not a
reliable guide.
The failure of the tile itself will depend on
many different factors, including the original
manufacture, the make-up of the material
within the tile and its firing in the kiln.
Because tiles are much thinner in section
than brick, they are less susceptible to variations
in firing. Nonetheless, there will always be some
tiles that are from the cooler parts of the kiln
and therefore more vulnerable to early failure,
particularly as a result of frost damage. That
said, as a rule handmade tiles tend to have
great durability and, if well-fired, tend not to
be particularly vulnerable to frost damage.
Only after many years will the best examples
eventually weather, exposing the softer and
more porous clay body below to frost damage.
Other factors which can influence the
longevity of tiles (and, in fact, any roof
covering) will be the orientation of a building,
the steepness of the roof and indeed the microclimate around the building. Clay tiles are
best used on roof pitches of 40° but some single
lap tiles can be used down to 25° pitches.
Other more controllable factors include
such matters as tree branches brushing up
against the roof covering and dislodging or
breaking tiles, climbing plants being allowed
to grow over and into tiling to dislodge and
damage it, and clumsy workmen treading on
the tiles.
Due to the rough texture of a clay tile
surface it is likely to harbour lichens and
mosses. These plants should not necessarily be
regarded as harmful. Although lichens produce
acidic secretions and moss can hold moisture
and lead to frost damage, they are unlikely to
cause much damage. Indeed moss can provide
a protective layer and lichens contribute to the
characteristic colouring of tiled roofs. However,
significant moss growth can increase the weight
on the roof structure generally, and when it
dies and rolls into the gutter it can cause quite
serious gutter blockages.
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| One of the most common problems of a tiled roof is slippage due to rusting metal fixings and decaying battens. The
best solution is to re-lay the tiles, salvaging and reusing as many of the originals as possible. The alternatives (see below)
can be disastrous. |
If moss is to be removed, care should
be taken. Simply pulling moss from the roof
surface is more likely to cause damage than
by letting it die naturally or by appropriate
chemical removal means (biocidal treatment).
However, care should be taken with chemical
removal methods to ensure that the chemicals
do not run down to the gutter and into the
surface water system.
The defects that most often affect tiled roof
coverings are in fact the sort of defects that
affect all roof coverings: failure of battens (rot,
woodworm etc); failure of the batten fixings
(nail corrosion); deterioration of the tile fixings
(rotting pegs, corroding nails or crumbling
torching); failure of or defects to the roof frame;
defects to perimeter details (soakers, flashings,
etc); defects to roof details (valleys, verges, eaves,
etc); and wind uplift.
Problems that can affect the tile surfaces,
apart from those rare occasions when moss or
lichen cause damage, are usually brought about
by matters such as pollution, the premature
failure of poor quality tiles, saturation from
leaking pipes or drips from overhanging details
such as TV aerials.
Frost damage can occur where moisture
is retained on the surface and this sometimes
happens at the laps. Sometimes localised frost
damage can cause a tile to break at the head
lap. Machine-made tiles are particularly prone
to frost damage as the surfaces are more even
and regular, enabling moisture to be trapped
on the underside. Handmade tiles on the other
hand have a natural variation which is both less
moisture-retentive and more pleasing to the eye.
Other problems can arise due to poor
laying in the first instance. Such problems
include inappropriate detailing at verges and
hips, poor setting and laying of the ridge and
poor detailing of abutments such as chimneys
and walls. Abutments need particular care. Local
vernacular may dictate the use of tile creasings,
or else mortar fillets, or lead. Lead flashings
are usually the more reliable and mortar fillets
the least. Whichever detail is used, lead soakers
should always be incorporated between each tile
to resist the passage of rainwater horizontally.
Another common failure with clay-tiled
roofs is brought about by the failure of the
fixings or battens due to rot or rusting. The
battens often use sapwood, which is much
more vulnerable to decay than heartwood.
Pressure-treated battens should always be
used for repair and replacement. If care is
taken, many of the tiles themselves can usually
be salvaged and reused. A word of warning,
however: because peg tiles tend not to be
pegged every course and therefore rely on
friction and/or the torching, there is a risk of
mass failure and slippage if a careless roofer
steps onto the roof. Before attempting to repair
a clay tile roof it is important to check the
fixings below in case the attempt at repair itself
causes more damage.
When repairing a tiled roof it is important
to obtain as close a match as possible to the
original in terms of texture and colouring. Non-ferrous fixings should be used to reduce
corrosion risks. Any lime torching should be
continued across new areas of work, and with
the existing torching properly reinstated.
Most roofs can be satisfactorily patch-repaired
rather than having to be completely
stripped and re-covered. However, if complete
re-covering is to take place, every attempt
should be made to salvage the tiles and as
a rule of thumb one would hope to salvage
approximately 70 per cent.
Complete stripping and re-covering
requires the new work to comply with building
regulations, and this would often mean the use
of a lining over the rafters beneath the battens
and tiles. Such linings restrict airflow into the
roof space, and the roof space then has to be
positively ventilated or a modern breathable
lining used.
It should be noted that where there is
a double-lap roof covering, a lining is not strictly necessary for weathering purposes. Homeowners often attempt
to line a roof because they believe it to be appropriate or perhaps to
stop unnecessary draughts. However, attempting to line a roof from the
underside (within the roof space) can lead to a number of problems.
Because the lining is then not laid over the rafters it will direct any
penetrating water into the eaves where it will cause rot and damage.
Careful thought and installation is needed with regard to retrospective
lining and it is best avoided.
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| John & Jane Penoyre, Houses in the Landscape, Faber and Faber, 1978 |
In recent years there has been an increase in the use of expanded
foam applications to the undersides of tiles. These are often marketed as
providing a solution to insulation problems, securing loose tiles in place
and reducing draughts. However, the use of such material should be
viewed warily and it is suggested that such material should be regarded
as a last resort only, particularly for historic buildings. Foam stuck to the
underside of the tiles means the tiles cannot be salvaged for re-use at
a later date. The practice also makes it very difficult to undertake patch
repair in future because of the difficulty in getting individual tiles out.
There is also a possibility of reducing the life of the tiles or slates if they
are a bit porous, as it reduces the evaporative surface area: water absorbed
when it rains will no longer be able to evaporate from the lower surface.
The risk of frost damage is therefore greater.
Spray-on foams also perform poorly as a means of insulating roofs.
The blocking up of ventilation and the lack of a moisture barrier can lead
to condensation problems. Building regulations require a ventilation gap
above insulation or a vapour membrane under the insulation, but with
spray-on foams neither are provided.
From an aesthetic point of view these foams can also be a problem,
as it is often difficult to prevent the foam spilling out between gaps in the
tiles (particularly pantiles). Such foams are therefore a short-term form
of repair that could increase the long-term cost of later work. If tiles are
slipping it is better to undertake a proper repair.
Of course these negatives should always be balanced against the
difficulty of access and perhaps the expected future lifespan of the roof.
If the building is listed, however, such work would require consent and
many conservation officers would probably refuse consent for use of such
products.
Traditional clay tiles create beautiful roof coverings that are full of
character due to the individual nature of the tiles. Provided they are
carefully and properly maintained there is no reason to expect them to
perform poorly. Many of the typical problems found can be resolved
without loss of the tile itself. Before embarking on any work to a roof seek
professional advice on what is required. If altering or extending the roof of a listed building,
ensure the appropriate consent has been obtained.
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