Repairing Clay-tiled and Slated Roofs
Hand-cut Welsh slates laid to diminishing
the Church Institute, an Arts &
Crafts building of 1912
by Herbert Luck
North in Llanfairfechan, North Wales
There is evidence of the use of slate,
stone and clay tile roofing materials in
Britain dating back to Roman times.
Although for much of the early medieval
period thatch of one form or another
predominated, by the 12th century the most
important buildings tended to be roofed in
more permanent materials once again. Church
and cathedral architecture in particular
demanded roofing materials of the very highest
quality, and slate, stone and fired clay fixed
to timber battens or sarking boards quickly
emerged as the standard roofing system from
the later Middle Ages to the present day.
Nationally, the appearance of roofs varied
widely, from large stone slabs in the lake
district, to the smaller rough stone-slates of
the Cotswolds, from the fine blue and purple
slates of North Wales, to the green-hued
slates of Edinburgh and Cornwall, and from
the plain clay peg tiles of the South East, to
the pantiles of Lincolnshire and Humberside.
Detailing too was affected by the materials
used as well as by local traditions, until the
advent of the canals and then the railways
brought access to new ranges of materials.
Prior to the mechanisation of Britain’s slate
quarries and the laying of the railway systems
in the early 19th century, natural slate was
generally produced in varying sizes which
provided a ‘random’ slate roof. This meant
that all sizes produced by the quarry in the process of cleaving the rock from the face
were used, eliminating unnecessary wastage.
Roofs were fixed with the largest slate at the
eaves diminishing to the smallest slate at
the ridge. This ensured that the area of the
roof that carried the most water made use
of the slates that provided the most lap.
Once the quarries were furnished with
machines and linked by rail, it became far
more efficient to transport slate in standard
sizes, despite the fact that this produced
much more waste (evidenced by the huge
slate tips we see today). Fixing standardised
slate also saved many man-hours on site and
didn’t require the same level of expertise.
The vast majority of slate produced
around the world today is sized
slate, but it is still possible to source
random slate from UK quarries.
Stone-slates have been used in many
areas across the UK for centuries and were
generally laid in a similar fashion to random
slate. However, these tend to be formed in much
thicker and heavier pieces than true slate to
allow for their relative weakness and porosity.
Double-lap clay tiles were the main alternative
to slate in most areas of the UK. Produced in
the Midlands and the South East in particular,
these tiles were laid in much the same way
as slate, being double lapped. In other words,
because plain tiles do not overlap their
neighbours on either side, there needs to be a
double thickness of tile over every part of the
roof to keep it water-tight. The tiles therefore
overlap not only those in the course below
but also those in the course below that.
Many tiles were produced with ‘nibs’
at the top in order to hook them onto the
fixing battens. This lessened the need for
additional fixings such as nails, and made them
relatively quick to install given their small
size, which was generally around 10" x 6".
Clay tiles are still produced and used on
a large scale throughout the UK, and a few specialist firms can still produce tiles by hand,
using clays and firing techniques to match
the less regular appearance of older tiles.
Single-lap clay pantiles were traditionally
seen all down the eastern coast of the UK.
This is because they were originally produced
in countries such as Holland and Belgium
and were imported across the Channel
and North Sea, tiles being an ideal ballast
material for ships returning from exporting
British goods to the continent. Bridgwater,
Somerset was another centre of production.
Because they overlap neighbouring
tiles in the same course, single-lap pantiles
only need to overlap a single course below
them to keep the roof water-tight.
|Tingles (metal clips) pepper the roof of a church at Llangollen in North Wales,
indicating widespread failure of the nails fixing the slates.
||New handmade nibbed clay tiles being laid across new battens on breathable
felt (Photo: Jamie Moore, Recclesia)
MY CHURCH ROOF IS FAILING: WHAT ARE THE LIKELY CAUSES AND IS IT REPAIRABLE?
Slate, stone and fired clay are all extremely
long-lasting materials, and fixed to modern
standards can last for centuries. However,
one weakness is common to all types of
traditional pitched roofing: fixing failure.
Metal fixing nails are particularly
vulnerable. Once exposed to atmospheric
conditions, corrosion causes ferrous fixings to
fail far more rapidly than the roofing material
itself. This is why slipped slates and ‘tingles’ are
a common sight on old slate roof slopes. Tingles
are narrow strips of lead, copper or other metal
which are nailed to the batten at one end and
hook under the lower edge of the slate at the
other to form a simple, if unsightly, temporary
fix. However, It isn’t just the slate nails that
can fail prematurely, batten nail failure can
lead to whole courses of slate or tiles slipping
down the roof as gravity slowly takes over.
||Two samples of tiles being compared with the weathered originals to
ensure the most suitable match (Photo: Jamie Moore, Recclesia)
Clay nibs and riven oak pegs, although
more durable than galvanised steel nails,
also present a weak point. Clay nibs can
fracture due to thermal stresses in the roof
covering, and oak will eventually decay.
However, after nails it is the failure of metal
flashings that is the most common reason for roof failure on historic buildings. Usually of
lead but sometimes of other materials such
as copper, these flashings were designed with
prolonged exposure in mind, and well-detailed
leadwork can last for centuries. The most
common reasons for failure are not defects
in the material itself but poor design, bad
installation and inadequate maintenance. Roof failure will normally make itself known
where the rainwater is most concentrated such
as the lower portions of valleys, parapet gutters
and outlets. Internally, problems will generally
become evident on the outer walls. It is vital to
investigate the source of these problems at the
earliest opportunity if outbreaks of dry rot and
extensive structural damage are to be avoided.
Repair or replace? In many instances it
will be quite possible to carry out localised
repairs in order to prolong the life of
an entire roof. However, if nail failure is
advanced then a complete re-roof may be
the only answer. If water ingress has gone
undetected or ignored for long periods then
structural work may also be necessary.
Simple maintenance Whether there is
internal evidence of water ingress or not, it
is always prudent to carry out regular visual
inspections of all roof areas and attend to
any minor defects such as the odd slipped
slate or tile, damaged flashing or blocked
rainwater channel. Parapet gutters, valleys
and rainwater channels must be regularly
cleared of leaves, plant growth and even dead
pigeons. Keeping up with simple regular
maintenance in this way can prolong the life
not just of the roof, but the entire building.
REPAIR AND REPLACEMENT OF SLATES AND TILES
Individual slates can be successfully replaced
but it is not usually quite as simple as repairing
clay tiles, which can often be slotted into
place without too much technical know-how.
By far the best solution is to remove all the
slates above the offending area and re-fix in
same manner as they would have been fixed
originally, usually with two clout nails.
|New polychrome clay tiles made by Dreadnought for the chancel of All Saints’, Nocton near Lincoln (by George Gilbert Scott, 1862). Clay which was naturally the
right colour was sourced to match the originals, rather than dyed, and those original tiles found to be still serviceable were reused on the south face of the nave.
(Photo: Dreadnought Works, Hinton, Perry Davenhill)
It may not always be practical to remove
large areas of roof just to replace one or two
slates near the eaves, so a little skill can be
required to make a successful replacement.
Unfortunately, many individual slate repairs have
been carried out in the past using inappropriate
methods such as adhesives and tingles, neither
of which can be considered a permanent fix.
Successful, permanent and invisible slate
repairs can be achieved in a number of ways.
For example, copper straps can be threaded
through pre-made slots in the slate and
fashioned to create a wedged spring, which
is flattened while the slate is being pushed
up into place, then springs out behind the
fixing batten when arriving at the correct
position. Because the slate is not secured
into the batten, as with nailing, it wouldn’t be
appropriate to fix a whole roof in this way but
as an individual repair it is effective because the
surrounding nailed slates will prevent uplift.
There are a number of products on the
market that work in a similar way such as
the ‘Jenny-Twin’ fixing, which consists of a
pair of aluminium clips that clasp the sides
of the slate and are secured by a tongue that
passes through a drilled or punched hole in
the slate and a swivel ‘tail’ that rests on the batten. Installation can be a bit fiddly but
the clips are very effective once in place.
While tingles have had a relatively
useful role in signalling whether a roof needs
replacing, they are only a temporary solution
and need to be replaced by something
more permanent. As long as repairs are
recorded it will still be possible to monitor
the overall condition of the roof and
determine when replacement is necessary.
With double-lap clay tiles, the first indicator
of the end of the roof’s useful life is finding
whole rows of tiles slipping due to corroded
batten nails. These tiles generally have either
‘nibs’ or oak pegs which allow them to hang
onto the batten even if the tile fixing nail has
corroded. Only a complete re-roof should be
considered when this defect is identified as it
will not otherwise be possible to carry out an
effective, lasting repair. As with slates, it may
well be possible to reuse some proportion of
the existing tiles if the aesthetics and historic
interest of the roof are to be preserved.
However, specifiers and clients should be
realistic when considering reusing roofing
materials and should not expect to save more
than 50 per cent in most instances even if the
material appears to be in good condition.
If renewing a roof completely, it is usually
best to replace materials like-for-like in order
to maintain the authenticity of the building.
Fortunately we are blessed in the UK with
a stable supply of traditional materials such
as Welsh slate and clay tiles and therefore
replacing these can be relatively simple
logistically. However, difficulties may be
encountered finding some stone slates, and
Scottish slate is now rare. In such cases it may be necessary to use material of similar geology.
Clay tiles can be made by specialist
manufacturers to match almost any profile,
colour or texture. Some may be available from
standard ranges, but specials are often required
to match regional variations, particularly
where pantiles and romans (flat roofing tiles
with one or more rolls) are concerned.
Beneath the slates and tiles themselves
things can be quite different where a roof
covering has been renewed, as modern fixings
and methods have evolved to last far longer
than their predecessors. Also the introduction
of insulation and ventilation systems to comply
with modern regulations can have a major
effect on detailing. It should be remembered
that it is now mandatory to install insulation
to strict current specifications if replacing
25 per cent or more of any roof area.
MODERN VERSUS TRADITIONAL FIXING METHODS
There is no practical reason to install slates
and tiles using historic fixing methods.
Materials should be fixed according to
current best practice. This shouldn’t mean
that the roof will look any different to the
original, but it will last longer. Laying slates
or tiles using the most appropriate modern
fixings will also allow future generations to
date the work and assess its suitability.
Historic Churches, 2012
JONATHAN GREENOUGH is a Heritage
Roof Master and a Fellow of the Institute of
Roofing with over 27 years’ experience. His
heritage roofing expertise has been called
upon for projects throughout the UK and
beyond, including at Buckingham Palace.
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Communications Limited 2014