Repairing Scottish Slate Roofs
|Figure 1 Scottish slate roofs, constructed to traditional patterns and details, have proven to be durable.
||Figure 2 A squared and shouldered Scottish slate
with a single nail hole
Scottish slate roofs have a number
of characteristics which make them
well suited to both the local climate and
the nature of the material produced by slate
quarries in Scotland. These include features
such as diminishing courses, random lengths
and widths of slate, single nailing and laying
onto sarking board rather than battens
When repair work is being carried
out to Scottish slate roofs it is important that
these differences are understood and work
specified accordingly. It is also important to treat the roof
as an integrated whole rather than a series
of distinct elements.
As well as the slate
covering, the sarking board onto which the
slate is fixed, lead flashings, ridges and a range
of other features are all integral to the proper
functioning of a roof. All of these elements
should be considered when repairs are being
specified and carried out.
SCOTTISH SLATE AND SLATING
While some Scottish quarries produced
slates in standard sizes, most produced
slates in random sizes and thicknesses.
The latter were generally intended for
the domestic market although some were
exported. Slates of all sizes and grades,
including those of random width and length
were, on occasion, also imported from
other parts of the United Kingdom which
saw Scotland as a market for this output,
although this was never common practice.
Variations in the size of slates from
Scottish quarries led to the practice of laying
the material in ‘diminishing courses’, ensuring
the most efficient use of the quarry output. This
means that the largest slates are laid at the base
of the roof with the smaller slates laid nearer
the ridge. The slates were fixed in place using a
single nail in the centre of the slate head. Slates
were trimmed at the shoulders of the head to
make it easier to move slates aside to access any
that were broken or damaged. This type of slate
is known as ‘squared and shouldered’ (Figure 2).
The pattern of a Scottish slated roof is dictated
by the size of the slates and although it has
a distinctive character, this varies from one
building to another.
A further difference in Scottish slating
practice is that Scottish slate roofs are almost
always laid onto sarking boards rather than
onto battens. Such boards were laid with a
‘penny gap’ between each board – a gap of
about the thickness of an old penny. The gap
performed the twin function of allowing
for any expansion and contraction within
the wood, and keeping the roof space below
well ventilated and allowing the dispersal of
occasional wind-driven rain from under the
slates, keeping the adjacent timber dry and
free of decay.
SURVEY AND INSPECTION
||Figure 3 Scottish slate has decorative as well as
functional applications, as the treatment of this
||Figure 4 (left) A detailed inspection is key to correct
specification of repairs. Figure 5 (right) New slate should match the characteristics
of Scottish slating practice as shown on this new roof
with its distinctive diminishing courses.
(Photo: WC Cameron Slaters)
As with any building element, regular
inspection of Scottish slate roofs will allow
emerging repair issues to be tackled early
(Figure 4). Repairing small problems as they
arise will help avoid the need for larger, more
costly repairs in the future.
A detailed roof survey should be carried
out prior to any repair work taking place to
ascertain whether localised patching or full
re-roofing is required. This will include taking
photographs to ensure that the pattern of the
roof is maintained in any required work. The
scope of work should then be agreed with a
contractor, allowing for the likelihood that
when work starts it may be discovered that
slightly more slates than first anticipated will
need to be replaced.
A common cause of a need
to re-slate a roof is ‘nail sickness’, the rusting
away of the nails which hold the slates in place.
This might be due to poor quality or simply
age. Depending on the quality and durability of
the slate, decay and softening of the top of the slate can also take place around the nail hole.
As a general rule, if more than 25 per cent of
the slates on a roof are loose or defective then
re-slating will be required (Figure 7).
When considering repairs, adjacent areas
and elements should be investigated, including
lead flashings, lead valleys and related details
as well as masonry work, chimneys and skew
copes (the coping stones that cap a pitched
gable parapet). If there are external signs that a flashing has becoming loose or misaligned,
repairs should be carried out as soon as
Flashing defects can allow significant
amounts of water to penetrate into the fabric
leading to decay of timber and a build-up of
moisture in masonry. An inspection internally
of the underside of the roof will also show
whether there is any water ingress and aid the
identification of problems which may not be
SOURCING SLATE FOR REPAIR WORK
As no Scottish slate has been produced for
over 50 years, sourcing suitable replacements
for use in repair work can be challenging.
While salvaged or reclaimed Scottish slate
may be available in some instances it is likely
that new slate will be required for many jobs
due to the scarcity and cost of reclaimed
When procuring new slate, care
should be taken to match as far as possible the
dimensions, thickness, texture and colour of
the original slates (Figures 5 and 6). It is also
important to replicate the original pattern
as far as possible, using the same number of
courses, sidelap and treatment to elements
such as skews and dormers.
Several producers offer a range of sizes to
allow the replication of a Scottish slate roof
with its characteristic diminishing courses.
However, mechanised production means that
the range will not be a full mixture of random
widths and lengths.
|Figure 6 (left) In this case a good match has not been achieved. Figure 7 (right) Where a roof has suffered the loss of many slates and is showing signs of further deterioration,
re-slating is likely to be required.
SMALL SCALE REPAIRS TO SLATE ROOFS
Roof access and forming a working
platform should be considered carefully
to ensure the safety of the operative and
to avoid damaging adjacent slates. The
replacement of single slates in a Scottish
slate roof is a relatively easy task as they
are normally single nailed (Figure 8). The
broken off head of the slate is removed with
a slate ripper (Figure 9) and a new slate is
dressed to size and nailed in its place.
The issues outlined above regarding
the sourcing of slate for repair work are as
relevant for small patch repairs as for larger
scale re-slating work.
RE-SLATING SCOTTISH ROOFS
Assessing whether a slate roof needs
repair or complete replacement has to be
made based on the amount of repair, the
condition of the slates and other plans for
the building. In some cases the existing
sarking boards may be in sufficiently
good condition to allow the new slates to
be fastened onto the existing boards.
If the sarking boards are damaged and
require renewal they should be re-laid with a
small gap to allow for ventilation. The use of
bituminous under-slate felt or other vapour
barriers under the slates which prevent water
vapour movement can lead to high humidity
in the roof space and can have detrimental
effects on the roof in the long term.
membrane or vapour control layer is being
specified it should be of a type which allows
moisture to move through the sarking board
(Figure 10). Modern proprietary roofing
papers are designed for this purpose, and
if correctly specified remove the need for
obtrusive roof vents on the pitches.
|Figure 8 Individual missing slates can be patch-repaired
fairly easily using single nailing. The double
lap characteristic of Scottish roofs is also evident.
||Figure 9 Removing the head of a broken slate using a
|Figure 10 These sarking boards are in sufficiently
good condition for replacement slates to be laid onto
them with the addition of a breathable membrane.
(Photo: WD Cameron Slaters)
||Figure 11 Lead detail at a gable parapet
(or ‘skew’) installed during re-slating work
(Photo: WD Cameron Slaters)
In Scottish practice, slates are double
lapped so that each slate covers part of the
slates in two courses below (Figure 8). The
head and side lap must always be large enough
to prevent wind driven rain penetrating
underneath the slate.
The head lap or ‘cover’
is the distance by which the leading (bottom)
edge of a slate overlaps the nail hole of the slate
two courses below and is usually 50-75mm, decreasing to under 50mm at the top of the roof
to allow the smaller slates to lie properly.
The varying widths of slate mean that an
even side lap cannot always be maintained.
(The side lap is the distance between the
edge of a slate and the edge of the slate which
it partially covers in the course below.)
Generally the side lap is worked out by placing
the perpendicular joint between two slates in
a course approximately centrally to the slate in
the course below.
In practice, this is difficult
to maintain with slates of varying sizes so it
is usually assumed that the side lap will be at
least 50mm. One and a half slates are often
used on the edges of the slopes, while narrow
bachelor or ‘in-bands’ slates are used mid-row
to regulate the side lap.
Slates should be single nailed as in
traditional practice. Fixing with non-ferrous
nails is likely to prove more durable in
the long term and copper nails are often
specified. Traditionally it was usual to
‘cheek nail’ (or side nail) every sixth course
to help keep slates in place and T-shaped
nails were manufactured specifically for
this purpose. In exposed areas or on turrets
this can be increased to every three courses.
This practice is likely to be beneficial
in repair work. In some cases slates in
exposed areas are bedded in mortar to keep
them in place, particularly when working
with smaller sizes nearer the ridge.
REPAIR OF LEADWORK ON
SCOTTISH SLATE ROOFS
The correct repair of leadwork is a subject
in its own right but it in the context of slate
roofs it is an important part of the works
and should always be done at the same
time. As mentioned, lead is often used in
situations such as valleys and at points where
masonry such as parapets and chimneys
meets slate roofs. Where such leadwork
is being repaired it is important that it is
correctly detailed and secured and that lead
of a sufficient thickness (or ‘code’) is used
to ensure a durable repair. For example,
Code 7 lead is recommended for valleys.
Relevant trade association guidance
should be consulted and Scottish practice
in this area followed when repairs are being
carried out. On many traditional roofs mortar
skews were used, and while their replacement
with lead is likely to result in a more durable
repair, the change in material, and the
angles in which the slates sit will change the
appearance of the roof (Figure 11).
||Figure 12 New slate should match the characteristics of Scottish slating practice
as shown on this new roof. (Photo: WD Cameron Slaters)
The ridges of slate roofs are an area of
vulnerability and are treated in a number of
ways. Lead is often employed along ridges,
applied over a timber former called a ridge
roll. Where cost is an issue zinc ridge of
standard length can be used. In some cases
ridges are formed of terracotta or stone. Any
repair or replacement should be carried out
using like-for-like materials to ensure that
the visual integrity of the roof is maintained.
Whatever material is used it is important to
ensure a firm fixing or bed to aid the securing
of the smaller slates at the top of the roof.
WHY DOES IT MATTER?
As with all natural building materials, the use of slate changes
regionally, according to the properties and characteristics of the
material, according to climate, and according to fashion. Often
seemingly minor variations in construction technique or material have
a disproportionate impact on colour, texture and pattern, shaping the
appearance of a building or structure, and contributing to distinct
regional styles of architecture.
Slate roofs are an integral part of our heritage, and a detailed
understanding of the material and its traditional use is a prerequisite of
successful and sympathetic repair work. Both the replacement material
and the pattern in which it is laid must match the original as far as
possible to preserve not only the roof’s character but also its function.
F Bennett and A Pinion, Roof Slating and Tiling, Donhead,
G Emerton, The Pattern of Scottish Roofing, Historic Scotland, 2000
English Heritage, Practical Building Conservation: Roofing, Ashgate,
English Heritage, Stone Slate Roofing, Technical Advice Note, 2005
T Hughes, ‘Detailing and Conservation of Vernacular Slate and
Stone Roofs’, The Building Conservation Directory, Cathedral
Communications Limited, Tisbury, 2013
T Hughes, ‘Sourcing Roofing Slates’,
The Building Conservation Directory, Cathedral Communications
Limited, Tisbury, 2009