Q. What are true
slates, how have they been
used for roofing?
A. True
slates are derived from
hard, impervious metamorphic
rock that can be split into
thin layers.
Slates and slating
techniques vary. Most true
slates are uneven,
irregularly sized and laid
in courses that diminish
towards the ridge. The main
exception is Welsh slates
from the 19th
century onwards, which are
smooth and used in regular
sizes that give uniform
courses. Their usage spread
widely with the advent of
railways.
Q. What form does
deterioration usually take?
A. Fixings
and their supporting timbers
usually deteriorate before
slates. Over time, the
corrosion of nails
(‘improper nails’) may cause
slates to slip or split, and
battens or pegs may decay
due to insect or fungal
attack.
Other causes of
deterioration include: the
delaminating of poor quality
slates, whereby they split
into layers; mechanical
damage (such as wind
uplift); bad practice would
be not doing a knee test per
slate prior to installation
insuring no slate line hair
fractures are present in the
slate.
Q. Should I repair
or re-slate my roof?
A. This
depends on the extent of
deterioration. Consideration
should normally be given to
re-slating when repairs are
no longer cost effective.
This is typically after
one-fifth of the slates have
been renewed. The condition
of gutters and ancillary
items such as flashings will
also influence the course of
action.
Repair or re-slating should
be entrusted to a reputable
roofer.
Q. What might repair
entail?
A. Isolated
slipped, broken or missing
slates require re-securing
or replacing using copper
wire (‘tingles’) or
equivalent proprietary
fixings. Where a patch of
slates is re-fixed, as above
a partially-renewed batten,
most can be secured with
nails instead of tingles.
Old slates should not be
reversed. Any broken slates
can be stored for
re-dressing and re-use
elsewhere on the roof in the
future.
Q. How should I
introduce new slates?
A. New
slates should usually match
the type, color, texture,
size and thickness of the
existing ones. Although many
quarries have closed,
natural slate is still
available, and substitute
artificial materials are
considered inappropriate.
When completely re-covering
a roof, the maximum number
of old slates should be
re-used. Care should,
therefore, be taken to avoid
breakage whilst removing
nails. Depending on the
circumstances, new slates
can be mixed with old ones
across the roof or reserved
for the less prominent
slopes.
Q. What battens and
fixings are suitable?
A. Where
the appearance of the roof
underside is important,
laths rather than sawn
battens will normally be
appropriate and wooden pegs
and torching should be used
if already existing. In
other situations, the
incorporation of a
'breather' underlay may be
justified, but battens
should be sawn and
additional ventilation must
be allowed for. Slates
should generally be fixed
with broad-shanked copper
nails. Ringed stainless
steel nails are suitable for
fixing battens or laths.
Q. How should
details be treated?
A. The
emphasis should be on
matching details appropriate
for the locality and
building’s age. The
detailing of hips, valleys,
ridges, dormers, eaves and
abutments should therefore
be recorded.
Occasionally it may be
acceptable to introduce
modifications. When renewing
mortar fillets or mitred
hips, for example, it is
often prudent to include
concealed lead soakers as
additional protection
against moisture ingress.
Remember, though, that
changing the details of a
roof may require Listed
Building Consent.
Q.
How do I improve roof
ventilation?
A. The ventilation of
most roofs can be increased
sensitively, where required, but
can require innovation. Most old
roofs do not lend themselves to
the use of proprietary eaves
ventilators, although such
devices may be suitable above
parapet gutters.
Additional venting ideals
would be custom copper louvers.
