Page 77 - The Building Conservation Directory 2013

t w e n t i e t h a n n i v e r s a r y e d i t i o n t h e b u i l d i n g c o n s e r vat i o n d i r e c t o r y 2 0 1 3 7 7
3.1
Structure & Fabric : Roofing Detailing and Conservation
of Vernacular Slate and
Stone Roofs
Terry Hughes
Presented with a roof like the one
shown in Figure 1, owners and contractors
often decide to strip it and dump it
in the nearest skip. More than just the loss
of a historic roof, this represents a missed
opportunity. Buildings like this which have
been neglected or poorly maintained can
provide valuable (and increasingly rare)
information about historic roof construction.
Although we may disparage Turnerising
(covering the roof with mesh and bitumen) as
the ruination of a good roof, and even be a bit
sniffy about grouted roofs (which are covered
with mortar) in spite of their contribution
to landscapes, they are flies in amber: they
preserve the slating for recording and authentic
reconstruction more or less as constructed.
Happily the Pembrokeshire cowshed
roof shown in Figure 1 has benefited from
the care of a conservation-minded owner
and a sympathetic contractor. The outcome
has not only seen the roof conserved
but the process has also advanced our
knowledge of vernacular detailing and
provided training for tomorrow’s slaters.
Recording roofs
Any intervention in a roof should start with an
understanding of the existing roof. This can
be very difficult because the slating may have
deteriorated to the point where the original
construction is completely hidden or where
movement in the structure might mask an
inherent flaw in the construction. It isn’t always
clear whether a lath failure has resulted in
slates slipping, or the slates slipped first causing
the laths to rot.
For these reasons and because he or she
will recognise any special details as they
are exposed, the person doing the work
must thoroughly understand how slating
works (Figure 2). If the roof has deteriorated
substantially, if for example the laths and slates
have moved, the best option may be to try to
establish the original slater’s intention when
the roof was laid rather than what is there now.
Photographs taken perpendicular to the slope
and including a scale and course numbers will
show the present condition. By manipulating
the image it is also possible to work out the
original detail (Figures 3 and 4).
The reason for the survey is probably that
the roof needs attention but it is important
to give the surveyor a clear explanation of
what is required. It is a much bigger job to
produce a detailed description of a roof and
its importance than to decide whether it is
Figure 1 This Pembrokeshire cowshed has been reconstructed with all its original detailing. This was only
possible because it was carefully recorded before dismantling.
Figure 2 To understand an old roof’s construction,
the surveyor must first understand how slating
works. Water penetration is prevented by having
adequate laps between slates but in some traditions
additional precautions such as placing small pieces
of slate or shales over the slates’ heads are used to
keep out water.
Figure 3 Where the slating has become displaced
it is possible to digitally ‘reconstruct’ the original
arrangement. Slates 21, 22, 23, 24 have slipped off
their laths.
Figure 4 Slipped slates digitally repositioned onto
their laths to show the original triple lap construction
– course 21 overlaps slate course 24 etc.
beyond repair and maintenance. In practice, it
is rare for a formal survey to be requested until
the roof is beyond repair and needs stripping
and re-laying, so besides recording the present
condition and the original detailing it will be
vital to decide whether any changes are needed.
Leaving aside the issues of installing
underlays or insulation and ventilation, the key
question is whether the roof was weather-tight
as originally constructed and whether the same
specification will be satisfactory for the future.
This will require experience and judgment on
the part of the surveyor and specifier and was
a good deal easier to determine before our
BS/EN codes of practice began to anticipate
an increase in the frequency and intensity of
deluge rain.