128 / 208
1 2 6
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 5
T W E N T Y S E C O N D E D I T I O N
3.3
STRUCTURE & FABR I C :
ME TAL ,
WOOD & GLASS
Quality of material
Much of our historic
joinery was constructed from wood that was
slow grown. This wood generally has a fine,
close-grained texture and, because much of it
was from old stands, it tends to be fairly clear
of knots and vertically grained, giving it good
durability and stability.
Today, managed softwood plantations
aim to produce timber as quickly and as
economically as possible. This faster grown
timber is not as durable as that from the mature
trees that were more common up to the start
of the 20th century. Much of the modern fast-
grown softwood will be used in construction
once it has been pressure impregnated with
preservatives. Generally this type of timber is
not suited to quality repairs of historic joinery.
The quality and closeness of grain of repair
timber should match that of the original as
closely as possible. This will reduce differential
movement at the junction of old and new wood.
The last remaining virgin forests in the
world are almost all protected, and timber
which matches the quality of that used in the
past can be difficult to find. Nevertheless, it is
still possible to obtain quality material from
legal and sustainable sources (see Defra’s
Timber Procurement Policy, 1 June 2013,
Due to the difficulties of fixing to end-
grain, some popular repair methods should be
avoided. In particular, some literature illustrates
the use of a 45-degree angle cut to join together
two pieces of joinery, such as extending the
stile of a frame. This amounts to almost end-
grain to end-grain fixing and has no life.
Preservative treatment
Due to the
generally less durable nature of redwood (but
not necessarily Douglas fir), the repair timber
may be specified as preservative-treated. If
this is the case, all the sections of wood that
are to be used in the repair would need to be
cut and fitted before being disassembled and
sent for treatment, so that all the surfaces
are properly protected from insect and fungi
invasion. Before fixing, it is essential that
the preservative-carrying fluids have fully
evaporated from the wood allowing it to be
glued. With or without preservative treatment,
when the repair is complete all the wood must
be primed and sealed, especially the end-grain.
Cost
Because the success of the repair
relies on good quality material, it is well worth
paying a little extra for it. Repairs are labour
intensive and when set against the labour
cost the extra expense of high quality repair
material is relatively small.
Longevity
Repairs undertaken with quality
materials and good craft practices become
part of the original structure. There are many
excellent carpenters and joiners who can, once
shown the methods, complete an extensive
repair to these historic elements while drawing
personal satisfaction from the process.
Recommended Reading
RG Albion,
Forest and Sea Power: The Timber
Problem of the Royal Navy 1652–1862
, Harvard
University Press, Cambridge (Mass), 1926, p182
J Bispham,
Wood Focus
, Institute of Wood
Science, issue 4, Spring 2001, pp4–5
HM Colvin (ed),
Building Accounts of King
Henry III
, Clarendon, Oxford, 1971
P Dollinger,
The German Hansa
, Macmillan,
London, 1964, p73
English Heritage,
Practical Building
Conservation: Timber
, Ashgate, Farnham, 2012
H Forrester,
Timber Framed Houses of Essex
,
Regency Press, London, 1976
O Rackham,
The Ancient Woodland of England:
The Woods of South-east Essex
, Rochford
District Council, Rochford, 1984
WG Simpson and CD Litton,
‘Dendrochronology in Cathedrals’
in
The
Archaeology of Cathedrals
, T Tatton-Brown
and J Munby (eds), Oxford University
Committee for Archaeology, no 42, 1996, p197
FH Titmuss,
Commercial Timbers of the World
,
Technical Press, London, 1965
JOSEPH BISPHAM
MSc PhD served a five-
year indentured apprenticeship (1962–1967)
as a carpenter and joiner. He has since built
up a business specialising in the repair and
conservation of historic timber buildings
and runs historic joinery repair courses for
the Society for the Protection of Ancient
Buildings and others. He lectures for a
number of organisations including the
Architectural Association and is a member
of the Institute of Wood Science.
available at
www.gov.uk) through established
timber merchants. Redwood (
Pinus Sylvestris
)
from Sweden, Finland or Russia is a good choice
for most joinery repairs. A good alternative
is Douglas fir (grade no 2, clear or better).
Selection is important and all repair timber
should be chosen and inspected piece by piece.
Moisture content is particularly important. It
is likely that the stocks sourced will have been
kiln dried. At the inspection stage moisture
readings should be taken of the historic joinery
in the zones of sound wood, especially in the
areas where the splicing of new to old is to take
place. The moisture content in the selected
repair timber and in the original wood should
correspond to within one or two per cent to
avoid subsequent movement.
Craft practice
Minimal intervention is a
key principle of good conservation practice.
However, in the case of joinery repairs some of
the sound wood must be removed so that the
new wood can be fixed to a sound material,
otherwise the repair will fail at the joint
between old and new.
Each repair will need to be assessed on its
own merit with regards to quality and the run
of the grain. Close inspection of the original
construction will reveal not only how it was
made but also the quality and species of wood
used. Any new sections of wood used in the
repairs need to be of the same dimensions and
matching profiles as the original joinery.
In joining a new section of wood to the
original, a method of repair should be chosen
so as to remove as little as possible of any
mouldings, and this may mean cutting into
original timber behind in order to preserve
the detail. The success of the repair relies on
joining the two pieces of wood together so that
new and old wood work together as a single
unit. The splice/scarf joint should be cut at
a shallow angle running along the grain of
the wood. The cut gives a larger surface area
with the run of the grain to glue the two faces
together. Wherever possible fixings should be
to the inner faces or sides. If exterior fixings are
required, stainless steel and wooden dowels for
tenons should be used.
Repairing a sash with typical decay at the base: the new section of the stile was spliced to retain as much of the original
as possible. This late 18th-century window was made in pine and retained original hand-blown crown glass panes.
The repaired sash ready for priming and painting