Page 59 - The Building Conservation Directory 2013

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Building Contractors Structural
Timber Repairs
Robin Russell
Timber has been used in building
structures for centuries, whether for
roofs, floor beams and joists, posts and
lintels for windows and doors, or for complete
timber framed buildings, including load-
bearing walls and screen partitions.
The use and form of the timber elements
and signs of how they were shaped are
useful in dating historic buildings because
the type of joints, framework design and
tool technology changed over the centuries.
Whether or not they are visible, timbers are
intrinsic to the historic and archaeological
interest of the building.
Structural timbers may deteriorate
as a result of decay, over-loading,
or as a result of poor design and
alterations carried out in the past.
A common problem is decay brought
about by moisture, often owing to a leak, poor
maintenance or condensation. This allows
fungi (dry rot for example) or wood-boring
insects (such as woodworm and deathwatch
beetle) to colonise the timber and by their
action reduce its strength.
Over-loaded structural members fail
by cracking, bending or crushing. The
over-loading may arise either as a result of
weakening following decay, or because they
were designed poorly or frugally, or because
they were meant to take a different set of loads
than they are currently bearing. For example,
a roof structure designed for thatch or slate
may not be capable of supporting the weight of
heavy tiles.
Repair options
Repairing failed structural timbers is, of
course, not a new practice. For centuries
repairs have been fashioned using carpentry
methods or with blacksmith-made splints,
brackets and ties, and these ancient repairs
certainly add character and help tell the story
of the building.
In more recent times we have also
utilised modern materials such as steel, epoxy
resins, carbon fibre rods and wire rope to
reinforce structures. Building repairs can
also be effected by completely replacing
timbers with new timber or, where used
appropriately and sympathetically, materials
such as steel or reinforced concrete. It may
also be possible to reduce the loads through
the design of secondary structures and
in-fills such as brick panels, or packing-
up under partly decayed timbers.
The relative merits of each form of repair
largely depend on the situation. Although
there is no right or wrong method, there is
always a solution that is most appropriate to
the circumstances presented by the building.
The art is to identify it.
When dealing with historic structures,
the principles which come into play are:
• Structural integrity – ensure the
structural members are capable of
taking the loads they may have to bear
• Minimal intervention – retain the
maximum amount of historic timber
and minimise alteration or introduction
of new elements. Consider the impact
of the repair process on the component
and the structure, including access
issues, the viability of moving the timbers
for repair, and the extent of damage
which might be done to other building
elements in executing the repair
• Reversibility – try to ensure that
alterations and additions can be
undone without harm to the fabric
• Like for like – where possible,
use the same materials and
techniques as previously used.
• Honesty – make the solution honest but
Final tightening of side-planted timbers which were bolted to the ends of joists to re-establish their bearing into the masonry.
The ends were removed following a severe dry rot attack.