Page 61 - The Building Conservation Directory 2013

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Building Contractors
bearing, it is essential to increase the junction
between the two. Extending the end of the
timber can be done with side-planting or
splicing-in but the alternatives are many
and varied. The bearing can be extended by
introducing steel or timber bolted under the
beam; by forming a whole box section steel
shoe attached into the beam; by adding a
timber, steel or masonry post under the end of
the beam, down to the ground; or by creating a
timber or steel corbel on the wall beneath the
end of the timber. Similarly, a beam pulling
out of an adjacent beam can be picked up with
a fabricated strap like a joist hanger.
Another place for simple splints like
these is where overloaded purlins have
split. Side-planted steel sections or timbers
bolted across the failure and attached
to sound timber often work well.
Steel and resin options
Where beams are in need of a little more
assistance, flitch plates can be inserted along
part or the whole length of the beam. This
method involves cutting a slot into the timber
and making the beam a composite of steel
and timber. The plate may also take a T-shape
either the right way up (top of the beam) or
upside down (underside of the beam). From
an engineering viewpoint, the steel section
is best let into the underside of the beam and
fixed upside down, so the wide part of the T is
positioned to carry the greatest tensile forces.
From a practical viewpoint, these are difficult
repairs to carry out in situ, requiring overhead
cutting of the slot by multiple drillings or
chain-morticer or chain-saw cutting. The risk
assessment alone takes some serious thought.
Despite its higher cost, stainless steel is
highly suitable for this kind of repair: it offers
higher strength for its size than plain steel,
and it withstands corrosion in oak beams.
(Even in ancient timbers, the tannic acid
present will corrode many metals.)
The downside to using any steel is
that the parts which show may not be as
aesthetically pleasing as with other solutions.
However, with a bit of forethought elegant
solutions are possible (albeit at a slight extra
cost), such as recessing nuts and bolt heads
and plugging the holes with timber, or just
rounding the ends of brackets. Adding a
simple blacksmith-style decoration can
make the repair much more elegant, while
keeping the repair ‘honest’ and its history
clear. Where the ends of fastenings are on
show, square plates can be neatly welded to
the ends of threaded bar or bolt-heads and
rebated into the surface of the timber.
Another method of strengthening timbers
in a similar way is to let in steel bars or carbon
fibre rods. These are usually fixed with an
epoxy resin. The letting-in can involve cutting
slots or drilling holes lengthwise from the end
of the timber or diagonally across cracks.
When the timber to be repaired is
historically significant, a partial replacement
is vital. To preserve the character of the
original timber, one option is to remove the
face as a veneer for later reapplication over a
newly inserted piece. Where there is limited
access or risk of damage to the surrounding
plaster, it is possible to treat a beam which
has been eaten away to within an inch of
its life by building up a laminated resin and
timber beam in situ. The face can then be
consolidated with resin poured into the
beetle flight holes before being reattached
to the beam. Such solutions tend to be very
satisfying, but obviously complex, expensive
and unnecessary for most repairs.
A combination of replacing rotten
or failed timber with new timber and
strengthening with resin bonded steel or
carbon fibre rods can be adopted to good effect
in many situations. Some companies will even
supply pre-shaped repair pieces with the rods
already bonded in place, ready to fix to the
prepared end of a failed timber.
Resins offer a variety of advantages:
minimal loss of fabric, versatility, gap-filling
and the opportunity to carry out certain
repairs which are impossible by other
methods. However, they are not particularly
reversible and can be prone to failure in humid
environments.
Spreading roof structures
Turning our attention to roof structures,
spreading out of the A-frame is often
indicative of inadequate ties or decay at eaves
level. Inserting additional, often lower collars
(tie beams) between the trusses or rafters,
assists greatly in reducing roof-spread, and
this consequently restrains the outward thrust
on walls. Timber is commonly used but an
alternative is steel wires bolted in a substantial,
but sensible way to the timber and tensioned.
Improved rigidity can also be achieved
by securely bolting a gusset made of ply
to a nearby truss or rafter and to a ceiling
joist below. This strengthens the joint and
triangulates the roof. The technique is
especially useful in roof spaces where the
collars are necessarily high for headroom. It
is an effective solution but shouldn’t really be
on show.
Where the junction of trusses and collars
have weakened or slipped, a simple steel
Y-shaped plate bolted in place can stabilise the
structure.
Scarf repair to the base of a post: note the inverted V-shape to ensure water is not channelled into the joint (left) Hoisting a
replacement support beam into place (right)
Typical solutions where a beam end has decayed
Timber scarfed
to existing, wedged
and bolted top to
bottom
Stainless steel
flitch plate let into
beam and bolted
side to side
Timber splints
planted on either
side and bolted
through