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IRONWORK REPAIRS IN TIMBER-FRAMED BUILDINGS.
Why do timber frames
2.5 Decay - Rot -
Beetle Attack .
A significant factor
in timber frame failure is the decay of the stressed timbers resulting in
a loss of their structural integrity. There are essentially two causes of
decay: rot and beetle attack. Rot is usually confined to dry or wet rot.
Dry Rot is one of the
most serious fungal 'diseases' in the built environment and occurs in timber
infected by the causative organism Serpula lacrymans. This affects mostly
softwoods and infestation typically occurs where wood comes into contact with
stone or brickwork. Dry rot has the ability to grow through masonry. Strands
can transport moisture from damp areas enabling the fungus to spread. Damage
characteristics are that the infected wood is dull brown in colour (Fig 2.12)
and deep cracks appear along and across the grain. The timber becomes light
in weight and crumbles easily.
Dry rot is visible in this sole plate in the Wheat Barn at Cressing Temple.
Its cause is both rising damp and an ineffective gutter system which vented
onto the ground. Remedial work included re-routing the drainage and lowering
the exterior ground level.
There are many fungal
species causing wet rot (Cellar Fungus, white rot, brown rot, oyster fungus,
white spore or mine fungus) and remedial treatments are required for all of
them. There may be signs of a darkening of the timber or a bleaching. Certain
types are found only in buildings and only in specific areas. Fungi exist
and grow by feeding off dead wood, causing it to break down, changing its
weight and colour. The most common wet rot, the cellar fungus (Coniophora
puteana), must have very damp conditions, and is most easily recognise by
its dark brown threads spreading over the surface of the timber.
Both types of rot demand
an unusually high water content that might be the result of, for example,
a leaking rain gutter or rising damp. They prefer warm conditions but will
not thrive over 25 degrees centigrade.
Beetle infestation is limited in this country to the Deathwatch (Anobiidae,
especially Xestobium rufovillosum) and the wood boring beetle or wood worm
(the common name for the larval stage of certain wood-boring beetles). Dead
or injured trees are their natural target but they also attack structural
timber and furniture. Included are the furniture beetle Anobium punctatum,
which attacks older timber and the powder-post beetle genus Lyctus, which
attacks newer timber.
All wood boring beetles
digest cellulose and they will eat timber which has more than 10-12% moisture
content. Left unchecked they will completely consume a timber until it collapses.
In the case of rot or
beetle decay, ironwork will be introduced only if the infestation has been
checked at an early stage and there is enough sound timber to anchor the iron
to. Normally in an advanced stage of decay removal and replacement is the
Rot and beetle infestation
can be totally eradicated by drying the timbers out completely and exposing
them to daylight to kill any spores.
Erosion of timber by
constant water action or exposure to high winds normally occurs only in wind
and watermills and even then is rare. Erosion by animal action (Fig. 2.13),
especially rodents is very common and, historically, may have led to metal
plating and strapwork to reinforce the damaged members.
The corner of the porch of the Wheat Barn at Cressing Temple. The timbers
have been exposed to water and animal erosion. The yard contained long-horn
cattle who enjoyed licking the sodden timber. This eroded the woodwork far
quicker than normal.