Page 139 - The Building Conservation Directory 2013

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3.4
Structure & Fabric : External Works
the water to flow away from the wooden
glazing bars towards the centre of the pane and
increase the rate at which rainwater is shed.
Heating
From the early 18th century decomposing
dung or bark was used to produce
background warmth in hot frames and
pit houses. The sunken houses were
surrounded by pits to receive the dung,
which was periodically renewed.
Heating using stoves or other means
of burning solid fuels dates back to the
17th century. During the Victorian era,
boiler technology advanced from simple
stoves at the start of the century to very
sophisticated segmental cast iron boilers
using pressurised systems by the end of
the century. Coke was preferred as it was
cleaner-burning than coal and therefore
produced fewer smuts on the glasswork.
Edwardian systems relied on thermo-cycling
and pumped systems were not used in
glasshouses until later in the 20th century.
Experimental heating systems using oil
lamps and gas were also produced but these
were comparatively rare and used in smaller
glasshouses. The use of stoves in glasshouses
proved unpopular mainly because the fumes
were detrimental to the plants and the
distribution of heat was very uneven.
Common Problems
Timber decay
As with any timber construction, and
particularly in the warm and damp conditions
encountered in glasshouses, the threat of
decay was always present. This tended to
result from neglect rather than poor design,
manufacturers having made every effort to
minimise the number of exposed joints and to
introduce internal and external weatherings
wherever possible.
Biological decay in timber takes several
forms with wet and dry rot and beetle attack
being the most common.
Wet and dry rot are both caused by
forms of fungi. Their spores germinate on
damp timber and grow into fine threads of
mycelium which spread through the timber,
drawing nutrients and causing it to decay.
Usually the first visible sign of decay is surface
crazing of the timber. The mushroom-body of
a fungi, the sporophore, only appears in the
most advanced stages of decay.
Dry rot (Serpula lacrymans) requires
moisture contents in excess of 28 per cent,
but once established the fungi can remain
active at moisture contents of more than 20
per cent, and it can remain dormant at still
lower levels. More problematically, dry rot
can transport moisture from the source,
enabling it to colonise dry timber further
afield, often travelling between the masonry
plinth and the timber above. This makes
eradication very difficult in a glasshouse,
and the removal of some timbers may
be required. However, the application of
fungicides such as ethylene glycol (common
anti-freeze) is an effective preventative as
well as killing the fungus.
Wet rot is caused by several different
forms of fungi. Generally its spread is not
as rapid as dry rot, and it requires higher
moisture content level (50 per cent and above)
as well as consistently damp conditions. It is
therefore easier to eradicate.
Beetle infestations, such as common
furniture beetle or ‘woodworm’, cause damage
when the larvae bore through timber as they
feed on it. Fungal decay often promotes beetle
infestation because it changes wood into a more
digestible form. Insecticide treatments are
readily available, but ensuring the environment
is low in moisture and humidity and that the
conditions are light, well ventilated and clean
will prevent infestation.
Corrosion
When exposed to air and water, cast iron,
wrought iron and steel corrode in an
Above-ground heating pipes and remote operating mechanism for front ventilators in a vinery
A lean-to conservatory with top-hung roof ventilator and, below, the quadrant operating gear for a 100-foot
roof ventilator
electrochemical process. The result is a layer
of hydrated iron oxide known as rust. Unlike
aluminium and lead which form a passive
oxide layer protecting the metal from further
corrosion, rust is porous, admitting both water
and air, so the metal continues to corrode
beneath the surface.