Page 137 - The Building Conservation Directory 2013

t w e n t i e t h a n n i v e r s a r y e d i t i o n 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 3 1 3 7
3.4
Structure & Fabric : External Works
wall (often the garden wall) and fitted with
full-length front and ridge ventilation
• three-quarter span: requiring a
lower back wall than a lean-to and
therefore less brickwork, this design
has the advantage of enabling longer
periods of sunlight penetration
• clear-span house: a free-standing
building with full length ventilation
along both sides and the ridge.
• Small designs include forcing pits, hot
and cold garden frames (many of which
were movable), and glazed shelters
(fixed to the wall with brackets and
projecting above trained fruit trees).
Siting and aspect
Ranges of glasshouses were often sited far
from the main house and encircled by kitchen
garden walls. These high walls were frequently
used to support lean-to and three-quarter
span houses as well as providing walls for
glazed shelters.
The aspect of glasshouses is of paramount
importance and they generally followed these
principles:
• span houses should have their
ridges running north-south
• lean-to and three-quarter span
houses should face south
• ferneries and other houses for shade-
tolerant plants should face north.
The inclination of the roof is also important.
Roof slopes varied from 30-45 degrees
depending on the time of year at which the
most sunlight was required, with the angle
increasing where the requirement was for
greater light penetration earlier and later in
the year. At these times the sun’s rays will
strike more squarely on a steeper pitch as the
sun is closer to the horizon.
General principles
Victorian glasshouse manufacturers aimed
to produce durable structures that would
give maximum infiltration of sunlight with a
minimum of shadowing, good ventilation and
heating, and minimal maintenance. With the
exception of heating technology, the general
design principles were set by the mid 1800s
and changed very little thereafter.
Most glasshouses used frames that
incorporated both wood and metal, with only
a small proportion being entirely cast and
wrought iron due to the high initial cost and
the more intensive maintenance requirements.
Exclusively iron-framed glasshouses tended
to be used for curvilinear work because this
is difficult to produce in wood. Metal houses
were also difficult to seal making them cold
and difficult to fumigate.
As cast and wrought iron became
more readily available, they were combined
with wood to produce rigid, lightweight
constructions. For example, the use of cast
iron truss brackets and wrought iron ties
enabled far shallower sections of timber
to be employed which reduced shadowing
and painting requirements. Surprisingly,
small sections of timber also proved to be
more durable than larger sections in humid
conditions as the deeper sections retain more
moisture making them prone to decay.
The increasing use of iron simplified
glasshouse design because cheap but attractive
multi-functional components, which were
often surface mounted, could be used.
A fine example was the muntin produced by
Messenger & Co Limited which combined
a muntin with additional functions which
included a projection to receive hinges for the
front ventilators, together with mountings for
their operating mechanisms, support for the
eaves plate and rafter, a shoe to receive a roof-
tensioning rod and carriers for vine strainers.
Wherever possible, simple designs were
employed such as cup hinges for ventilators
(below right) which were virtually maintenance
free and reasonably resistant to corrosion.
Brickwork
A sound foundation was essential, especially
where stable high walls were required for
An original Foster & Pearson clear-span cold frame at the Lost Gardens of Heligan, and, right, a new example of the same design with beaver tail glass. The lever at the
end operates the ridge ventilator