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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 5
T W E N T Y S E C O N D E D I T I O N
3.3
STRUCTURE & FABR I C :
ME TAL ,
WOOD & GLASS
heartwood is more resistant to rot and beetle
infestation. When kept dry, English oak has
a longevity that no other native hardwood
species enjoys.
Historically, there has always been a
demand for English oak for construction and
periodically intense demand made it a scarce
commodity, particularly when demand for
timber for a major project removed all local
supplies. The supply of English oak for the
construction of the roof of one of the great
cathedrals during the middle-ages would have
placed a heavy demand on native resources.
In these circumstances, when home-
grown supplies were hard to come by, there is
evidence that local materials would have been
supplemented with oak from abroad. Examples
of imported oak timbers have been discovered
at Ely and York cathedrals alongside English
oak, and English merchants are recorded as
visiting and trading at the southern Baltic
ports of Elbing (now Elbląg, Poland), Danzig
(Gdansk, Poland) and Stralsund (Germany) as
early as 1350. By the 15th century Danzig was
one of the most important ports exporting
timber to England.
USE OF OAK
Much of the oak used in the timber frame
construction of dwellings was green oak. When
an oak tree is first felled moisture content
commonly accounts for half of its weight. The
dry weight of oak is constant at 650–800 kg per
cubic metre (40–50 lb per cubic foot) but it is
not unusual for a freshly felled oak to contain
the same weight again in water. In this case the
timber would be described as having a moisture
content of 100 per cent. (Moisture content is
measured by calculating the weight of the water
trapped in the cells of the wood as a percentage
of its dry weight, so wood can have a moisture
content in excess of 100 per cent.)
With such large quantities of water
trapped in the wood fibres it would have been
impossible to fully season the oak to a dry state
in the large sections required for use as beams,
girders, and posts, before the construction of
a timber frame building took place. Indeed,
the framing of many of the timber buildings
that survive today in East Anglia was cut and
fashioned with green oak. The carpenter would
have been very familiar with the benefits of
converting and working oak when it is green
(unseasoned) as it is much easier to cut and
fashion in this state. Oak continues to harden
as it dries and as part of the drying process it
will move and twist, tightening up the tenon in
the mortise joint and giving more strength to
the structure.
The size of the timber available was a
factor that influenced the scale of the building
or hall, in particular the width, which would
be governed by the length of timber required
for the tie beams. The height of the rooms
would also be influenced by the length of the
bole of oak for conversion to the required post
sizes. In the search for a large-section oak
with dimensions above 12 inches square for a
principal jowl post, the village carpenter had to
use a field oak.
Unlike the oak from managed woodland,
field oaks would have been much older as much
of its early growth was lateral, and the timber
would not be as straight. The timber was
converted by cutting or splitting the oak trunk
through the middle, lengthways, and upending
the timber to display the thickened end that
was originally towards the ground. The thick
section (or jowl) could then accommodate the
tenons of the tie beam and the wall plate. Each
tree chosen for this purpose would supply
a pair of jowl posts with enough length to
accommodate most secular dwellings.
THE DECLINE OF OAK AND THE
RISE OF SOFTWOODS
In the 17th century, the consumption of oak
for the Elizabethan housing boom, for fuel and
for naval ship-building placed even greater
demands on the native timber resource. The
absence of a reforestation policy compounded
Morden and Lea’s 1682 map of central London shows no timber wharfs on the banks of the Thames.
In contrast, John Rocque’s ‘plan of the cities of London and Westminster, and borough of Southwark’, dated
1746, shows the southern bank of the river lined with timber yards, timber merchants and framing yards, built
to supply England’s burgeoning demand for building timber.
Coppiced trees which have been cut back to the ‘stool’
then allowed to regenerate and, in the left foreground,
a ‘standard’ which has been left to grow on