Agriculture Reference
In-Depth Information
In respect of this within same notes on the practice in East Kent, Terence Paul
Smith, in his paper, 'Brick-Tiles in East Kent: an Interim Report', p. 5, states:
Window-heads are sometimes simple, with the tiles taken straight across a flat top.
Many buildings have tiles only on the top storey (of two) and here in the wooden
cornice or facia
(cf. infra)
double as the window-head. In quite a large number of
cases, however, 'flat-arch' windows of apparent gauged work are present: in a few
instances these use purpose-made tiles, which must have been very expensive…'
A good example of a gauged flat arch of brick tiles is 64, High Street, Salisbury,
Wiltshire, which dates from the mid-eighteenth century. The façade is built of
brick tiles laid header bond placed over an early seventeenth-century timber-
framed building.
Though the angle of 60º for the skewbacks remained the ideal, study of Batty
Langley's plate of the two straight gauged arches (1749) (see Fig. 102) reveals
another common bricklayer's practice. That of placing the 'striking point' along
the vertical centre line to the opening at window cill level; regardless of the
height. This gives Langley's fig. I a 70º skewback, whereas his taller fig. II has a
much steeper angle of 76º.
A long accepted method practised by craftsmen bricklayers, for determining a
skewback angle, is the One Third Rule, as explained by Nicholson (1823, 352-3):
The proper method of skewing all camber arches should be one-third of their
height. For instance, if an arch is nine inches high, it should skew three inches;
one of twelve inches, four; one of fifteen inches, five; and so of all the numbers
between those….
Using this method a consistent angle of 70º is achieved for all skewbacks, no
matter what the face height of the arch is.
Two other methods were used that involved setting-off a specified measure-
ment out from a jamb line extended above the springing line a face height of 1 ft
(306 mm) to the extrados. The first measured
1
⁄
8
th of the span of the opening along
the line of the extrados beyond the extended jamb line. So for a 3 ft (915 mm)
opening, this would be 1½ inches (39 mm) per foot, making the measurement for
the top of the skewback 4½ inches (117 mm) also creating an angle of 70°. The
second method measured out 1 inch for every foot of span (or 25 mm per 300 mm),
so for a 3 ft (915 mm) span the measurement at the top of the skewback would
be 3 inches (76 mm) beyond the extended jamb line, creating an angle to 75°.
Acute-angled skewbacks between 30° to 45° are sometimes seen on flat or
camber arches. These angles, however, are not only visually disturbing, but
they also contribute weakness to the arch, particularly on wide spans, reduc-
ing their effectiveness in being strong enough to accept, resist, and discharge
the thrust brought to bear upon it from the loading above. On the majority of
such arches there are frequently cracks to either side of the arch face directly
