Agriculture Reference
In-Depth Information
up from the jambs, due to the stresses created in the haunch or shoulder of
the arch as the unsupported central area of the arch is forced down through
gravity.
Though it is never stated in any craft topics, it has always been accepted best
practice to set the first or 'springing' bricks on either side of the arch from their
skewbacks with the same size putty joint as the rest of the arch. Frequently this
is not attended to, a larger bed joint of front mortar being used instead, seri-
ously detracting from the overall precision and appearance of the gauged arch.
Cutting skewbacks to the correct angle from the sliding bevel, obtained from
the drawing must always be precise. The shaped bricks must then be set into
position with the angle strictly maintained throughout the full length of the
skewback by working to either radial string lines, or using fixed wooden guides
(guns). The skewback bricks must also be perfectly flat across the full depth of
their cut and rubbed surfaces in order to take the fine joints of the springer
bricks. This is why skewbacks are frequently constructed of the same rubbers as
the arch - though their faces are not rubbed and they are laid as standard face-
work in front mortar - as they cut easily and accurately (Fig. 108).
Figure 108
Skewbacks cut precisely
from standard sized
rubbing bricks laid
in standard or 'front
mortar' to facilitate a
sharp cut facilitating a
tight 'putty joint' at the
springing points.
In concluding this examination of gauged camber arches, and concentrating
on constructional faults, Nicholson (1823, 352) highlights a poor craft practice
responsible for a common construction defect:
…the faults alluded to, are the bulging or convexity in which the faces of arches
are often found, after the houses are finished, and sometimes loose in the key or
centre bond. The first of these defects, which appears to be caused by too much
weight, is, in reality, no more than a fault in the practice of rubbing the bricks too
much off on the insides; for it should be a standing maxim (if you expect them
to appear straight under their proper weight) to make them the exact gauge [sic]
on the inside, that they bear upon the front edges; by which means their geo-
metrical bearings are united, and all tend to one centre of gravity.
