Civil Engineering Reference
In-Depth Information
floor joist bearings and invariably for
rafter bearings (Figure 2.66).
In some parts of the Western
Isles, turf walls were laid just as the
sods came from the fields, usually
grass down.
Separating walls and partitions
may be found constructed in pisé-de-
terre, but partitions were not usually
in cob because they would take up too
much space.
Abutments
Chimney breasts and stacks can be
constructed in their entirety from cob
or from brick and stone.
External corners
External corners built in cob are
occasionally found to be rounded,
and this may well be intentional and
not necessarily the result of exposure.
It has been said that this reduces the
tendency for the wall to crack. It is
not usual to find masonry quoins in
cob houses.
Figure 2.65
Old cob on the left, and new on the right. A house extension nearing completion
treading it down. Surplus material is
then pared down to finish up with a
vertical wall. The waste material is
recycled into the next mix. The first
lift is allowed to dry naturally for a
few days before the next lift is added.
Clay lump and stabilised soil
block walls are laid in a similar
manner to masonry with regular
bonded courses using a clay or fresh
earth mortar.
With wattle and daub walls the
earth is non-loadbearing, the daub
comprising subsoil which is clay
based, well mixed with animal or
vegetable fibre. The daub is laid
against the lattice of sticks using
plastering techniques. Mud and
stud walls are generally thicker, with
the mix beaten between the vertical
laths and completely covering all
the timbers.
Most areas of the UK with large
numbers of earth walled buildings
have local groups of enthusiasts
with a specific interest in reviving the
techniques and encouraging
traditional repair methods
†
.
Reinforcement may be found in the
form of timber planks laid within the
wall thickness. Sometimes chicken
wire was used. Continuous wall
plates are sometimes necessary for
Continuous roof wall plate
Openings and joints
Ancient practice was to saw out
openings for windows and doors after
the cob had dried, depending
naturally on whether stones had been
incorporated into the mix, and it may
still be possible to do this when
alterations are needed. However,
more modern practice, certainly with
pisé-de-terre, is to use formers strong
enough to withstand the ramming,
which are withdrawn when the
material is dry.
450 mm
Continuous
floor wall
plate
Cob
Limewash or lime
render both sides
300 mm
Main performance
requirements and defects
Strength and stability
The strength of a cob wall relies on its
moisture content remaining stable.
Wetting through a roof
failure, or by moisture being held
behind an inappropriate render can
result in a build up of moisture at
the base of the wall. The earth may
then shear and a total collapse may
follow. However the thickness
ensures that loads are low, with
compressive stresses normally being
less than 0.1 N/mm
2
.
Compression tests carried out in
Figure 2.66
Typical section through a cob wall
† The ICOMOS UK Earth Structures Committee can supply
addresses of contacts.
