_Composition._--Cob is a mixture of shale and clay, straw and water.
Shale is a common and widely distributed stratified formation of a slaty
nature, and there are few types of clay soil that would not serve for
cob-making.
The precise relative proportion of the first two ingredients varies,
depending on their individual peculiarities.
Local custom as to the composition and preparation of the mixture will
generally be found to have adjusted itself to the peculiarities of the
soil.
The following extract is from an analyst's report on a sample of typical
old cob walling:
* * *
"The material when placed in water fell to pieces. On analysis, it was
found to consist of:
Per cent.
Stones (residue on 7 by 7 mesh sieve) 24.40
Sand, coarse (residue on 50 by 50 mesh sieve) 19.70
Fine sand (through 50 by 50 mesh sieve) 32.50
Clay 20.60
Straw 1.25
Water, etc. 1.55
------
100.00
------
"The material is a conglomerate of slaty gravel with a very sandy clay,
to which mixture a small proportion of straw has been added.
"The clay acts as an agglutinant, and the straw as a reinforcement.
"Efficient protection from frost and rain would be necessary before such
material could be considered weatherproof."
* * *
[Illustration:
+A Devonshire Cob Farmhouse, probably between 200 and 300 years old.+]
[Illustration:
+A Cob-built Village.+]
(N.B.--Lime is occasionally added to the clay-shale, but this is not
usual.)
_Mixing._--The old method of mixing by hand is as follows: A "bed" of
clay-shale is formed close to the wall where it is to be used,
sufficient to do one perch. A perch is superficial measurement described
as 16-1/2 ft. long, 1 ft. high, and the amount of material will vary
according to the thickness of wall required. Four men usually work
together. The big stones are picked out. The material is arranged in a
circular heap about 5 or 6 ft. in diameter.
Starting at the edge the men turn over the material with cob picks,
standing and treading on the material all the time. One man sprinkles on
water, and another sprinkles on barley straw from a wisp held under his
left arm. The heap is then turned over again in the other direction,
treading continuing all the time. "Twice turning" is usually considered
sufficient. Straw bands may be wrapped puttee-wise around the legs of
the men to keep them clean, and these are removed at the end of the day.
More rarely the mixing is done in a rough trough, whilst a power-driven
"pan-mill" has also been tried with success; though one would think that
the use of such a machine might tend to diminish the binding strength of
the straw submitted to its grinding.
[Headnote: Implements]
[Illustration: COB PICK (Measured from example at Great Fulford).]
_Building._--In building a man stands on the low base-wall, and lays the
material handed up to him on the cob picks, treading it into position.
Thorough treading is important, and the heels should be well used. The
material is allowed to project each side an inch or so beyond the stone
base to allow for paring down afterwards. The courses are usually about
2 ft. high. The cob should be laid and trodden in diagonal layers, as
shown in the diagram: this is to secure proper bonding. It takes from
two to three weeks for a course to dry, according to the weather, and
five or six men would be required to build the walls of an ordinary
cottage. This would not keep them continuously employed, however, and
they would require to have several buildings in hand at the same time,
so as to be able to turn from one to the other while the courses were
drying.
[Illustration: COB COURSE, OR SCAR, SHOWING DIAGONAL LAYERS.]
At the completion of a course the corners are plumbed up from the stone
base below, a line is stretched through and the wall is then pared down
"plumb" with the "paring iron" by the man standing on the wall.
Sometimes, however, the paring down is left until the wall is finished
and dry. Four men will do about four perches per day of a wall 2 ft.
thick, preparing and laying material.
The material is rarely laid between timber shuttering as in Pise work,
as the retaining boards tend seriously to retard the drying out.
[Illustration: PARING IRON (Measured from example at Great Fulford)]
_Drying._--If a course takes from two to three weeks to dry, it
naturally takes a long time for a whole cottage to completely dry out.
The walls can be built from about March to September. The internal
fitting, plastering, etc., can be done in the winter, but the external
rendering must not be done for at least a year, perhaps two years, to
allow the walls to become perfectly dry.
As unprotected cob is sensitive to frost, especially if not thoroughly
dried out, it should be given a good external rendering as soon as it is
really dry, and should in the meantime be protected from frost by some
temporary covering, straw-matting or what not. Also all cob-work must be
protected from the rain both whilst building and when built.
No artificial methods of drying are at present usual, beyond good fires
inside during the winter, though, as under such conditions a cob cottage
is not usually considered fit to live in for several months after
completion, some artificial means of drying might be worth considering.
_Foundations and Base._--The depth of the excavations required for the
foundations naturally depends upon the character of the site and soil,
as also does the spread of the footings, if any.
The base-course wall of brick, stone, or concrete should be carried up
some 2 ft. or so above ground level. In old days this walling was not
infrequently built "dry"--but good lias lime or cement should be used in
all new work.
The damp-course too was an unknown refinement to the by-gone builders,
and the introduction of this one improvement alone makes the new cob
cottage a very different dwelling from the old.
The usual forms of damp-course serve well for cob walls, though slates
laid butt and broken joint in cement are probably the best.
[Headnote: Walls and Roofs]
_Thickness of Walls._--The thickness of walls may be anything you please
from 18 in. upwards. There are old examples a full 3 ft. across, but for
an ordinary two-storied cottage a thickness of about 2 ft. is general.
Eighteen inches is certainly the minimum thickness, and would not
ordinarily be adopted for any but one-storied buildings.
The first-floor walls are made the same thickness as those below, for if
they were reduced in width, as is usual in a stone building, the extra
weight thus thrown on to one side of the ground floor walls would tend
to make them bulge, unless quite dry and thoroughly set.
There are old cob walls in existence fully 30 ft. in height, and there
is no apparent limit in this direction provided they are thick enough.
The upper layers compress the lower ones, and automatically render them
more dense and stone-like and fit to bear the load imposed above.
_Hipped Roofs._--As a general rule, however, it is found expedient to
hip back the roof rather than carry it up in a tall gable, partly
because cob-building at a great height above the ground in short and
diminishing layers is a somewhat tedious process, partly because a
hipped roof with good eaves is very welcome for the protection that its
projection affords the walling.
_Masonry and Carpentry._--The bonding of cob to stone and brick is
sometimes liable to leave an open joint that will require filling when
the cob dries and shrinks. Many of the chimneys in old cob houses are of
brick or stone, and brick and stone jambs are sometimes to be seen in
cob walls, but they are probably by way of repairs to damaged corners.
It is considered better to have cob all round, so ensuring the uniform
settlement of the building.
The timber built into old cob does not seem to decay. The walls are
usually so dry, especially when plastered, that the wood is well
preserved. The straw in the interior of old cob walls is often as bright
as when put in. The straw in cob performs a similar function to hair in
plaster. Heather has sometimes been used instead of straw with good
results.
[Illustration: WALL COPINGS.]
The old practice was for beams, wall plates, joists, etc., to be just
bedded on the cob, and for the cob to be filled in between the joists.
In new work, particularly when the use of imperfectly seasoned timber is
unavoidable, it would be wise to take the usual precautions as to the
proper ventilation of all "built in" woodwork--especially the ends of
joists and so forth. Roofs must of course be tied and exercise no thrust
on walls. The roof plates are sometimes tied down by galvanised iron
wire.
[Illustration: LINTEL-BEARING CROSS-PIECE]
Door and window-frames are also fixed to wood blocks built into the
jambs and to the wood lintels above. The frames are sometimes near the
outer face of wall, sometimes near the inner-face. Where the door-frames
are on the interior face of a 2 ft. thick wall, a convenient porch
results.
[Illustration:
+A Devonshire Farm, Local Material (Cob).+]
[Illustration:
+Devon Country House, built of Devon Cob.+]
Other joinery is fixed to wood pins driven into the cob where required.
Corners are usually of cob, though stone quoins are occasionally met
with.
Lintels are usually of wood well tailed into the wall and resting on a
wood pad placed crosswise.
[Headnote: Protection]
_Protection._--Old buildings that have been neglected are often found to
be somewhat eroded towards the bottom of their walls through the action
of rain and frost.
Protection is less here than higher up under the projecting eaves, and
the Achilles' heel of the cob wall is undoubtedly its base.
This vulnerable part, exposed as it is to driven rain, back-splash, and
the casual kicks, should be given special protection.
Where the base is of cob and not of masonry, the traditional method is
to provide a good deep skirting of pitch or tar, or a mixture of both,
applied hot to the face of the rendering that should completely cover
the exterior of all cob work.
This rendering is usually composed of lime and hair mortar, though
Portland cement has come into use to some extent recently.
Cement, however, is apt to be rather too "short" and brittle, and it
does not always hold to the cob walling very securely.
A rendering consisting of an equal mixture of cement and lime with three
parts of sand adheres well to cob, however, and is probably the best
coating that can be given to it.
This coating can be colour-washed or lime-whited in the usual way. The
granular surface of rough rendering or of "slap-dash" on the slightly
wavy surface of cob walling perhaps gives to whitewash its very highest
opportunity and charm.
Certain it is that the old cob cottages of Devon with the pearly gleam
of their white walls, their heaving bulk of thatch and their trim black
skirtings, are as gracious and as pleasant to the eye as any in all the
length and breadth of England.
Within, lime-and-hair mortar plastered straight on to the cob makes an
excellent lining.
_Chimneys._--Nowadays, chimneys are commonly built up in brick or stone,
but numerous good examples survive of flues and stacks constructed in
cob. The insides of these are pargeted with lime and cow-dung in the
usual way, brickwork being only introduced immediately around the
fireplaces.
_Rats._--Where the surface rendering of cob-walls has been omitted or
has been allowed to fall away, an enterprising rat will sometimes do
considerable damage by his tunnelling.
A little powdered glass mixed with the lower strata of a wall will
discourage any such burrowing, but the best preservative for any cob
building is a thoroughly good skin of rendering, especially if this be
reinforced by fine-mesh wire-netting secured to the wall.
_Strength._--The strength of cob walls is surprisingly great so long as
they are vertical, and are not subjected to undue lateral thrust or
tension.
Beams as large as 12 in. by 12 in. may be seen supported by old cob
walls, and there is nothing likely to be asked of the material in the
way of strength to which it cannot easily respond.
_Design._--Cob, like every other material, should have a certain say in
the design of any building in which its use is intended.
The chief desiderata are a plain straightforward plan and broadly
treated elevations where voids and solids are carefully disposed with an
eye to getting as large unbroken blocks of cob as possible.
[Illustration:
+Cob House temp. Elizabeth, Lewishill.+
Walls from 3 ft. to 4 ft. thick. A wing was added in 1618.
This farm has been occupied by the family of the present holder
between 300 and 400 years.]
[Illustration:
+Another Devonshire (Cob) Farmhouse, Weeke Barton.+]
The cracks that are sometimes found in old cob buildings are almost
entirely attributable to unsuitable design in such respects, or to bad
foundations.
Cob walls built up in the ordinary way are not very suitable for
internal partitions on account of their considerable width and the
consequent waste of space, though in old work cob was sometimes used as
a filling for stud and lath partitions which were finally plastered over
in the usual way.
The sun-dried clay-lumps so much used for walling in Suffolk would seem
to be admirable for forming the partitions in a house of cob.
Cob work is usually repaired with rubble, stone, or brick.
New openings are easily cut through cob walls, and this fact has
occasionally led to the collapse of an old building through the zeal for
light and air of some new occupier exceeding his caution, and causing
him to cut away the substance of his walls in cheerful disregard of the
laws of gravity.