[Ill.u.s.tration: 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.
[Ill.u.s.tration: 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 r.e.t.a.r.d the drying out.
[Ill.u.s.tration: 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 b.u.t.t 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.
[Ill.u.s.tration: 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.
[Ill.u.s.tration: 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.
[Ill.u.s.tration: +A Devonshire Farm, Local Material (Cob).+]
[Ill.u.s.tration: +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 gla.s.s 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.