This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The size of the stones should vary according to the heaviness of the building, and a clause governing this point is often inserted in the Specification, such as, " No stone shall be more than 9 inches or less than 3 inches in depth."
The stones, as will be seen in Fig. 208, are built up to courses, which are broken at irregular intervals by large stones, the length of these joints being governed also by a Specification clause, such as, "No horizontal joint shall be longer than 6 feet."
Fig. 208. Irregular Coursed Rubble, Ashlar Quoins.
Small stones, called Snecks, are introduced at irregular intervals, marking the stopping of the courses and giving an overlapping appearance. The facings look best when hammer dressed or worked to any other rough texture.
Rubble built up to Courses is composed of stones built up to form straight horizontal joints at irregular intervals of from 1 foot to 18 inches, as shown in Fig.
Fig. 209. Random Rubble built up to Courses.
209. The quoins and dressings are carefully dressed and laid with care to serve as gauges for the courses, lines being stretched between them, and the intermediate portions of the wall being built up to these lines.
When the stones are squared and the headers run the entire depth of the course, as in Fig. 210, it is sometimes termed Coursed Header Work, concerning which the following remarks of Professor Rankinel are well worth noting: - "One-fourth part at least of the face in each course should consist of bond stones or headers; each header to be of the entire depth of the course, of a breadth ranging from one and a half times to double that depth, and of a length extending into the building to from three to five times that depth, as in ashlar. Those headers should be roughly squared with the hammer, and their beds hammer dressed to approximate planes; and care should be taken not to place the headers of successive courses above each other, for that arrangement would cause a deficiency of bond in the intermediate parts of the course. Between the headers each course is to be built of smaller stones of which there may be one, two, or more in the depth of the course. These are sometimes roughly squared, so as to have vertical side joints; sometimes the stones are taken as they come, so that the side joints are irregular; but no side joints should form an angle sharper than 60 degrees. Care should be taken not only that each stone shall rest upon its natural bed, but that the sides parallel to that natural bed shall be the largest, so that the stone may be flat, and not be set on edge or on end. Howsoever small and irregular the stones may be, care should be taken to make the courses break-joint. Hollows between larger stones should be carefully filled with smaller stones completely embedded in mortar."
Fig. 210. Coursed Header Work.
When rubble walls are built of stones occurring in shallow beds of regular depth the method of regular coursing is adopted. When they are less regular in depth they are sometimes sorted into sizes, so that the courses may all be one stone in depth.
1 Rankine, Civil Engineering, p. 386.
Picturesque boundary walls may be formed of any of the above kinds of rubble work laid without mortar, save polled flint or Kentish rag work, in which the adhesion of the mortar is essential to stability. Random rubble (see Fig. 211), or random rubble built to courses, is most generally used for this kind of work. Considerable care should be taken in bonding these walls, and in setting and pinning the unwrought stones. Dry walling should increase in thickness towards the bottom, the average thickness being about 4 inches for every foot in height of the wall.
Sometimes weathered copings are bedded on top of these walls, but more often the top is finished with a number of irregularly shaped stones, set on edge in mortar, puddled clay or even sods.
Fig. 211. Random Rubble setDry.
Mortar Joints in Rubble Work may be of any of the forms already shown for brickwork, the degree of perfection thereof being in accordance with the degrees of fineness in the working of the stones. When the joints are wide small chips of stone or pebbles are pressed into the mortar while it is yet soft. This is called Galleting, and although its appearance is rather pleasing it is not to be recommended, as it causes the joints to hold water and the wall to be damp.
2. Block-in-course. - When walls are required with a fairly fine face and of considerable strength a class of masonry known as Block-in-course (see Fig. 212) is employed.
The stones are squared and brought to a fine joint, while the faces are wrought to various degrees of fineness according to taste; but hammer dressing is most usual. The courses vary in thickness from 3 to 12 inches, but are chiefly built of Shoddies (stones less than 12 inches deep), and it is in this respect that brick-in-course masonry differs from ashlar.
Hard stones occurring naturally in beds of the above sizes are most suitable for this class of masonry. Chisel drafts are often run round the edges of the stones on face to enable close joints to be formed. Block-in-course masonry is difficult to define, as it closely resembles coursed rubble when no great amount of labour is expended upon it; while it resembles ashlar when it is more carefully wrought.
Fig. 212. Block in Course , Hammer-dressed Quoins rock-faced with drafted Margins.
3. Ashlar is the term applied to the finest class of masonry, formed of blocks of stone usually over 12 inches deep, squared and worked to joints 1/8 inch thick (see Fig. 213). Deep bedded stones are usually employed for ashlar work, these being sawn into blocks of the required dimensions. It is named according to the work upon the face of the stone, which may vary in degree of fineness from Quarry-Faced Ashlar to Wrought Ashlar. When the face of the stones are rough, chisel draughts are run round the edges to ensure the close fitting of the joints. Every stone should be fitted into position before setting in mortar, and if the joints are not sufficiently true to give a thin joint, the untrue sides should be taken out of winding. The beds and joints should not be worked too smooth, otherwise the mortar will not adhere to them properly.
Fig. 213. Ashlar with vermiculated Quoins.
The stone, having been properly trued and fitted in position, is raised off its bed and a bed of mortar is carefully spread for it; the edges of the stone are buttered as in brickwork, and it is carefully set in position again. The side joints are then grouted, the grout being worked about with a piece of hoop iron to ensure the joint being completely filled.
It is very rarely indeed that walls are built entirely of carefully wrought stones. They are, as a rule, faced with ashlar work and backed with brick or rubble, as in Figs. 214 and 215. When brick backing is used the facings should be cut to brick dimensions, so that the work may be properly bonded, and the stones should be arranged like bricks in Flemish bond, the headers tailing well into the wall. The backs of the stones should be roughly squared to prevent awkward cutting of the bricks.
When rubble backing is used the rubble and the courses should be levelled up with the courses of the face work. The number of bed joints in the backing should be made as few as possible, as the extra number of joints in the backing will cause the work to settle unevenly. It is always better, therefore, to use cement or other quick-setting mortar for brick or rubble ashlar, so as to reduce this unequal settlement to a minimum.
Where squared stones are used for masonry work the length, depth, and breadth should be proportioned according to the hardness of the stone, in order to give it the maximum amount of strength. With the weaker sandstones and granular limestones the length should not exceed three times the depth, and the breadth one and a half times the depth, while with harder stones the length may be as much as four to five times the depth and the breadth three times the depth.1
Fig. 214. Rubble Ashlar.
1 Rankinc, Civil Engineering, p. 242.