Elements of Stone Masonry

Chapter 2: Stone Masonry

General Considerations

The stonework entering into the construction of building may be divided into three classes: rubble, ashlar, and trimmings. Before describing these, however, a few general observations, applying to all classes of stone masonry, are necessary.

Whatever may be the quality of mortar used, the wall should contain as much stone and as little mortar as possible, as the former is the stronger material. In  rough walling, if the stones are pressed together until the more prominent angles on the faces come almost into contact, the interstices being filled with mortar, there results better work than if a thick, yielding mass of mortar is allowed to remain in the joints. Absolute contact, however, is not advisable, as the mortar in drying shrinks and may leave the stones bearing only on the projecting angles.

The joints in stonework vary in thickness from 3/16 to 1/2 inch. A 1/2-inch joint is probably the best for ordinary work, while a 1/2 inch joint should be used for rock-faced work only.

Stone being of a brittle nature, the longer pieces in a wall must be properly supported and well bedded in order to prevent them from breaking. It is also best to avoid extremely long stones, although the length of a stone should be greater than its height, especially in ashlar work, on account of the vertical bond. There is a certain medium that should be observed; and while a compact mass, broken as little as possible, is most desirable in a stone wall as well as in brick walls, the mason will often find it better to break a very long stone into two or more shorter ones, even though by so doing additional joints are made. However, in laying very long stones, as in steps or copings, it is customary to bed them only at the ends, so that when the mortar joint shrinks there will be no danger of the stones being broken by bearing on some obstruction at their middle.

The best stones should be used for piers, jams, sills, lintels, cornices, band courses, etc.. in the order mentioned; and all stones in which the length of the face is greater than its height should be so quarried that they can be laid on their natural beds, except, of course, piers and long jambs, which necessarily have the bed of the rock vertical.

Defective Methods. A stone with a hollow cut in it, as shown at a, Fig. 17, should never be used in a wall, because when the mortar shrinks, the stone will bear only at the edges and is liable to spall, or chip off, with the result shown in the illustration. If not closely watched, careless stone masons are tempted to cut stones in this manner, as it is much easier than cutting them to a true bed.

Another improper method often carried out by masons is to cut the stone as shown in Fig. 18 and underpin the back with spalls. this practice is also liable to lead to disaster, as the stone may split as shown at a.

On account of the liability of spalling, as illustrated in Fig. 17, rusticated joints are often used in the basement and first story of tall buildings.
 

Rubblework

Rubblework consists of stones in which the adjoining sides are not required to be at right angles. It is used for rough masonry, as in foundations, backing, etc., and frequently consists of common field stone, roughly dressed; but whenever possible, quarried rubble should be used, as better bedding can thus be secured. Conglomerate and slate stones abound in many localities, and are cheap and durable, but they do not cut easily. Such stones are often used with good effect, however, in walls with cut-stone or brick trimmings; or when good lengths can be had, they are used for rock-faced sills, lintels, and trimmings.

Rubble Walls. Fig. 19 illustrates a good rubble wall, the stones being bonded about every 4 or 5 feet as shown at a. The largest and best stones should be placed at the bottom and at the angles, as indicated at b, and should be laid up in alternate courses of headers and stretchers. Such work is generally laid with beds and joints dressed but very little, and rough angles only being knocked off. The stones are set irregularly in the wall and the interstices are filled with spalls and mortar. If better work is desired, the joints and beds of the stonework should be hammer-dressed. Such walls are frequently pointed with colored mortar, showing raised joints.

Fig. 20 shows a form of rubble masonry much used for country and suburban work. The quoins or corner stones, a are hammer-dressed on top and bottom, and may be either cut stone or rock face. The latter finish harmonizes well when stones similarly dressed are in the body of the wall. All joints should be hammer-dressed, as shown at b, and no spalls should show on the face, while the mortar joints should not exceed 1/2 to 3/4 inch in thickness. This makes an effective wall, especially for country churches, lodges, and other small buildings; but the work is expensive, owing to the labor required in dressing the joints.

Field-Stone Walls. In Fig. 21 is shown a field-stone wall. Walls of this kind are built of small, uncut boulders, and are frequently employed for fences and rustic-house work. Such walls should be made quite thick on account of the round and unstable shape of the stones used in their construction.

Walls With Brick Quoins.  Fig. 22 shows a rubble wall with brick quoins, or corners, at a. In this case, all the top and bottom joints of the rubblework have level beds, as at b. This kind of construction makes a very effective wall, and can be built quite cheaply when the stone used splits readily, or can be laid on its natural bed, thus requiring but little dressing.

Coursed Rubble, in walls of coursed rubble, some effort is made to produce a coursed effect. Stone of random sizes is used, but little or no attention is paid to uniformity of height in the different courses. For such walls, the stones are generally roughly dressed before the wall is begun. Care should be taken to get as nearly parallel beds as possible, and to bring the face of each stone to a fairly even surface at approximately right angles to the beds. the quoins in coursed rubble are usually dressed and laid with more care than the remainder of the work' they also serve as gauge courses. Coursed rubble, when well built, makes a very solid wall and is extensively used.

Fig. 23 illustrates a coursed rubble wall, the rubblework being shown at a' the quoins at b' the bond stones running through the walls, at c; and two of the course joints, at d e f and d1 e1 f1.
 

Ashlar

Stonework that is cut on four sides so that the adjoining sides will be at right angles to each other, is known as ashlar, no matter whether the face is dressed or not.

From Fig. 23 it is evident that some stones of this form are also found in coursed rubble. The latter may therefore be considered as the connecting link between rubble and ashlar stonework.

In the following description it should be understood that the style of ashlar designated has nothing to do with the finish on the face of the stone, but simply the manner in which it is laid, although certain kinds of ashlar are generally made with the styles of dressing shown in the illustrations.

Ashlar is usually laid either in regular courses with continuous joints, as shown in Figs. 24, 25 and 26, or in broken courses, without regard to continuity of the joints, as shown in Figs 28 and 29. All ashlar should have straight and horizontal bed joints, and the vertical joints should be kept plumb. If the work is not done in this manner, ashlar walls will present a poor appearance.

Coursed Ashlar. A class of stonework in which the blocks are uniform in size and the bed joints are continuous is known as coursed ashlar. When such stones can be obtained readily, this kind of work is not very expensive. A coursed-ashlar wall is shown in Fig. 24, in which 12" x 36" ashlar is shown at a, and the backing, which consists of 12-inch rubble, at b.

A good effect is produced by making the courses of two heights, but cut in regular sizes, and having the vertical joints in alternate courses directly over one another. This class of work is illustrated in Fig. 25. in this figure, a 14-inch course is shown at a; a 6-inch course, at b; and the backing, at c. The latter may also be brick, as the ashlar can be well bonded into it. If the narrow band course b is rock-faced, or has some different finish than the wide courses a. the appearance of the work will be further improved.

The stonework of many public and office buildings has rustic quoins and base or band courses, as shown in Fig. 26. Here, the quoins, which have a 1-inch bevel, or chamfer, at the joints, are shown at a; the plain, rubbed, or tooled stones forming the face of the wall, at b' the rustic band course, having a 1 1/2 inch chamfer cut on it, so as to project beyond the quoins, at c; and the stone or brick backing, at d. This method of construction is very expensive, owing to the great amount of dressing required.

Block-In-Course Ashlar. In block-in-course, or blocked-course, ashlar work, all blocks of stone are cut the same height but in different lengths, and no attempt is made to have the joints come over one another. The length on the face is usually two or three times the height, and about one-fifth of the face should show headers, as at a, Fig. 27. These headers should rest on long stretchers below them, in order that the wall may be better bonded. As a rule, this style of work looks best in rock-faced finish, but any finish desired may be used. Many quarries have stratified stone that is just the proper thickness for this class or work, but unless the stone can be found in such shape, block-in-course ashlar work is generally quite expensive.

Random-Coursed Ashlar. The method of laying random-coursed ashlar walls is illustrated in Fig. 28. In  this class or work no attempt is made to have the vertical joints over one another, and it has only general arrangement in courses, as shown.

In regard to the best methods of proportioning the blocks and arranging the same so as to produce a harmonious effect, it is first necessary to consider what the various heights of the blocks must be in order to form good longitudinal bond. Assume the lowest height at 2 inches- as a stone any thinner than this presents an appearance of weakness - and the greatest height at 16 inches - as any higher than this looks to heavy for random-coursed ashlar. The gradations may then be 4,5,6,7,8,9,10,11,12,14 and 16 inches, thus giving eleven distinct heights - a variety that, when well arranged, produces a most pleasing effect.

If the three highest numbers are taken as jumpers,  or course levelers, combinations may be made of the other stones so that their combined thickness will equal that of the jumper.  In this manner, several arrangements are possible.

The next point to be considered is the lengths of the blocks. the bond, or the lap of the stones over one another, should be, for the thinner blocks, at least 6 inches, and for the thicker ones, 8 inches.

Broken Ashlar. In broken-ashlar stonework, no attempt is made to have the stone run in courses, but each block is cut for the location in which it is to go. It generally takes more time to build than broken ashlar than coursed work; hence this kind of wall is more costly, owing to the increased amount of labor required to fit and lay the different sizes of stone. Broken ashlar, when properly executed, presents a pleasing appearance. It is generally laid up as rock-faced work, but in some cases, it is tooled or hammer-dressed. It should have no horizontal joints more than 4 feet long, and several sizes of stone should be used. Fig. 29 shows an ordinary broken ashlar wall, 2 feet thick, the sizes of stones used being 4,6,8 and 12 inches in height. The quoins are shown at a, and the body of the wall at b.

Best Stone for Ashlar. The hardest kinds of rock are the best suited for ashlar masonry, as, in pitching, the spalls fly off more easily and leave the fracture in sharp lines; whereas, with the softer kinds of rock, the fracture has a bruised and crushed appearance, which is not at all pleasing. The best stones to use are the granites and the most compact bluestones and sandstones.

Laying Out Ashlar. If ashlar in regular courses and sizes is to be used, drawings should be made showing each stone of different size, the heights of the courses and other necessary details. The drawings for public and office buildings usually show every stone, unless broken ashlar is used, in which case it is only necessary to show the quoins and jambs, together with enough of the ashlar to indicate the character of the work desired. it is almost impossible to follow carefully a drawing showing all the stones laid as broken ashlar.

Backing. The expense of ashlar masonry is such that it is commonly used merely as a facing, being backed with either rubble masonry or brickwork. It is only on works of great importance and solidity that ashlar masonry is used throughout the whole thickness of the wall. In general, the term ashlar applies to the facing, or veneering, of stone, or to the stones that constitute the facing. Both stone and brick are used as backing. but in most cases, brick is the cheaper and is therefore more extensively employed. When using brick for the backing, the joints should be made as thin as possible, employing cement mortar so as to avoid shrinkage. Backing of this kind, however, should never be less than 8 inches thick.

Both stone and brick are used as backing, But in most cases, brick is the cheaper and is therefore more extensively employed. when using brick for the backing, the joints should be made as thin as possible, employing cement mortar so as to avoid shrinkage. Backing of this kind, however, should never be less than 8 inches thick.

When a hard, laminated stone with flat, parallel beds can be obtained, it should be used, as it is considered to be a stronger backing than brick. Irregular rubble backing should not be used for dwellings higher than two or three stories, unless the walls are made at least one-fourth thicker than when brick backing is used. All backing, whether or brick or of stone, should be carried up at the same time and built in courses of the same thickness as the ashlar. This kind of construction is illustrated at a, Fig. 30 (a) and (b).

If the courses are not over 12 inches high, they are usually bonded sufficiently to the backing by making every other course wider, and by having one through bond stone to every 10 square feet of wall, as shown at b, Fig. 30 (a) and (b). This method is called toothed bonding.

Method of Fastening Thin Ashlar. Although not so strong as a toothed bond, an ashlar facing of from 2 to 4 inches in thickness is often used, especially when marble or other expensive stones are employed in the construction. In such cases, each piece of ashlar should be tied to the backing by at least one iron clamp, or anchor, similar to that shown in fig. 31, while if the stones are more than 3 feet long, two anchors should be used. all iron clamps, or anchors, should be either galvanized or dipped in hot tar or asphalt, to prevent the formation of rust on them.

Belt courses extending 8 inches or more into the wall should also be laid about every 6 feet in height, so as to give support to the ashlar. When a wall is faced with thin ashlar, the effective bearing strength is only that given by the thickness of the brick or stone backing, the facing not being relied on for that purpose.
 

Care of Stonework

Pointing. The effects of the weather on the exposed edges of the joints in masonry usually cause the mortar to crumble and fall out. For this reason, it is customary to refill the joints to a depth of from 1/2 to 1 inch, with specially prepared mortar. This operation is called pointing.

In work that is to be pointed, no mortar should be placed within an inch of the front edges of the stone, as this saves raking out the joints preparatory to the pointing. Sometimes, strips of wood the exact thickness of the joint are set on the edges of the lower course. Then, in setting the stone, the superfluous mortar is pressed out and the stone rests on the wooden strips, which are removed when the mortar is hard.

Pointing is generally done as soon as the walls are completed, but, if the season is too far advanced, it should be deferred until spring. Under no circumstances should pointing be done in freezing weather, nor in extremely hot weather, as then the mortar will dry too rapidly.

The most durable mortar for pointing is made of equal parts of Portland cement and sand. These materials are mixed with just enough water to give a plastic consistency, add to this mixture a little slaked lime to make the mortar stick and such coloring matter as may be desired.

Portland and Rosendale cements discolor most limestones and marbles, and some sandstones. However, by exercising care, the mortar may be kept from the face of the stone, and the joints may be pointed afterwards with mortar that will not stain. A cement made of plaster of paris, lime, and marble dust, called Lafarge cement, is sometimes used for setting marble and limestone; it is claimed that this cement will not cause discoloration.

Cleaning. After pointing, it is usually necessary to remove the mortar stains, etc. from the face of the wall. This may be done by scrubbing the stonework with water containing muriatic acid, the proportions being about 20 parts of water to 1 part acid. For cleaning granite and limestone, wire brushes are used, and for sandstones and other soft stones, stiff bristle brushes usually serve the purpose. The stonework should be scrubbed until all mortar stains are removed.

As previously stated, the sand blast, operated by either steam or compressed air, does the work of cleaning walls very effectively and rapidly. It not only removes the outer layer of the discolored stone, but leaves a fresh, bright surface. Even fine carvings have been very successfully cleaned by this method.

Stone Defects. Granite may contain cracks, black or white lumps known as knots, and a brownish stain called sap. when such defects are found, the stone should be rejected, provided the importance of the work justifies it. Cracks are the main things to guard against, however, and they may be detected by the absence of the clear ringing sound when  the stone is struck with a hammer.

Sand holes are frequently found in sandstone. These are bodies of uncemented sand, that become dislodges by jarring or by the action of water, and produce a pitted appearance and an uneven color. Attention must also be paid to securing uniformity of color as sandstone from different parts of the same quarry may vary greatly in this respect.

Faults in Dressing Stone.  - The common faults of cut stone are coarseness and poor workmanship. In dressing stone, builders will avoid any work beyond that necessary to make the material barely acceptable to the inspector.

Frequently, the ends of cornices, belt courses, etc. will not match properly. It should be strictly required that the utmost care be taken in cutting all similar pieces to the same pattern, and that the abutting surfaces be closely dressed.

Laying of Stonework. In erecting stonework, care should be exercised to have the stone set on the natural bed, with good joints, and not in too small nor in too thin pieces. The bed joints in ashlar work should be square to the face of the work, and not less than 4 inches wide at both top and bottom. The proper bonding of the walls, especially from the ashlar and for the trimmings, should be given very careful attention, as should also the placing of lintels, copings, wall anchors, etc.

Another point that requires attention is the formation of the joints on which great pressure comes; also the mortar should be kept back from the face, so that the edges of the stones will not be chipped off. In pointing, the joints should be well raked out and the pointing mortar properly laid. Many other precautions for the good performance of the work will be doubtless suggest themselves to the careful superintendent.