Finish of Stonework
Stereotomy. - The science of making patterns, or templates, to which a stone is to be cut to fill a certain place in an arch or other complicated piece of stonework, is called stereotomy. In practice, the engineer makes a drawing of the intended stonework, showing where the joints in the face are to be located, and the stone cutter then details each block and cuts it to fit exactly with the others. It is therefore important for the engineer to understand the different finishes to which stone is dressed, but it is not necessary for him to be able to make the templates for each stone.
Rock-Faced Work. In Fig. 3 is shown rock-faced, or pitch-faced work, and the method of using the pitching chisel. The face of the stone is left rough, just as it comes from the quarry, and the joints, or edges, are pitched off to a line, as shown at a. As this finish requires very little work, rock-faced dressing is cheaper than any other kind, especially when granite or hard limestone is used.
Margins. Building stones are often faced an inch or so from their edges. This dressed strip, shown at a, Fig. 4, is known as the margin, or draft line, to distinguish it from the rock-faced work at b. This margin is cut on soft stone with a chisel, but on extra-hard stone, such as granite it is usually cut with an ax, or peen hammer.
Pointed Work.In producing pointed work a pointed chisel is run over the face of a stone to knock off any large projections. This work is called rough- or fine-pointed work, according to the number of times the work is gone over. In Fig. 5 is shown an example of rough-pointed work, while in Fig. 6 is shown an example of fine-pointed work that is also margined.
Tooth-Chisel Work. The finish called tooth-chisel work is produced by dressing stone with a tooth chisel. The surface of a stone finished in this way resembles pointed work, but it is not so regular. Working stone with a tooth chisel is one of the cheapest methods of stone dressing known.
Broached Work. Fig. 7 illustrates what is knows as broached work. In this kind of work, the stone is dressed with a point so as to leave continuous groves over the surface. At a is shown the margin, or draft line, and at b, the broached center, which is cut in two directions in order to illustrate right-and left-handed broaching.
Tooled Work. For tooled finish a tooth chisel from 3 to 4 1/2 inches wide is used. In this kind of work, the lines are continued across the width of the stone to the draft line (when one is used). When well done, tooled work makes a very good finish for soft stones.
Drove Work.The finish known as drove work is somewhat similar to tooled work, but is generally executed on harder stone. There are two general classes of drove work, namely, hand drove and machine drove, the former being shown in Fig. 8 and the later in Fig. 9. Machine-drove work, as will be noticed, is more regular than hand drove. Also, the cuts are a little deeper, although this is hardly apparent from the illustration. For a large quantity of cutting, machine work is cheaper than hand work; it is not so pleasing in appearance, however.
Crandalled Work. In Fig. 10 is shown crandalled work, which, when well done, gives the stone a fine, pebbly appearance. This finish is especially effective for the red Potsdam and Longmeadow sandstones. In the Eastern States, it is used for sandstones, probably more than any other finish sandstones, probably more than any other finish.
Rubbed Work.In producing the finish known as rubbed work, the surfaces of stones are rubbed with a piece of softer stone, together with sand and water, until perfectly smooth. Sandstone and most of the limestones are finished in this manner, and if granite, limestone and marble are rubbed long enough, they will take a beautiful polish. The operation of rubbing can be performed either by hand or bymachine. If the rubbing is done soon after the stones are sawed into slabs and are still soft, it is cheaply and easily performed, as the sawing makes the face of the stone comparatively smooth.
Brush-Hammered Work.In Fig. 11 is shown the finish of a stone after having been brush-hammered. This finish, which leaves the surface of the stone full of points, is a very attractive one for hard limestones and sandstones, but should not be used in dressing the softer kinds.
Patent-Hammered Work.A stone finished by a patent
hammer,which is generally used on granite and hard limestone, is shown in Fig. 12. The stone is first dressed to a fairly smooth surface with the point and then finished with the patent hammer. The degree of fineness in the finish is determined by the number of bladesin the hammer, the usual number being eight or ten. The ax may be used instead of the hammer, but more time is required to obtain an equally good finish.
Vermiculated Work In Fig. 13 is shown a stone having a somewhat elaborate finish, which is known as vermiculated from the worm-eaten
appearance. Stones cut in this manner are used principally as quoins, or corner stones, and in base courses. Owing to the cost, this style of dressing is not often used in the United States.
A simple method of obtaining the vermiculated effect is by the use of a patented sand-blast process. The sand employed in this process is carborundum dust, which is one of the hardest substances known. It is blown against the stone with high velocity by means of compressed air. While this sand will rapidly cut and wear away hard surfaces, such as stone it will not cut soft, yielding surfaces, because the latter do not suddenly stop its motion but, by giving way slightly, permit it to sink in a short distance and then rebound. For this reason, the nozzle of the blowing machine is made of soft rubber and those portions of the stone that it is desired to have raised are covered with beeswax, asphalt, or even heavy paper. The remainder of the face of the stone is eaten away by the sand blast. When the proper depth has been reached, the sand blast is stopped and the material used to cover the raised part of the stone is removed. It is then necessary to put on a few finishing touches with a pointed chisel, when the stone is ready to go in the structure. The sand blast is also used to clean stonework that has become soiled and stained by smoke and dust.
Scale Work A pleasing and novel method of stone dressing, presenting a striking effect of light and shade, is illustrated in Fig. 14. The finish shown at a, known as scale work is obtained by cutting out rows of shallow flutes between the drafts of the stone with about a 1-inch tool. The flutes are about 1 inch wide, and are alternated so that each successive course "breaks into" the preceding one and forms with it a series of hexagonal hollows,
giving a honeycombed appearance. The application of this finish to a window jamb is shown at b. This unique method is applicable, of course, only to soft stones, such as limestone, but to these it gives a beautifully crisp and varied surface. The cutting can be done either by hand or by machinery.
Rusticated Work.The term rusticated work is generally used to designate sunken or beveled joints. Two examples of this finish are illustrated in Figs. 15 and 16, the former showing the stones with recesses a having sharp edges and the latter with recesses a having rounded edges. This style of work is expensive, and is usually employed in the finish of basement work or to emphasize piers and other projections.
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 underping 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 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.
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 ashlarwork 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.
The term trimmings, as generally used, includes moldings, belt course, sills, caps, and other cut stone (expect ashlar) used for ornamental purposes.
The stones for such work should be of good quality, having the beds closely dressed and the ends square and properly matched. The faces may be pitched off, but all washes, soffits, etc. should be cut or rubbed. When a brick building is trimmed with stone, great care should be taken to have the trimmings set properly, so that it will not be necessary to split the courses of brick below or above, for such a procedure will spoil the appearance of the building.
Bond Stones and Templates.All piers above a certain size require bond stones, that is, stones the full size of the pier, to prevent them from splitting. The course of brick placed underneath should be brought to an exact level to receive the stone. Otherwise, the weight above may cause it to crack or become displaced. Only strong stones, such as granite, bluestone, and hard trap rock, should be used, and they should be cut to the full size of the pier.
Bearing stones placed under the ends of beams and girders to distribute the weight more evenly on the wall are called templates. The pressure per square inch allowed on the brickwork or stonework in the wall under the templet, as specified by the building lawn of the town in which the building is being erected, governs the size of the template required, and is usually from 100 to 200 pounds. It is better, however, to make templates too large rather than too small. A hard, tough stone should always be employed, and the usual rule is that the thickness of the stone should be one-third of the smallest surface dimension, except when very large stones are used; but the least thickness should be 4 inches. When a wooden girder rests on a templet, a good plan is to place a flat stone above the end of the girder so that the wall will rest on the stone and not on the wood. This is advisable for the reason that when the wood shrinks, the settlement may cause cracks in the wall.
Strictly construed, bond stones and templates are not ashlar, but as they require more or less dressing, they are considered as being ashlar.
Quoins. The corner stones of a wall, as already inferred, are known as quoins. They are often dressed differently from the other stones in order to make them more prominent. Quoin stones should always be equal in size to the largest stone used in the wall; otherwise, the effect of strength and solidity that they are intended to produce will be lost. Sometimes, the quoins of a rubble-stone wall are built of brick.
Jam Stones. The stones used in the sides of a door or window opening are calledjamb stones.the alternate ones should extend through the width of the wall to insure a good bond. Fig. 32 illustrates cut-stone jambs in a rubble wall. The jamb stones bonding into the wall transversely are shown at a; those bonding longitudinally, at b; the stone window sill, at c; and the rubble wall, at d.
Occasionally, when stone piers or pilasters are built on the outside of the building, the windows are recessed so that the projection of the sills and lintels will not be so
noticeable. This is illustrated in Fig. 33, in which a shows the lintel; b, the sash; and c, one of the jamb stones.
Jambs and quoins are often finished with a draft, or angle, line, especially when the softer stones are used. Fig. 34 illustrates this method of finishing, the quoin or jamb stone, as the case may be, being shown at a; the angle draft, at b; and the broken ashlar wall, at c.
Washes and Drips. The tops of all cornices, belt courses, etc. should have an outward and a downward pitch from the walls, as shown at b, Fig 35. If the top is level or slopes inwards, rain will collect, and in time will cause the disintegration of the mortar in the adjacent joints and finally penetrate the wall. The beveled surfaces are called washes. On the under side of the cornices, etc., drips should be made to prevent rain water from flowing down the face of the wall. At a, Fig. 35, is shown the drip; at b, the wash of the cornice' and at c, the stone cut to a sharp angle, so as to shed part of the water from that edge.
Window sills should also have a drip cut in them, as shown at a, Fig. 36, so as to keep the walls below from becoming discolored by dirt washed off the sills by rain.
A lintel, often called a cap, is a stone that supports the wall over a door or a window opening and, as it must resist bending stress, it should be a strong, tough stone having an ample cross-section. The ends of stone lintels should not be built into the walls more than is necessary to give sufficient bearing 4 to 6 inches at each end is the usual allowance. There should be a little play allowed at each end, so that the lintels can yield slightly without cracking if the walls on either side settle unevenly.
Relieving Lintels. Often, when a long lintel is used over an opening, the stonework
above the lintel is arranged as illustrated in Fig. 37, in which a shows the lintel, and b the relieving lintel, or stone above it cut with two diagonal joints, as at c. In this way, some
of the load is taken off the lintel and transferred to the wall on both sides of the opening.
When a lintel extends through the wall and is not supported by angles or beams, the strength may be increased, provided the stone is stratified, by cutting it in such a manner that the layers will set on edge, as shown at a, Fig. 38. This procedure, however, may cause the face of the lintel to flake off if the layers of stratification are thin and not securely joined together.
When considerable weight rests on a stone lintel, a brick relieving arch may be used but unless much skill is exercised in its construction, this arch will detract from the appearance of the building, especially if it
extends through the entire thickness of the wall. To avoid this result, if a stone of sufficient depth cannot be used, the lintel may be strengthened by the use of iron beams or angles. When the lintel is of moderate length, it is sufficient to use a piece of angle iron, as in Fig. 39, in which a shows the stone lintel; b, the angle, which should have its longer side vertical; c, a wooden beam to which the interior woodwork is nailed; d, the brick wall; and e, the window reveal, or side.
I-Beam Supports. When the width of the opening is considerable, stone lintels should be supported on the I beams. If only the weight of the lintel and the wall is to be carried, a single I-beam may be used, as shown in Fig. 40, in which the stone lintel is shown at a; the I beam, at b; the wooden beam to which the wood finish is attached, at c; the reveal, at d; and the brick wall, at e.
If, in addition to the walls, the floorbeams over openings must be carried, it is best to use two I-beams, as in Fig 41. Here, the stone lintel is shown at a; the I beams, held together by bolts and separations, at b; an iron plate on which the wall rests, at c; a floor beam, at d; the window reveal, at e' and the brick wall, at f.
When it can be avoided, the best plan is not to support the weight of a wall on both stone and steel or wooden beams, as the deflection of each material is different, making it practically impossible for each to carry its proper share of the load. The weight should preferably be borne by the steel beams alone.
Built-Up Lintels. - It is sometimes necessary to use a stone lintel that is 10 or 12 feet long. Since it is difficult to obtain a single piece of stone of this length, the lintel may be made in sections, as in Fig. 42. At least three stones should be used, and the joints should be cut as shown at a. when cut in this manner, the stones are self-supporting. The end pieces may be built into the wall for a considerable length, so as to act as cantilevers supporting the middle section. If such long lintels are used, however, it is better to carry them on I-beams, as shown in Figs, 40 and 41.
In stonework it is best to avoid placing a pier directly on top of the lintel. All openings should preferable be directly above one another.
Log and Slip Sills. In mason work, sill is the name given to the stones that form the bottom of the window and door openings in stone or brick walls.
Lug sills have flat ends, or lugs, built into the wall. These lugs should not enter the walls a distance of more than 4 inches, and should be bedded on mortar only at the ends. If a sill is bedded solid and settlement occurs, it will probably be fractured at the jamb line, as the pier or side walls will likely settle more than the wall under the opening. The joints un under the sills should be filled when the finished
walls are cleaned down.
Slip sills are made just the width of the opening, and are not built into the walls, being put in place after the frame is set. such sills are cheaper, but they do not look so well as lug sills. Besides, there are exposed vertical joints at the ends into which water will penetrate. However, any settlement of the masonry is not liable to break up a slip sill, and they are therefore often used in the lower parts of heavy buildings.
All sills should have a bevel, or wash, about which 1 inch of the foot, extending to the back of the reveal, as shown in Fig. 43. They sometimes have a beveled surface the full length of the sill, the brickwork being made to fit the stone. The later construction, however, is not good practice, as it permits water running down the jamb to enter the joint between the brick and the stone the slopping upper face also forms an insecure bearing for the wall resting on it. In Fig. 43 is shown the proper method of cutting the surfaces. as shown at a, the flat end of the lug sill carries the brickwork reveal c. At b is shown the bevel, or wash, and at d, the drip.
If no cover is put on the top of a wall, rain will wash out the joints. For this reason, the parapet walls are capped with a wide stone called coping. Terra cotta is also occasionally used for this purpose. The upper surface of the coping should be pitched, as shown at a, Fig. 44, and should have a drip on the under side, as shown at b. The coping should be about 3 or 4 inches wider than the wall. horizontal coping stones are often clamped together at their ends to prevent them from becoming displaced.
G ab le copings should be anchored either by bond stones or by long iron ties. A form of coping that is extensively used is shown in Fig. 45 in which the coping is shown at a, and the corbel, at c. The bottom stone b, sometimes known as the kneeler, should always be well bonded into the wall. In some cases, the coping is cut in steps, so that each stone will have a horizontal bearing on the wall. This method of coping is objectionable, however, on account of the increased number of joints. It is well to have long pieces of coping, so as to reduce the number of joints - a common length is 6 feet.
A short piece of coping cut as shown at a, Fig. 46, should be inserted at intervals to bond the coping securely to the wall.
Gable copings do not necessarily have to be pitched on top, bu they should project on both sides of the wall and should have a drip at each edge so as to shed rain water.
In laying stone steps, it is important to see that they are firmly supported at each end, but left free in the middle. If the stones forming the steps have a bearing along their entire length, they might, after a slight settlement in the foundations, rock from side to side when stepped upon, or they might crack. In order to strengthen extra long steps, however, it is sometimes necessary to insert a middle bearing. Great care must then be taken to have the middle and two end supports exactly on a line. Each step should overlap the one below at least 1 1/2 inches, and should have an outward pitch of about 1./2 inch. Steps having a nosing, as shown at a, Fig 47, make a good appearance, but they are more expensive than the ordinary steps.
A hard stone, such as granite or bluestone, should be used for steps; but for private residences, where the wear is not great, limestone or a fairly hard sandstone may be employed.
Stone stairs are sometimes made with only one end supported. This end is built solidly into the wall, and each step is carried on the next lower one, as illustrated in Fig. 48. As shown at a, the landing is bappetted into the tread of the top step. The manner in which each step is cut and supported by the lower one is shown at b. To be safe, the bearing dimensions should not be less than are indicated in the illustration. The bottom step should be firmly held in place by dowels set into the floor, as shown at c, as this step must sustain the thrust of the whole flight. The stone blocks forming the steps are usually cut in the triangular crossection shown, which method of cutting gives a good appearance to the soffit, or ramp, of the stairs.
Iron staircases are extensively used in fireproof construction. In such cases, the treads, and sometimes the risers, consist of marble slabs, while slate, which is cheaper, is also used. Staircase railings for stairways having stone or iron steps are often elaborately finished. They are generally made of iron, which is doweled into the ends of the steps.