Elements of Stone Masonry

Chapter 3: Trimmings

Special Stones

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 template, 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 template, 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 called jamb 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.

Lintels

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 floor beams 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.

Sills

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.

Coping

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.

Gable 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, but they should project on both sides of the wall and should have a drip at each edge so as to shed rain water.

Stone Steps

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 cross-section 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.