This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
(Contributed by Walter Hooker)
The technical terms used in connection with stairs, and the methods of proportioning the risers to the treads, and of setting out stairs, are explained in Chapter VI (The Geometry Of Masonry). Part III. of Volume II.
The fliers of plain stairs are usually worked with the soffits parallel to the treads, in section presenting a series of rectangles as shown at A, Fig. 134. In more important work, and where the soffits are visible, they are splayed as at B, forming winding surfaces on the under side, whence they are termed " spandrel steps."
With staircases where no winders are used the fliers rise to quarter or half-landings, and a section of the stairs will show a raking surface when splayed on the under side, forming a continuous line parallel to the line of nosings. Where winders are used the soffits of the winders will have twisted surfaces or curves in section, and will require a series of bevels accurately cut to suit them. The soffit of each winder will flatten out as it recedes from the centre, owing to the greater width of the steps as they approach the outer wall of the staircase. A portion of stone of square section is left on to form a bed in the wall.
Fig. 135 shows an example of a turret stair. Here the well is circular on plan. It is assumed here that the height from floor to floor is 12 feet, while the diameter of the well is 5 feet 6 inches. Each step is made of one solid stone, with a circular piece left on the smaller end, which forms, when the stair is built up, a continuous central newel. The first operation in setting out full-size working drawings is to determine the rise and, tread of the steps. In this case the total height from floor to floor is divided into 24 equal parts, making the risers each 6 inches deep, while the circumference of the turret is divided into 15 parts, thus giving a comfortable tread as well as providing ample headroom. This headroom should not be less than 6 feet 6 inches, and where possible it should be more. In small turrets, however, it is not possible to give the steps a comfortable rise and tread as well as an ample headroom, and a compromise should be made with due regard to all three factors. In Fig. 135 the entire plan and elevation of the stair is shown, but in practice it is only necessary to calculate graphically or mathematically the exact sizes of one step, as each is the exact counterpart of the next.
When the steps are simply rectangular on plan the rise and the back face of the steps are made tangential to the central newel, as shown on the separate " Plan showing Tangential Steps "(Fig. 135),on which the back of step No. 1 is shown in dotted lines. This saves labour, and makes the step stronger at the smaller end.
To set out one step on plan, divide up the circumference of the turrets into the required number of equal parts, and draw lines radiating from the centre of the turret to each of these points as shown dotted on steps Nos. 1 and 6. These dotted lines represent the positions of the several risers. Draw a circle to represent the central newel. Then draw on the nose and the part beneath the adjoining step. These are drawn respectively parallel to the riser lines of the stone itself and the stone above it. The wider ends of the steps are built into the wall. The bed mould for each step is shown shaded on step No. 10. The under surfaces of the steps are splayed, thus forming a continuous spiral, intersecting the walls and the central reveal in spiral lines as shown upon the elevation.
In setting out individual steps upon the stone a zinc template is formed of the shape of the shaded step in Fig. 135, or two laths are nailed together with cross pieces, giving the angle of divergence from the parallel, and gauging the extra width of the wall end from the well hole end, as is shown in the lowest drawing on the right-hand side of Fig. 135.
Another detail of this figure shows the soffit of one step, the stone being turned upside down, and illustrates the twisting surface required to form the soffit when built up into a spiral plane.
To set out the twist, divide the two ends into the same number of equal parts. By working drafts down from the square section, from a to a, b to b, etc., and of equal depth from the square face, the soffit face will be reached. The intermediate surface can then be brought down to the same plane, and will give the finished soffit as shown.
The square portion left on at the end is built into the outer wall. The winders for connecting two straight flights are treated in the same manner, moulds for the outer ends being formed by developing the elevation of the curved portion.
Fig. 136 represents the plan, elevations, and part section of a double flight of stone steps leading up to the entrance of a modern mansion designed by Mr. Robert W. Carden, A.R.I.B.A. A reference to the plan will show the steps so arranged that the two flights abut on a broad landing, which is further increased in area by a projection beyond the line of the stairways, the balustrading being returned round to enclose it.
In this instance the steps are spaced 11 1/2 inches from nose to back, and with 6 inches rise, giving easy and convenient "going." They are supported on strong walls, the ends being carried well over and bedded solid therein. The moulded strings are cut in sections as shown in detail at Y, the upper portion (forming the rake) and the bottom bed being coincident and in the same plane as the beds of the masonry in the supporting wall.
The balusters are cut in one piece, and can either be secured with dowels (see Chapter XII (Stone Columns). Part II. Volume I.) or, if the stone is of such a nature as to admit thereof, by tenoning the balusters and sinking a corresponding mortice in the stone below. The handrail is furnished with mortices corresponding with tenons on the heads of the balusters in the same manner, and the latter are secured in the usual way with cement. In heavy work, where moulded balusters are employed, the lower member is worked on to the upper stone of the raking plinth, and the upper on the under side of the handrail, forming a base and cap respectively to the mouldings of the baluster as shown at Z in Fig. 136. The landings may be in two or three slabs, joggled together as already illustrated (see Chapter XII (Stone Columns). Part II. Volume I.), the stones being without joint from front to back and having a bearing on the outer and main walls.
As an additional precaution, a flat arch of equal camber to that supporting the outer wall might be carried from the supporting walls of the top steps of each flight to support the landing.
The centre bay of the front is made up of a flat arch with salient voussoirs, springing from low piers with abutments on the outer faces to counteract the thrust.
This admits of an opening for light and air to a vault below, should convenience require it.
The alternate voussoirs project beyond the general face of the arch, and are carried up to the under side of the string, additional stability being thus ensured. The intermediate voussoirs are moulded.
As regards the system employed in bringing the rough material to its necessary shape for setting into position in the structure, it will be sufficient to detail the process as regards one or more of the more important stones.
As an example, the selection of one of the stones forming the raking string may prove most appropriate.
It is customary in all cases to commence by bringing one of the faces to a plane surface, usually one which forms a bed. At X, Fig. 136, the top plan or bed mould of the stone A on the general drawing is represented, while Y illustrates the side elevation, and Z represents the end elevation.
A rectangular stone is required, of a length equal to the extreme length, a height equal to the distance from the bottom bed to the apex of the top baluster base, and of width sufficient to contain the stone from back to front plus the salient moulding.
The modus operandi is as follows -
Bring the bottom bed to a plane surface, and work the internal vertical face to a plane surface at right angles to the bottom bed. By means of the bevel, scribe on this face the outline of the baluster bases. Next, work in the two joints, taking care to cut them accurately at right angles to the raking surface. Lastly, indent the mouldings on the two joints and work them through, together with the set back of the bases. Cut down the superfluous stone between the bases already indented on the inner face. The splays are then drafted and the whole stone fine dragged. The operation is then completed.
In a stair with the supporting walls curved on plan, face moulds would be required both externally and internally, and the stone would have to be first brought to a convex form on the outer and concave on the inner surface by means of boning lines from top to bottom.
The stone would then form a segment of a hollow-cylinder. The moulds would be found by developing the external and internal elevations, as explained in the case of arches circular on plan. For the outer mould it is necessary to cut templates of contrary outline, i.e. to make the mouldings recessed instead of salient on the templates, as shown at Z1. By this means and the frequent application of the hollow moulds to the work any inaccuracy is avoided.
It is important to note that in a great deal of modern work stone is not used structurally, but the appearance of solid masonry is given by supporting the stonework upon iron joists and girders, as explained in Volume IV.
The methods of stone-cutting, etc., explained in Chapters VI. to XII., are those enunciated in Gwilt's Encyclopedia, Purchase's Practical Masonry, and other books. These principles are well recognised, and it has been impossible to depart from or improve upon them. Although the examples given are in many cases fresh, acknowledgment of indebtedness to previous writers on the subject is due.