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.
Having once selected the wall and marked its extreme points, a sight-rail or "gallows" should be put up beyond it at either end, quite clear of the building. This is formed of two stout stakes driven firmly into the ground, one on either side of the centre line of the wall and sufficiently distant from that centre to allow the whole width, not only of the wall itself, but of its concrete foundations, to pass between them. A cross-rail of inch boarding should then be nailed to the stakes, and upon this the thickness of the wall itself, the width of the extremity of the footings, and the outline of the trenches should be marked in the same way that the bench-marks have been, by means of slight saw-cuts and paint. In a wall of great length, intermediate gallows may have to be erected for temporary use, and the saw-cuts marked upon these by sighting between nails driven into those on the extreme gallows. Lines are now stretched tightly from gallows to gallows, attached either to the saw cuts or the nails, and these give the outline of the wall, footings, or trenches, as the case may be. This, as already said, is work which the builder should do, and it is only for the Clerk of Works to check its accuracy, while he should see that the gallows are securely fixed, and are not disturbed until the work has progressed sufficiently for the walls to be independent of them.
A great many Builders trust to small stakes driven into the ground in place of gallows; but these are much more apt to be moved about or kicked over, and are thus liable to cause a good deal of trouble. In all important works gallows should be preferred, and in order that there should be no mistake at all, the length along the cross-rail which denotes the thickness of the wall should be painted, say black, the projection of the footings red, and the farther projection on either side to the outline of the trenches white, as shown in the illustration.
Once main walls have been built up to the gallows level and carefully tested they may be taken as a basis for subsequent measurements, and it is generally necessary to redimension the various drawings from these walls, measuring wherever possible from them, and not trusting to minor measurements. This often means a good deal of calculation upon the plans - the introduction of figures upon them in some distinctive colour, as from one side or other of the main base walls. It then becomes of the greatest importance to see that these walls are carried up perfectly regularly, with truly vertical faces and angles. This, of course, applies to all walls, though such extreme care is not always necessary in internal walls as it is in base walls, such as these, and internal walls.
A word of warning may be here given to select as a base wall one which really goes up through the building from basement to roof, for occasionally what looks upon the ground plan to be the best wall for the purpose is not carried up to any great height, or is broken or varied on upper floors, when a good deal of trouble arises through having to select a fresh basis to work from. Verticality can be perfectly easily tested by means of a heavy plumb-line, and truth of direction by the ordinary stretched line, while horizontal coursing is tested by a carpenter's level, the more accurate instrument being very rarely used above the ground floor and for the purpose described last week.
Just as surface dimensions are taken off a base, so are the vertical dimensions, and in this instance the base used would be the ground line shown on the drawings, and considered to be 10 feet above datum. By means of the level and staff, erected for the purpose upon rising ground near or upon a steady piece of the scaffolding, it is quite easy, as soon as the building has proceeded sufficiently far, to determine points upon the walls, in several different places if necessary, all of which are at some known height, say 15 feet, above •datum, by the same method as was described for checking the levels of the trenches. These points may be marked in pencil or in some permanent manner upon internal walls which are subsequently to be plastered; but upon external walls the marks, though sufficient for identification, must be of such a character as not to disfigure the work. As the building proceeds other marks can be made, by using the 5-feet rod, at intervals of 5 feet, and from these all heights can be most accurately determined - much more so than is possible by mere measurements from floor to floor. It may be pointed out that it is by no means an uncommon practice for a Contractor to save himself expense by deducting an inch or two from each floor as it goes up, in order, as he may say, that the wall-plates carrying the joists may rest upon brickwork without packing. Everyone recognises that it makes much better work to thus course with the bricks; but a Clerk of Works should be careful to see that if an inch is lost on one floor it is made up on another, if he permits any variation at all. About this, however, he ought to consult the Architect, as a very little difference might upset a scheme of internal decoration. This is easily checked if all the dimensions have been scaled and figured on the section as from the ground line (which may have to be done by the Clerk of Works), and if standard marks on all the principal walls have been made at every 5 feet above this ground-line.
The gallows system of setting out walls is also applicable to drains, only in dealing with them the cross-rail should be so fixed, by careful usage of the level and staff, that its upper edge is, for instance, at a fixed height, say 8 feet above the invert of the pipe. A long rod is then cut exactly 8 feet in length, and a cross-piece nailed to its head and another short piece to its foot. Every pipe that is laid can then be tested by erecting the rod, known as the boning-rod, so that the little piece at the foot rests in the pipe, while the crossbar is steadied between the sight-rails until it exactly comes in line. The position which the pipe is to occupy may be painted on the sight-rail, just as the thicknesses of the walls have been painted on the gallows-rail, and by dropping plumb-lines from these markings it can easily be seen whether the pipes occupy the positions which they ought to do.