This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
The first necessity in the construction of drains is a ground plan of the premises, showing the position of the whole of the sanitary appliances from which the waste-water has to l>e conveyed away, and also the position of the sewer or cesspool with which they have to be connected. In the case of a house standing in its own grounds, the plan should show the shrubberies, plantations, and paths, so that the line selected for the drains may be one that will cause as little disturbance as possible, and as far as practicable avoid the neighltourhood of the roots of trees. On the plan the drains must be marked to comply with the conditions settled in a previous chapter, taking care that manholes are fixed at all important bends, and that the branch-drain- are as short as possible. and converging in manholes, so that access may be had to all of them.
It has hitherto been far too prevalent to entrust the construction of a drainage-system to odd labourers with only intermittent supervision; the result is that the pipes are huddled together anyhow in the trench without uniformity of line or gradient, imperfectly laid, and without proper foundations being secured, so that in course of time mischief inevitably ensues. Again, many houses are drained without any plan of the system having been previously prepared, the course of the drain being staked out upon the ground in what is considered the most convenient position, angles and corners being "negotiated by sweeping curves, the idea l>eing no doubt that they will facilitate the flow of water through the pipes, such a thing as subsequent inspection or the necessity of obtaining access to the drain being entirely disregarded. The difficulty of discovering the course of drains constructed in this manner is palpable, and the whole method cannot be too strongly condemned.
It is the custom in all works of main sewerage to prepare sections from actual levels, and upon these sections to record the levels of the various places from which sewage is to be picked up; and while it is important that this should be done, it is equally as important that the same pains and the same care should be taken with regard to the drainage about a house, where the difficulties to be overcome in regard to levels are frequently far more intricate than in the case of main sewers. It is likewise impossible to determine, without the aid of carefully-plotted sections, the requisite fall to be given to the drain, or to select the size of pipe that will convey away the whole of the sewage and rain-water. Without the taking of levels, it is also impossible to select the most economical lines for drains - that is to say, the levels will enable us to determine the line that will involve the least amount of excavation. It may be necessary to make a slight detour, which would increase the length of the drain; but possibly by reason of the surfaee-levels showing that considerably less excavation in the trench would be required, economy may be thus effected without at all reducing efficiency. A section is also valuable in that it shows how it may be possible to effect changes of level in the drain; and as these will be brought about at the manholes, it may be necessary to slightly change the position of these from the places which had previously been selected upon the plan. This method of changing the levels of the drains effects considerable economy in construction without in the least deteriorating the efficiency of the drain, providing the minimum self-cleansing gradient is not departed from. The importance of this cannot be too much insisted upon. Quite recently I had to certify that a certain system of drainage which had been carried out was efficient, and I found that if this method of changing levels had been adopted, the scheme would have worked perfectly satisfactorily, and that at least £70 would have been saved in excavation alone. The levels required are not only the surface-levels along the line of the main drain, but also the discharging levels of the outgoes from all gullies in connection with waste-pipes; and care must be taken to see whether any of these would be required either below the ground-level or in the basement. It is obvious that these levels can only be accurately taken by someone who is intimately acquainted with the operating of the usual type of surveyor's level. The levels should bear a relation to some well-defined point which is not likely to be obliterated, and which may be used for the purpose of a datum. When the situation of a house admits of it conveniently, it is preferable to commence levelling from a bench-mark of the ordnance datum, as nearly all sections of public sewers are plotted with reference to that datum, so that the laying down of the intended levels of the house-drainage upon the required section is very much simplified. The section may be plotted on any convenient scales, according to the length of line, and the variation between the highest and the lowest level recorded. Care sdould be used at all bends in the line and at the positions of the junctions for branch-drains to take a level upon some fixed point which ran be again referred to. The value of these levels should be clearly shown upon the section at their proper positions.
The depths of the drains shown in Plate XIV. are governed by the depth of the basement floor, and the levels at which ■ connection may be made to the main sewer. Where then are no cellar drains, the level at the extremity of the longest branch will govern the depth and gradient of the main drain. As there are waste-pipes in the basement borth in front and at the back of the house, it will be necessary for the drain to be kept at an uniform depth from the surface; but assuming that there had been no waste-pipe at the back of the house, the proper course would have been to have changed the level of the main drain at the manhole (No. 2). In completing the section it Lb preferable to show tin-drains by red lines, and all letters and figures relating to sizes and gradients and reduced levels of inverts should be clearly marked in red; the height of the datum line above the ordnance datum or other fixed point should be recorded, and along the datum line, the distance of the various points where manholes are to be fixed or changes of direction and gradients occur, should also be clearly marked.