Installing Above-ground Drainage

Leaky and insanitary drainage systems, coupled with equally unsatisfactory sources of water supply, were directly responsible for the epidemics of typhoid fever and cholera with which Britain was plagued until towards the end of the nineteenth century.

Victorian sanitary and social reformers noted the coincidence of bad drainage and disease but, lacking modern knowledge of the means of spread of infection, they drew the wrong conclusions. They became obsessed with the idea that bad drains were responsible for all the ills to which the flesh is heir. Not only the diseases already mentioned were attributed to this cause but such unlikely candidates as diphtheria, scarlet fever, smallpox and tuberculosis were blamed upon the malign influence of ‘drain air’.

The results of this misapprehension were almost wholly beneficial. The provision of watertight, self-cleansing drainage systems prevented the contamination of water supplies and water-borne cholera and typhoid were virtually eliminated. Improved domestic ventilation-intended to get rid of ‘drain smells’-did something to reduce the incidence of air-borne droplet infections such as scarlet fever and diphtheria.

However, their determination to ‘keep drain smells out of the home’ resulted in the cumbersome and obsolescent ‘two pipe’ system of drainage which remained with us until well on into the 1960s. It is, in fact, only in the last few years that the modern ‘single stack’ drainage system has ceased to be regarded with suspicion by at least some plumbers and environmental health engineers. The purpose of the two-pipe system was to keep as much of the drainage system as possible outside the walls of the house. Barriers were interposed to remove the least possibility of gases from the drain or sewer entering the home. Since the majority of houses in this country were built prior to the 1960s, the two-pipe system of drainage is the one most likely to be encountered in existing buildings,

The two-pipe system makes a firm distinction between ‘soil’ and ‘waste’ appliances. W.c.s, urinals and slop-sinks are soil appliances although, of course, only the first of these is likely to be found in the home. Soil appliances could be connected directly to the drainage system. The trap built into the appliance afforded a barrier against ‘drain air’. A further precaution is that the w.c. compartment must be adequately ventilated-by means of a window opening to the external air or by mechanical means ensuring at least three air changes per hour. Furthermore there must be an ‘intervening ventilated space’ between the w.c. compartment and any room used for living, sleeping or for the cooking and preparation of food. In most homes a passageway or landing provides this intervening ventilated space though in some instances a separate lobby has to be provided.

These provisions relating to ventilation must still be observed in modern building design. Most authorities today would accept that these requirements are essential for aesthetic, rather than for health, reasons. Ground-floor w.c. outlets connect, by means of a branch underground drain, to the nearest drain inspection chamber. Outlets of upper floor w.c.s are joined to an external soil-pipe by means of a short branch. The soil-pipe is continued, open-ended, to above eaves level to provide the drain with means of ventilation.

Waste appliances-sinks, baths, showers, wash basins and bidets—could be situated in, or immediately adjacent to, habitable rooms. Two barriers are therefore provided by the ‘two pipe’ system against the possible ingress of drain air. There is the trap provided at the outlet of the appliance and, as a further precaution, the waste pipe from the appliance is required to discharge over an external, trapped yard gully. Only the gully outlet could be connected directly to the drain.

At one time indeed the byelaws of some local authorities went even further. Waste pipes from sinks, baths and basins were not permitted to discharge directly over a yard gully but into an open channel, at least 18in long, connected to the gully. The simpler the plumbing system the more satisfactory the two pipe system of drainage was, and is, likely to prove.

In the 1920s and 1930s the great majority of houses intended ‘for the working classes’ had just two plumbing fittings—an external w.c. and a shallow stoneware kitchen sink. If a bathroom was provided it was likely to be on the ground floor adjacent to the kitchen. The w.c. outlet connected directly to the underground drain. The sink and bath wastes discharged over a yard gully outside the kitchen. There were no problems. Difficulties arose when upstairs bathrooms became commonplace. An upstairs w.c. could discharge into the external soil/vent pipe, but how should the bath and wash basin wastes be dealt with?

The usual solution, in the provinces, was to run a length of rain water down-pipe discharging over a yard gully up the external wall of the house to a point just below the floor of the upstairs bathroom. A rain water hopper head would be inserted into its open upper end and the bath and basin waste pipes would discharge over this hopper head.

This was never a satisfactory arrangement. Soapy water would dry and decompose on the internal surfaces of the hopper and down-pipe. Smells resulting from this process would be carried up the down-pipe to discharge within a few feet of bathroom and bedroom windows. Thus, the very drain smells that the two pipe system was designed to eliminate would find their way into the house. The risk to health may have been minimal but the nuisance from smell was undeniable.

Recognising the defects of the rain water hopper head, the drainage byelaws of some local authorities forbade their use. They required the main waste pipe to discharge over a gully but insisted that the upper end should be treated in exactly the same way as the soil/vent pipe-it had to be taken open-ended to above eaves level. Branch waste pipes were connected to it in the same way that branch soil pipes were joined to the main soil and vent pipe. This arrangement was acceptable for one and two storey buildings with basic plumbing fittings.

Its disadvantages for multi-storey blocks of flats and hotels are obvious.

Prior to World War II, what was known as the ‘one pipe’ system of above-ground drainage was becoming commonly adopted for buildings of this kind. With the ‘one-pipe’ system —as with the ‘single stack’ system that has superseded it-the distinction between soil and waste appliances is abolished and all branch soil and waste pipes discharge into a single main soil and waste pipe. The danger that attends any installation in which a number of appliances drain into a single pipe is that of loss of seal from the trap beneath the appliance.

All sanitary fittings are provided with a trap at their outlet which retains sufficient water to prevent smells from the waste pipe or drain from escaping. W.c.s and yard gullies have a built-in trap that forms an integral part of the appliance. A separate metal or plastic trap is fitted to the waste outlet of sinks, baths, basins and bidets.

The simplest kind of trap is a U bend, usually made of copper, brass or lead, having an access eye with a screw-in cap fitted into its base. More attractive in appearance is the bottle trap, the whole of the lower part of which can be unscrewed for cleaning or clearance. Traps are described as ‘P’ traps if they have a more-or-less horizontal outlet and as ‘S’ traps if they have a vertical outlet. The ‘seal’ of the trap, that prevents the passage of drain air, is the vertical distance between the outlet of the trap and the upper part of the bend. With the two-pipe system it was usual for traps to have a Wz in or 2in seal. In order to ensure that drain smells cannot pass the trap it is essential that this seal should be maintained at all times.

Loss of seal may occur from momentum—a bucket of water thrown quickly and accurately down a w.c. may go straight through the trap leaving the seal broken-by evaporation, by ‘waving out’, by self-siphonage, by induced siphonage or by compression. Momentum and evaporation rarely present serious problems. Waving out is also a relatively rare phenomenon though it may be observed at times when a gusty wind is blowing across the top of a soil and vent pipe. The aspirating effect of the wind reduces pressure in the soil pipe and the water level in any w.c. attached to it can be seen to rise and fall with the gusts of wind. This movement creates its own momentum which, in time, can break the seal of the trap.

The connection of a number of appliances to a single drainage stack substantially increases the risks of self and induced siphonage and compression-and the unpleasantness that can result from these phenomena. Some self-siphonage occurs when any sanitary appliance is discharged. Water overflowing from the trap of a bath, sink or basin completely fills the waste pipe, taking air with it and producing the partial vacuum that induces siphonic action. This is not too important where baths and sinks are concerned. They have a relatively wide diameter-37mm -waste pipe. After the siphon has been broken by air passing under the trap sufficient water will flow from the appliance to remake the seal. Basins are another matter. They have a small diameter-31mm -waste pipe that quickly fills with water as the basin discharges. There is relatively little subsequent drainage of water to recharge the trap.

With a two pipe drainage system temporary loss of seal from the trap of a waste appliance will not have serious consequences. The second line of defence, the seal of the yard gully, will remain intact. Only air from a relatively short length of waste pipe will be able to enter the room. With a one pipe or single stack system the situation is very different. Loss of seal can mean that smells from the main soil pipe, the underground drain and, in all probability, the sewer, can pass into the house. ‘Induced siphonage’ means siphonage of the water in the trap of one appliance resulting from the discharge of another appliance. Supposing, for instance, that a short length of branch waste pipe from a wash basin is connected to a waste pipe from a bath. The bath waste pipe will fill as the bath empties and water, flowing past the junction between the two waste pipes, will aspirate air from the basin branch. This will produce a partial vacuum within the branch and atmospheric pressure will push the water our of the basin’s trap, breaking the seal. ‘Compression’ can result in a temporary loss of seal. It can also produce some extremely disconcerting results! Suppose that a combined soil and waste pipe connects to the underground drain with a sharp ‘knuckle’ bend. The discharge of a bath or w.c. may completely fill the pipe at the base of the bend, if only for a few moments. The discharge of another upstairs w.c. at that time will compress the air in the soil pipe between the point of discharge and the point at which the soil pipe is temporarily obstructed. Air pressure within the pipe can then blow out the water seal of the trap of any appliance —perhaps a sink or a ground floor w.c.—connected to the soil pipe at low level.

Self-siphonage, induced siphonage and compression are prevented from occurring in a properly designed one-pipe system by ventilating the trap of each appliance. A small diameter vent pipe is taken from a point immediately behind each trap. This is connected to a main vent pipe which may be carried up to terminate open-ended above eaves level or, alternatively, connected to the main soil and vent pipe at a point at least lm above the highest soil or waste connection. Other precautions normally insisted upon in one-pipe drainage were deep seal traps and an easy bend connection between the soil pipe and the underground drain.

A typical one-pipe system installed prior to about 1960 would have been constructed in heavy iron pipe, suitably protected against corrosion, with caulked lead joints. The main soil and waste stack and the main vent pipe would normally have been against an exterior wall though there was a tendency, particularly in good class hotel construction, to situate these pipes in service ducts within the structure of the building. A first class early example of this kind of installation is to be found at the Cumberland Hotel, Marble Arch, London.

The ‘single stack’ system of above ground drainage is a natural development of the one pipe system. First introduced into this country from the U.S.A. in the years immediately following World War II it was used experimentally, and somewhat hesitantly, for some multi-storey building construction during the 1950s. Two factors that have led to its almost universal adoption for all above-ground drainage work are the development of plastic soil and waste systems and the requirement of the Building Regulations that all soil and waste pipes must be contained within the fabric of the building.

This regulation has led to a revolutionary change in the external appearance of British buildings. Virtually any building constructed before the 1960s will be seen to have its walls festooned with soil and waste pipes. The back walls of suburban domestic properties have the down-pipe and hopper head arrangement of the two pipe system. In contrast, the only visible evidence of drainage that can be seen on a building erected since the advent of the Building Regulations will be the rainwater guttering and downpipes and a short length of capped plastic vent pipe protruding a few inches above the surface of the roof.

The essential difference between one-pipe and single-stack drainage is the elimination of trap ventilation. It has been found that, provided proper attention is given to design, the risk of siphonage and compression can be eliminated without the need for an extensive, and expensive, complex of ventilating pipes.

For really successful single stack installation the building should virtually be designed round the plumbing system. Branch waste pipes should be short and laid at minimal falls. This is particularly critical where the wash basin waste is concerned. The maximum length of the wash basin branch waste pipe should be 1.68m. Where a longer branch is unavoidable it is usually necessary to ventilate the trap of the wash basin as with a one-pipe system. A small vent pipe is connected immediately behind the trap and taken upwards to connect to the main soil and waste stack at least lm above the highest connection to it. Possible alternatives might be the use of a patent self-sealing trap or the expedient of taking the outlet of the trap into a 40mm or 50mm diameter waste pipe instead of the 30mm pipe usual for basin wastes.

Deep seal traps should be used for all fittings. The w.c. branch connection should be ‘swept’ in the direction of flow and there should be an easy bend between the main stack and the underground drain.

Measures must also be taken to ensure that there is no risk of bath, basin or bidet outlets becoming fouled or obstructed by discharges from the w.c. No connection to the main stack should be made for a distance of 200mm from the centre of the point at which the w.c. branch connects to the main stack. This can pose a problem where bath, shower or bidet wastes are concerned. One solution is to offset these wastes so that they connect to the main stack below the level of the floor on which the appliance is situated. This is always inconvenient and can make a considerable addition to the cost of installation. It can be overcome by the use of the Marley collar boss. This fitting permits bath or bidet wastes to be discharged into an annular cavity between the collar and the connection taking the w.c. branch. /

Although single stack drainage is now commonly used for all waste drainage from all kinds of building it has few, if any, advantages over two pipe drainage for single storey development. Many designers prefer to limit the use of the single stack to upper floors in two or multistorey buildings.

Ground floor sinks, baths and basins can still discharge by means of short waste pipes over yard gullies. A very sensible provision of the Building Regulations requires that such waste pipes shall discharge above the level of the water in the gully but below the grid. This means that yards cannot flood with drainage water as a result of grids becoming blocked with leaves and other debris and also ensures that the full force of the waste pipe discharge is available to cleanse the gully.

Back and side inlet gullies are available to permit easy compliance with this requirement. Alternatively gully grids are available with slots in them through which the waste pipes can be passed.

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