It is a mistake to fix an air-extracting cowl on the top of a soil-pipe, as this checks the upward current in still weather, and in high winds prevents the inrush of air required by the trap-ventilating pipes when a fitting is discharged. A simple conical or mushroom top of galvanised iron, copper, or lead is better, but the open wire guard is now generally used. This may be of "dome" or "globe" shape, and can be obtained in sizes to fit pipes from 2 in. to 6 in. in diameter. The wire may be either galvanised-iron or copper, but the latter ought always to be used, as galvanised wire is soon corroded.

Disconnection

Some years ago the system of disconnecting soil-pipes from drains was introduced and was strongly advocated by many sanitarians. If the drains are long and foul, or if they are not disconnected from the sewer, a strong case can undoubtedly be made out in favour of the disconnection and foot-ventilation of the soil-pipes discharging into the drains; if the soil-pipes are not disconnected, the unsealing of a single trap will at once allow the foul air of the drains or sewers to enter the house, and how foul this sometimes is, all who have had any experience in sanitary work know. But if the drains are self-cleansirg and are disconnected from the sewer, and if proper precautions are taken to prevent the closet and hopper traps being siphoned, there can be no objection to utilising the soil-pipes as drain-ventilators. The cost of a separate drain-ventilating pipe is saved, and the disadvantages which are almost inseparable from disconnection will be avoided. These disadvantages include splashing, retardation of the flow due to the additional trap at the foot of the soil-pipe, and occasional puffs of foul air through the grating over the trap. The latter may be to some extent obviated by substituting for the grating an air-inlet pipe carried to a suitable position and fitted with a mica-flap ventilator arranged to prevent the egress of air. The disconnection of soil-pipes has not come into general favour, and is at the present time explicitly prohibited by nearly all sanitary authorities, including the London County Council.

Diameter

Until recently, soil-pipes 4 in. in diameter were almost invariably used, except in stacks serving a number of fittings, when still larger pipes were fixed, but pipes 3 1/2 in. in diameter are now considered large enough for ordinary stacks, and many sanitarians advocate still smaller pipes, as being more thoroughly cleansed by the small flush allowed and as being also less unsightly and less costly. There is, however, a greater risk of trap-siphonage when small pipes are used, and it is unwise to reduce the diameter below 3 in.; 3 1/2 in. pipes are the smallest allowed by the London County Council for water-closets, and 3-in. pipes for slop-hoppers and urinals. The outlets of many siphonic closets are only 3 in. in diameter, and the branch pipes may be of the same size. Very long soil-pipes subject the traps of the fittings to greater strains than short pipes, and ought therefore to be of larger diameter, but it is very seldom indeed that pipes larger than 4 in. are required even for lofty stacks serving a number of fittings.

Materials

The materials now almost invariably used for soil-pipes are cast-iron and lead; opinions have been sharply divided as to their relative merits, but there can be no doubt that cast-iron is now in the greater favour. The fact is that neither material is perfect. Zinc and stoneware have also been used, but are quite unsuitable, the former on account of its rapid corrosion, and the latter because of the number of joints, the impossibility of making the joints permanently tight, the difficulty of fixing, and other defects.

Cast-iron soil-pipes, unless thoroughly protected inside by an impervious coating, are certain to corrode, and if the joint at the foot is improperly made the rust may accumulate there and ultimately choke the pipe. The pipes must be painted outside every year or two to prevent external corrosion. Cast-iron is also a brittle material, and may be broken or cracked by a heavy blow, and the damaged pipe will be difficult to remove, and will be of no value when removed. Again, every junction in a cast-iron soil-pipe necessitates three joints, one for the branch pipe and one at each end of the junction-pipe, while only one joint is required for each branch in a lead soil-pipe. More joints are also required in the straight portions, as cast-iron pipes are made in 6 ft. (or, occasionally, 9-ft.) lengths, and drawn-lead pipes in 10-ft. lengths. On the other hand, lead pipes, on account of their softness, are easily dented by blows, and dragged and twisted by the sun and by gravitation; they are more costly, and require more skill in fixing. They are, however, practically incorrodible, and a cracked pipe can be repaired with solder or can be cut out and replaced by a new length without much difficulty, and the old pipe will fetch a fair price. A few years ago a lead-lined iron pipe was designed to combine the advantages of the two materials, and has already been extensively used. Some sanitary authorities allow either lead or iron pipes to be used externally, but insist on internal pipes being of drawn lead; this is now the case in London.

1. Lead. - Seamed-lead soil-pipes ought never to be used, as the soldered seams are often carelessly made, and are soon destroyed by the sewage gases. Lead soil-pipes ought to be of drawn lead, the metal to be of uniform thickness. They are generally manufactured in 10 ft. lengths, and can be obtained in sizes rising by 1/2 in. from 2 1/2 in. to 6 in., and in weights from about 5 1/2 lbs. to 10 lbs. per superficial foot.

Weight