The double T-Y is another fitting which should not be used on horizontal work, as the waste entering one side of the fitting will cross and enter the branch on the other side, instead of entering the main line only.

Vertical stacks should be straight whenever possible, but when offsets are necessary they should be made with 45-degree fittings.

Any building in which plumbing fixtures of any description are installed should have at least one stack extending through the roof.

Whenever a vertical line receives waste from a fixture on any floor, it should extend through the roof, if 10 ft. or more from the nearest stack.

The following sizes of soil and waste pipes should be followed:

Each soil pipe should be at least........................... 4 in.

Main soil pipes for water closets on two, three, or four floors. 4 "

Main soil pipe for water closets on five or more floors........ 5 "

Main soil pipe for tenement houses of more than three stories.. 5 "

Branch soil pipes....................................... 4 "

Main waste pipe for kitchen sink.......................... 2 "

Main waste pipe for sinks, lavatories, or laundry tubs on five or more floors ..................................... 3 "

Main waste pipe for six or more fixtures, not less than........ 3 "

The following sizes for main vent lines should be followed:

Main vent for 4-in. soil-pipe line.......................... 2 in.

Long branch vent lines ................................. 2 "

Main vent for stack serving sink, laundry tubs, and lavatories. 2 "

Main vent for line of water closets on three or more floors.... 3 "

Main vents for tenement houses of more than three floors.... 3 "

Additional main-vent sizes will be found under Plate 36.

The main vent line may be run independently through the roof, or it may be reconnected to the main soil or waste pipe above the highest fixture vent. The latter connection is shown in Fig. C, Plate 13, and it has certain advantages over the independent roof connection. In the first place, it saves cutting an extra hole through the roof, and the smaller the number of pipes passing through the roof the less will be the danger of leakage, and the less unsightly will the roof appear. In addition, the circulation of air through the vent system will be better, owing to the influence of the warmer air of the main stack in keeping the air in motion.

This connection may be made into the vent fitting shown in Fig. C, into an inverted Y-branch, and in the use of wrought-iron main vent by means of a tapped fitting on the main stack. When the pipe is to be increased through the roof, the vent line may enter the main stack through an increaser, such as shown in Fig. F, provided with a side hub or tapping. The lower end of the main vent should be reconnected to the main stack, as shown in Fig. D.

This connection allows all condensation and collection of rust and scale to be carried off into the drainage system, and in addition, it gives rigidity to the work, the danger from leakage due to accidental blows, settling, shrinkage, etc., being largely eliminated.

Fig. E shows a very common but undesirable method of connecting the lower end of the main vent to the fixture vent of the lowest fixture.

It will be plainly seen that all scale falling through the main vent will collect in the bend at the foot of the line, and such collections of rust and scale often present a serious difficulty.

In Fig. F is shown a common method of making the roof connection. Some plumbing ordinances require a 2-in. stack to be increased to 3 in. in passing through the roof, and a 3-in. stack increased to 4 in., that is, each pipe less than 4 in. in size shall be increased one inch in size.

Most ordinances, however, allow no pipe of less size than 4 in. to pass through the roof. The latter is the preferable method, for the reason that 2 and 3 in. and smaller sizes of pipe will sometimes entirely close up with hoar frost formed about the opening above the roof, this accumulation being produced from the steam rising through the stack. In increasing the size of pipe, long increasers, such as shown in Fig. F, should be used, and the increaser located not less than one foot below the roof.

Caps or cowls should not be used to cover roof pipes. In the case of roof pipes of tenement houses whose roofs are used by the inmates, the openings should be protected by the use of a wire basket, but under other conditions it is preferable to keep the opening entirely free, as even the wire basket gives opportunity for the collection of frost.

The roof pipe should extend two feet above the roof. Whenever the roof is used by the inmates, all pipes passing through it should be carried up at least 6 ft. above the roof. Roof pipes should terminate not less than 3 ft. above any window, door, or air shaft that may be within a distance of 12 ft., and such pipes should not terminate within 6 ft. of any chimney or flue.

When carried above the roof, pipes should be securely stayed to the roof. Many styles of roof flanges are in use, the most common probably being that of Fig. F, in which the hub is riveted to a flange of sheet copper, which may be slipped under the slate or shingles above the pipe, and over them below it. Adjustable roof flanges will fit a roof of any pitch. A very desirable form is one in the use of which the plumber is not required to go onto the roof to pour the lead joint.

A change has in recent years come about in the use of materials on the drainage and vent systems of the plumbing system. Years ago all piping of the plumbing system was of lead. This was followed by the use of cast iron on both main drainage lines and vent lines, with branch wastes and vents of lead.

Although much cast iron is still used on main vent lines, a large part of the main vents of modern plumbing systems are now constructed of wrought-iron pipe, and the branch vents as well, until at the present time a large majority of trap vents are of wrought iron, excepting in certain sections of the country that still adhere to lead work.

The present tendency, especially on large work in the large cities, is toward the use of wrought iron and brass for fixture wastes, and a very excellent feature to be noted in their use is that cleanouts at bends may be used, whereas this was not done in the use of lead wastes. The use of brass pipe for drainage purposes is excellent practice, but the cost of brass pipe is so great that, excepting on the higher grades of work, its use is limited.