Where vent stacks pass through a roof the openings around the pipes should be made perfectly storm-tight, by flashings of sheet lead or copper. A good method of flashing for use in most climates is shown in Fig. 24. This consists of a collar (a) of lead soldered to a flange (b) at the same angle as the pitch of the roof; the top edge of the collar is made tight around the pipe by working a bead (c) tightly around the pipe with a gasket (e) of asbestos packing and white lead in between. This makes not only a tight connection, but also a flexible one that is not affected by a settlement of the pipe.

In severe climates, where there is danger of hoar frost stopping the outlet of the stack, it can be checked to a great extent, and in some cases entirely prevented by flashing the opening in the manner shown in Fig. 25. In this method the pipe extends to a height of about 12 inches above the roof, and the opening through the roof is made 2 inches larger than the pipe, so there will be a 1-inch space all around it. The collar of the flashing is then made the same size as the hole in the roof, and is turned over and calked into the top of the stack as shown. This construction provides an air space around the pipe that is open to the attic and keeps the temperature of the exposed pipe at about the same temperature as the inside of the building. On tin or copper roofs the outer edge of the flange of either style of flashing should be well soldered to the metal; on tar, cement or asphalt roofs the flange should be placed between layers of roofing material and the finishing course laid upon it. On wooden or slate-shingled roofs the upper edge of the flange should extend at least two courses up under the shingles, and the exposed edges on wood-shingled roofs well tacked down with short nails having large heads.

Fig. 24

Sheet copper for flashings should weigh at least 16 ounces to the square foot, and sheet lead at least 6 pounds to the square foot. The roof flange of a flashing should extend at least 6 inches on all sides of the stack. Outlets to stacks above a roof should be free from caps, cowls or bends, as they obstruct the opening, easily choke with hoar frost, and interfere with the free passage of air. Outlets should be located well away from the windows, doors or ventilating shafts leading to the interior of a building, and, when practicable, should be a reasonably safe distance away from smoke flues. On buildings that have the ordinary pitch roof and on all kinds of roofs in cold climates vent stacks need only extend from 12 to 18 inches through the roof. On buildings with flat roofs having parapet walls the top of the stacks should extend at least 6 inches above the level of the walls. On tenement houses or other buildings where the roof is easily accessible, and used by the inmates, the top of all stacks should be at least 5 feet above the roof, and the outlet protected by a heavy brass wire basket securely fastened to the opening to prevent articles being dropped into the stacks.

Fig. 25

Size Of Soil And Waste Stacks

Under ordinary conditions, soil or waste stacks need not be larger in diameter than the largest branch discharging into them. However, in the case of large buildings many stories in height and with many fixtures connecting to a stack, the size should be determined by the rule for calculating the size of drain pipes, the head or fall being considered the same as for the corresponding sizes of main house drain.

Size Of Vent Stacks

Vent stacks should never be smaller than 2 inches in diameter. When the accompanying soil or waste stack is 4 inches or larger in diameter, the vent stack need have an area of only one-fourth (a diameter of one-half) that of the soil or waste pipe. The reason vent stacks can be smaller than soil or waste stacks is that air flows more than four times as readily as water, hence, enough air can flow in a vent pipe of given size to prevent a vacuum forming in a soil or waste pipe of four times its capacity. The proper size of vent stack to accompany any size soil or waste stacks can be found in the following table:

Table VII - Size Of Vent Stacks