Rainwater Drainage

Exterior sheet-metal gutters and leaders for rainwater drainage are not normally included as part of the plumbing work. Interior leaders or storm-water drains, however, are considered part of the plumbing work. Depending on local codes or ordinances in the locality, rainwater from various roof areas may or may not be led into the sanitary sewer (Art. 14.11). Where separate rainwater leaders or storm drains are used, the building drains are then called sanitary drains because they convey only the wastes from the various plumbing fixtures in the building.
Interior storm-water drain pipes may be made of cast iron, steel, plastic, or wrought iron. All joints must be tight enough to prevent gas and water leakage.
When a combined system is utilized, it is common practice to insert a cast-iron running trap between the storm drain and the building drain to maintain a trap seal on the storm drain at all times. Use of a combined system does not eliminate the need for separate drains and vents for wastewater. All codes prohibit use of storm drains for any type of wastewater.
Water falling on the roof may be led either to a gutter, from where it flows to a downspout (Fig. 14.10a), or it may be directed to a roof drainage device by means of a slope in the roof surface. Many different roof drainage devices, such as roof drains (Fig. 14.10b) and parapet drains, are available for different roof constructions and storm-water conditions.

Most plumbing codes include provisions to prevent the collapse of the building  structure due to water ponding on the roof because of a clogged storm drainage system. In most cases, these codes require installation of overflow roof drains or parapet overflow scuppers to relieve water from the roof in the event of such a condition. Local authorities should be contacted to determine what requirements apply in their jurisdiction.
When vertical leaders are extremely long, it is common practice to install an expansion joint between the leader inlet and the leader itself. Figure 14.10c shows an example of a connection of a building storm drain to a storm sewer. When the drain must be connected to a combined sanitary-storm sewer, a trap should be installed before the connection to the sewer (Fig. 14.10d).
Sizes of vertical leaders and horizontal storm drains depend on the roof area to be drained. Table 14.9 indicates the maximum horizontal projection of roof area permitted for various sizes of leaders and horizontal storm drains.
Semicircular gutters are sized on the basis of the maximum projected roof area served. Table 14.10 shows how gutter capacity varies with diameter and pitch.

Where maximum rainfall is either more than, or less than, 1 in/hr, refer to the plumbing code for suitable correction factors.
Drains for building yards, subsoil drainage systems, and exterior areaways may also be connected to the storm drainage system. Where this is not possible, these drains may be run to a dry well. When a dry well is used, only the discharge from these devices may be run to the dry well.

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