STORM-WATER CALCULATIONS

(c) Roof drains, overflow drains, and rainwater piping installed within the con­struction of the building shall be tested in conformity with the provisions of this Code for testing drain, waste, and vent systems.

Part В
Roof Drains

A 2 Materials: Roof drains shall be of cast iron, copper, lead, or other corrosion resisting material.

A 2.1 Strainers:

(a) Roof drains shall be equipped with strainers extending not less than four (4) inches (101.6 mm) above the surface of the roof immediately adjacent to the drain. Strainers shall have minimum inlet area one and one-half (IV2) times the pipe to which it is connected.

(b) Roof deck strainers for use on sun decks, parking decks, and similar occupied areas may be of an approved flat-surface type which is level with the deck. Such drains shall have an inlet area not less than two (2) times the area of the pipe to which the drain is connected.

(c) Roof drains passing through the roof into the interior of a building shall be made watertight at the roof line by the use of a suitable flashing material.

PartC

Sizing of Rainwater Piping

A 3.1 Vertical rainwater piping shall be sized in accordance with Fig. 8.3. Figure 8.3 is based upon maximum inches (mm) of rainfall per hour falling upon a given roof area in square feet (m2). Consult local rainfall figures to determine max­imum rainfall per hour.

A 3.2 Vertical Wall Areas. Where vertical walls project above a roof so as to permit storm water to drain to the roof area below the adjacent roof area may be computed from Fig. 8.3 as follows:

(a) For one (1) wall—add fifty (50) percent of the wall area to the roof area figures.

(b) For two (2) adjacent walls—add thirty-five (35) percent of the total wall areas.

(c) Two (2) walls opposite of same heights—add no additional area.

(d) Two (2) walls opposite of differing heights—add fifty (50) percent of wall area above top of lower wall.

(e) Walls on three (3) sides—add fifty (50) percent of area of the inner wall below the top of the lowest wall, plus allowance for area of wall above top of lowest wall per (b) and (d).

(f) Walls on four (4) sides—no allowance for all areas below top of lowest wall — add for areas above top of lowest wall per (a), (b), (d), and (e).

FIGURE 6.2 ■ Rainwater code requirements. (Courtesy of Uniform Plumbing Code)

and the illustrations here may not be up to speed with your current, local code. Check you own code requirements and use the tables here as examples of how to use what you have.

Now that you’ve had a chance to look over the illustrations, you may have a solid understanding of how to size a rainwater system. If you do, that’s great. But, maybe you have a little confusion that needs to be cleared up. Let me go over a few of the points that some plumbers from my classes have had trou­ble with. Start by looking at Figure 6.2, part C. In category A 3.2 of Figure 6.2, I want you to look at letter A. The code tells you to figure 50 percent of

Size of Horizontal Rainwater Piping

Size of Gutters

Maximum rainfall in inches per hour

Size of Gutters

The sizing example you have just seen is a good, step-by-step example of how to size a drainage system for storm water. You’ve seen actual code ex­amples and rulings, but remember that these codes are subject to change and may not be the codes being used in your area. Consult your local plumbing code for current, applicable code requirements in your region.

SECONDARY (EMERGENCY) ROOF DRAINS Secondary Drainage Required

Secondary (emergency) roof drains or scuppers shall be provided where the roof perimeter construction extends above the roof in such a manner that water would be entrapped should the primary drains allow buildup for any reason.

Separate Systems Required

Secondary roof drain systems shall have piping and point of discharge separate from the primary system. Discharge shall be above grade in a location which would nor­mally be observed by the building occupants or maintenance personnel.

Maximum Rainfall Rate for Secondary Drains

Secondary (emergency) roof drain systems or scuppers shall be sized based on the flow rate caused by the 100 year 15 minute precipitation as indicated in Fig. 8.12. The flow through the primary system shall not be considered when sizing the sec­ondary roof drain system.

CONVERSION OF ROOF AREA

General

Where roof drainage is connected to a combined sewer, the drainage area may be converted to equivalent fixture unit loads.

Less Than 256 Fixture Units

When the total fixture unit load on the combined drain is less than 256 fixture units, the equivalent drainage area in horizontal projection shall be taken as 1000 sq ft (92.9 m2).

Greater Than 256 Fixture Units

When the total fixture unit load exceeds 256 fixture units, each additional fixture unit shall be considered the equivalent of 3.9 ft2 (0.3623 m2) of drainage area.

Rainfall Other Than 4 Inches (102 mm) Per Hour

If the rainfall to be provided for is more or less than 4 inches (102 mm) per hour, the 1,000 sq ft (92.9 m2) equivalent in 1110.2 and the 3.9 sq ft (0.3623 m2) in 1110.3 shall be adjusted by multiplying by 4 and dividing by the rainfall per hour to be provided for.

VALUES FOR CONTINUOUS FLOW

Where there is a continuous or semicontinuous discharge into the building storm drain or building storm sewer, as from a pump, ejector, air conditioning plant, or similar device, each gallon per minute of such discharge shall be computed as being equivalent to 24 sq ft (2.23 m2) of roof area, based upon a 4-inch (102 mm) rainfall.

FIGURE 6.10A ■ Rainwater code requirements. (Courtesy of Standard Plumbing Code)

BACKWATER VALVES

Fixture Branches

Backwater valves shall be installed in the branch of the building drain which re­ceives only the discharge from fixtures located within such branch and shall be located below grade.

Material

Backwater valves shall have all bearing parts of corrosion resistant material. Back­water valves shall comply with ANSI/ASME A112.14.1 or CSA B181.1, CSAB181.2.

Seal

Backwater valves shall be so constructed as to insure a mechanical seal against backflow.

Diameter

Backwater valves, when fully opened, shall have a capacity not less than that of the pipes in which they are installed.

Location

Backwater valves shall be so installed to be accessible for service and repair. APPENDIX REFERENCES

Additional provisions for storm drainage are found in Appendix В-Roof Drain Sizing Method. These provisions are applicable only where specifically included in the adopting ordinance.

FIGURE 6.10B ■ Rainwater code requirements. (Courtesy of Standard Plumbing Code)

APPENDIX В