Storm Drainage Systems

Learning Objectives

After completing this chapter, you will be able to:

4.Identify the scope and applicability of storm drainage code provisions within the 2021 IPC and IRC.

5.Distinguish between combined, separate, and storm sewer systems, and apply the correct design criteria for each.

6.Calculate roof drain leader sizing, horizontal storm drain slope and capacity, and secondary (overflow) drainage requirements.

7.Apply the correct minimum number of roof drains based on roof area and rainfall rate.

8.Specify proper materials, fittings, and cleanout locations for storm drainage piping.

9.Navigate the code efficiently to locate tables, formulas, and exceptions during the open-book exam.

1.1 Scope and System Classification

Storm drainage systems collect and convey rainwater, melted snow, and other precipitation away from building roofs, paved surfaces, and foundations. The 2021 IPC covers storm drainage in Chapter 11, while the 2021 IRC covers residential systems in Chapter 33 (Part VII). The IPC applies to commercial, multi-family, and institutional buildings; the IRC applies to one- and two-family dwellings and townhouses.

Key distinction: The IPC permits a combined system (sanitary + storm) only where the public sewer is a combined sewer. The IRC prohibits combined systems entirely. A master plumber must verify local jurisdiction ordinances before connecting storm water to a sanitary sewer.

Separate systems are the default: storm water goes to a storm sewer, dry well, or approved drainage field; sanitary waste goes to a sanitary sewer or septic system. The code also addresses subsoil drainage (groundwater) separately from roof runoff.

1.2 Roof Drainage Design Fundamentals

Roof Area Calculation

The code requires that storm drainage be sized based on the projected horizontal roof area (not the sloped surface area). For a sloped roof, use the horizontal footprint. For walls that drain onto a roof (parapet walls, adjacent higher walls), add 50% of the wall area that drains onto the roof surface.

Formula:

Roof area (ft²) = Length × Width (horizontal projection) + 0.5 × (wall area draining onto roof)

Rainfall Rate (Intensity)

The IPC uses a 100-year, 60-minute rainfall rate (inches per hour, in/hr) from the local jurisdiction or from the code’s rainfall map/table. The IRC uses a 1-hour rainfall rate from Figure R301.2(1) or local data. For exam purposes, the rainfall rate is typically given in the problem statement.

Minimum Number of Roof Drains

The code requires at least two roof drains on roofs with a projected area greater than 10,000 ft². Roofs 10,000 ft² or less may use one drain, provided the scupper or overflow system can handle the full design flow. This is a safety redundancy requirement—if one drain clogs, the second prevents structural failure.

1.3 Sizing Roof Drain Leaders (Vertical Pipes)

Roof drain leaders (vertical conductors) are sized using Table 1106.2 in the IPC (or Table 33.2 in the IRC). The table provides the maximum roof area (ft²) that can be served by a given pipe diameter at various rainfall rates.

Key table logic: For a given pipe diameter, the allowable roof area decreases as rainfall rate increases. For example, a 4-inch leader at 1 in/hr may serve 7,300 ft², but at 6 in/hr only 1,220 ft². The table assumes the pipe is flowing full, not the roof.

Exam trap: Do not confuse leader sizing with horizontal drain sizing. Leaders are vertical; horizontal drains use a different table (1106.3) and require slope.

Procedure:

32.Determine the tributary roof area per leader.

33.Determine the design rainfall rate.

34.Select the smallest diameter leader from Table 1106.2 that serves ≥ the tributary area.

1.4 Sizing Horizontal Storm Drains

Horizontal storm drains are sized using Table 1106.3 in the IPC (Table 33.3 in the IRC). This table provides the maximum roof area served at three slopes: ⅛ in/ft, ¼ in/ft, and ½ in/ft.

Critical concept: Horizontal drains must be sloped to maintain self-cleaning velocity. The minimum slope is ⅛ in/ft for pipes 3 inches and larger. Smaller pipes (2 inches or less) require ¼ in/ft minimum.

Table logic: At a given diameter, a steeper slope allows a larger tributary roof area. For example, a 6-inch pipe at ⅛ in/ft slope may serve 13,600 ft², but at ½ in/ft slope it serves 27,200 ft².

Exam trap: The table is based on the pipe flowing half full at the given slope—not full. This is different from sanitary drainage design, which uses full-flow assumptions for sizing.

Combined vertical + horizontal runs: When a leader turns horizontal, the horizontal portion must be re-checked against Table 1106.3. The vertical leader may be one size, but the horizontal run may need to be larger due to slope limitations.

1.5 Secondary (Overflow) Drainage

The code requires secondary drainage for roofs where primary drains are blocked. This is a critical safety provision to prevent roof collapse from ponding water.

Two acceptable methods:

48.Overflow scuppers – openings in parapet walls or roof edges, sized per the code’s hydraulic requirements.

49.Secondary roof drains – piped separately to discharge at an approved location (often visible at the building exterior).

Sizing requirement: The secondary system must be sized to handle the full design rainfall rate, independent of the primary system. The combined capacity of primary + secondary must equal 100% of the design flow, with each system capable of 100% individually.

Discharge location: Secondary drains must discharge to a location where the flow is visible (e.g., at grade, or through a wall scupper) so building occupants or maintenance staff can detect a primary drain blockage.

Exam trap: Secondary drains cannot connect to the primary storm drain system. They must be independent and discharge separately.

1.6 Gutters and Rainwater Collection

Gutters (Eaves Troughs)

The IPC and IRC require gutters to be sized to the roof area they serve, using the same rainfall rate as the rest of the system. Gutter cross-sectional area must be proportional to the roof area. The code does not provide a single table for gutters; instead, it requires the gutter to have a cross-sectional area at least equal to the leader it discharges into, or be sized by engineering calculation.

Field practice: For rectangular gutters, the cross-sectional area is width × depth. For half-round gutters, use the area of a half-circle. A common rule: a 5-inch K-style gutter serves approximately 750 ft² at 1 in/hr, but this varies with pitch and rainfall rate.

Rainwater Harvesting

Where rainwater is collected for reuse (irrigation, toilet flushing), the code requires:

A label on the storage tank indicating non-potable water.

Cross-connection control – the rainwater system must have an air gap or reduced pressure principle backflow preventer before any connection to the potable system.

Overflow for the storage tank, sized to handle the full roof runoff.

The storage tank must be vented and accessible for cleaning.

1.7 Materials and Fittings

Storm drainage piping may be made of the same materials as sanitary drainage, with one important exception: storm water is not corrosive in the same way as sanitary waste, so lighter materials may be used where approved. Common materials:

Cast iron – for above-ground and underground, especially in commercial buildings.

PVC (Schedule 40 or DWV) – for underground and above-ground, where permitted by local code.

Copper – for small leaders and exposed work.

Galvanized steel – for exposed industrial applications.

Concrete or clay – for large underground storm sewers on the building site.

Fittings: The code requires drainage fittings (sweep elbows, wye branches) rather than pressure fittings (short-radius elbows, tee-wyes). A cleanout is required at the base of each vertical leader, and at intervals not exceeding 100 feet on horizontal runs.

Exam trap: Storm drainage cleanouts must be accessible. A cleanout at the base of a leader must be above grade or in an accessible pit. The code does not permit hiding cleanouts behind finished walls without an access panel.

1.8 Subsoil and Foundation Drainage

Where groundwater is present, the code requires subsoil drainage around footings and below slabs. This system includes:

Perforated pipe (4-inch minimum) laid on a gravel bed.

Filter fabric or gravel to prevent soil migration.

Discharge to a storm sewer, daylight, or sump pump.

Sump pumps are required where gravity discharge is impossible. The pump must be sized to handle the anticipated groundwater flow, and the discharge pipe must be protected from freezing.

Exam trap: Subsoil drainage cannot connect to a sanitary sewer. It must discharge to an approved storm water location.

1.9 Code Navigation

For an open-book exam, you must know where to look. Use this map:

Concept	IPC Location	IRC Location
Definitions (roof drain, leader, conductor)	Chapter 2	Chapter 2
Roof area calculation	Section 1106.1	Section 33.1
Rainfall rate data	Section 1106.1 (local data)	Figure R301.2(1)
Vertical leader sizing	Table 1106.2	Table 33.2
Horizontal drain sizing	Table 1106.3	Table 33.3
Minimum number of drains	Section 1106.4	Section 33.2
Secondary drainage	Section 1107	Section 33.3
Gutters	Section 1106.6	Section 33.4
Cleanouts	Section 1105 (via Ch. 7)	Section 33.5
Materials	Section 1103	Section 33.6
Subsoil drainage	Section 1108	Section 33.7
Rainwater harvesting	Section 1109	Not covered (use IPC)

Exam strategy: Memorize the table numbers first. The most common calculation questions reference Tables 1106.2 and 1106.3. If you can flip directly to these tables, you save 2–3 minutes per question.

1.10 Practical Field Points for the Master Plumber

89.Verify local rainfall data. The code map may not reflect your specific jurisdiction. Always check with the local building department before submitting plans.

90.Roof drain installation details. Roof drains must be installed with a flashing flange that extends up the roof membrane. The clamping ring must be set at or below the finished roof surface to allow positive drainage to the drain.

91.Expansion and contraction. Long horizontal runs of PVC or cast iron need expansion fittings where temperature changes are significant. Leaders on tall buildings may need expansion joints at each floor.

92.Ice and snow considerations. In Colorado, roof drains on unheated buildings may freeze. Consider heat tracing or interior leaders in conditioned spaces.

93.Coordination with structural. Roof drains must be placed at structural low points. The plumber must coordinate with the architect and structural engineer to ensure the roof slope directs water to the drains.

94.Testing. Storm drainage systems should be tested by filling with water (hydrostatic test) or by smoke test for leaks. The code requires testing of all drainage systems before covering.

1.11 Common Exam Traps

Using the wrong table: Vertical leaders use Table 1106.2; horizontal drains use Table 1106.3. They are not interchangeable.

Forgetting the 50% wall area addition: When a wall drains onto a roof, you must add half the wall area to the roof area.

Ignoring slope: A horizontal drain at ⅛ in/ft slope carries less than the same pipe at ½ in/ft. Always check the slope column.

One drain on a large roof: The code requires two drains above 10,000 ft². If the answer says one drain for a 12,000 ft² roof, it is wrong—unless a scupper system is provided.

Secondary drain connection: Secondary drains must be independent. Connecting them to the primary system defeats the purpose.

Cleanout spacing: 100 feet maximum on horizontal runs, and at the base of each leader. A 150-foot run without a cleanout is a code violation.

Combined systems: Only allowed where the public sewer is combined. In the IRC, never allowed.

1.12 Design Example (Conceptual)

A 200 ft × 150 ft flat roof (30,000 ft²) in a jurisdiction with a 4 in/hr rainfall rate. The roof drains to four leaders, each serving 7,500 ft².

Minimum drains: 30,000 ft² > 10,000 ft², so at least 2 drains are required. Four are provided—acceptable.

Leader sizing: From Table 1106.2 at 4 in/hr, a 5-inch leader serves approximately 7,600 ft². Use 5-inch leaders.

Horizontal drain: The 5-inch leader turns horizontal and runs 60 feet at ¼ in/ft slope. From Table 1106.3 at ¼ in/ft, a 5-inch pipe serves approximately 13,600 ft² at 4 in/hr. The 7,500 ft² tributary area is acceptable.

Secondary drainage: Provide overflow scuppers or secondary drains sized for 30,000 ft² at 4 in/hr, independent of the primary system.

Summary

Storm drainage is a safety-critical system. The code’s primary concern is preventing roof collapse from ponding water, which is why secondary drainage and minimum drain counts are mandatory. As a master plumber, you are responsible for ensuring that the design accounts for local rainfall, correct pipe sizing, proper slope, and independent overflow paths. In the exam, your speed depends on knowing the table numbers and the logic of the sizing procedure. Practice flipping to Tables 1106.2 and 1106.3 until you can locate them in under 10 seconds.

Storm Drainage Systems