Storm and Roof Drainage

Storm and Roof Drainage

Learning Objectives

By the end of this chapter, you will be able to:

1.1 Scope and System Classification

Storm drainage is addressed in Chapter 11 of the 2018 IPC. This chapter governs the collection, conveyance, and disposal of rainwater, groundwater, and similar clear-water discharges. As a master plumber, you are responsible for ensuring that the storm drainage system is separate from the sanitary drainage system unless a combined sewer is explicitly permitted by the local jurisdiction. The code presumes separation; any combination requires authority approval.

The IPC defines three distinct systems you must be able to differentiate:

The code requires that all roofs, paved courtyards, and similar surfaces be drained into a storm sewer or an approved point of disposal. The master plumber must verify that the point of disposal (e.g., dry well, detention basin, public storm main) is approved by the authority having jurisdiction (AHJ).

1.2 Roof Drainage Design Fundamentals

Roof drainage begins with the roof itself. The IPC requires that roofs be sloped toward roof drains or scuppers. While the structural design of the roof is not the plumber’s responsibility, the placement and sizing of roof drains is. The code requires that roof drains be placed at the lowest points of the roof, and that the roof structure be designed to support the anticipated water load if drainage is blocked.

Key definitions you must know:

The IPC requires that roof drains be sized based on the projected roof area and the rainfall rate for the locality. The rainfall rate is expressed in inches per hour and is determined by local climate data. The code provides a map of rainfall rates for the United States; you must use the rate applicable to your project location.

1.3 Sizing Roof Drains and Vertical Conductors

The sizing of roof drains and vertical conductors is based on Table 1106.2 in the 2018 IPC. This table provides the maximum roof area (in square feet) that can be served by a given pipe size for various rainfall rates. The table is organized by:

The table assumes a vertical conductor flowing approximately full bore. For a given rainfall rate, you find the allowable roof area for each pipe size. For example, at a 4-inch-per-hour rainfall rate, a 4-inch vertical conductor may serve a certain maximum roof area; a 6-inch conductor serves a larger area.

Critical exam point: The table is for vertical conductors. Do not use it for horizontal piping. Horizontal storm drain sizing uses a different table (Table 1106.3) that incorporates slope.

To determine the required conductor size:

Practical field point: When multiple roof drains serve a single conductor, the total roof area served by that conductor must be summed. Do not size each drain independently and assume the conductor can handle the sum unless the conductor is sized for the total.

1.4 Horizontal Storm Drain Sizing

Horizontal storm drains are sized using Table 1106.3 of the 2018 IPC. This table provides the maximum roof area that can be served by horizontal piping at various slopes (typically ¼ inch per foot, ½ inch per foot, and sometimes steeper). The table is similar in structure to the sanitary drainage table but uses different capacity values because storm water is clear and flows differently than sanitary waste.

Key points:

Exam trap: Candidates often confuse the horizontal storm drain table with the vertical conductor table. Remember: vertical conductors use Table 1106.2 (no slope consideration), and horizontal drains use Table 1106.3 (slope-dependent). Both tables are in Chapter 11.

Design example logic: For a 10,000-square-foot roof at a 3-inch-per-hour rainfall rate, with a horizontal drain at ¼ inch per foot slope, you would enter Table 1106.3 and select a pipe size whose allowable area equals or exceeds 10,000 square feet. If the table lists 8-inch pipe at 9,500 square feet and 10-inch pipe at 14,000 square feet, you must select the 10-inch pipe.

1.5 Gutters and Scuppers

The IPC addresses gutters (roof gutters) and scuppers (openings in parapet walls or roof edges) in Chapter 11. While the code does not provide a full design table for gutter sizing, it requires that gutters be sized to handle the same rainfall intensity as the roof drain system. The code references standard engineering practice for gutter sizing, and many jurisdictions accept the manufacturer’s published capacities.

Scuppers are often used as a secondary (emergency) drainage path. The code requires that scuppers be sized to prevent water accumulation on the roof during a design storm. The opening area of a scupper must be sufficient to pass the required flow. The IPC provides a formula or table for scupper sizing based on the head of water above the scupper opening.

Field responsibility: As the master plumber, you must coordinate with the architect or structural engineer to ensure scupper locations and sizes are consistent with the plumbing design. If scuppers are the primary drainage, they must be connected to conductors or leaders. If they are emergency overflow, they must discharge to an approved location (not onto walkways or public areas where ice can form).

1.6 Emergency Overflow Requirements

One of the most important code provisions in Chapter 11 is the requirement for emergency overflow drains or scuppers on roofs. The 2018 IPC requires that where roof drains are the primary drainage, an overflow system must be provided. This is to prevent roof collapse if the primary drains become clogged with debris.

The code states that the overflow system must be independent of the primary roof drain system. It can be:

The overflow system must be sized to handle the same rainfall rate as the primary system, but it does not need to handle the full roof area simultaneously. The code logic is that the overflow system must handle the flow that would occur if the primary system were completely blocked. In practice, this means the overflow system is often sized for the same roof area and rainfall rate as the primary system.

Exam trap: The overflow system must discharge at a point that is visible (e.g., above grade or at a location where a blockage would be noticed). It cannot discharge directly into the sanitary sewer or a below-grade storm connection without an air gap or other approved backflow prevention.

1.7 Materials and Installation Requirements

Chapter 11 references the general materials requirements of the IPC. Storm drainage piping can be made of:

The code requires that storm piping be installed with proper supports, that joints be watertight, and that the system be tested. The IPC requires a water test or air test for storm drainage systems, similar to sanitary systems. The test is typically performed by plugging the lower end of the system and filling it with water to a specified head.

Practical field point: Underground storm piping must be bedded properly to prevent crushing. The master plumber is responsible for verifying that the trench is deep enough to protect the pipe from frost and surface loads. The IPC references the minimum cover requirements for underground piping, which are typically 12 inches for non-metallic pipe and 6 inches for metallic pipe, but local conditions may require deeper burial.

1.8 Disposal and Discharge Points

Storm water must be discharged to an approved point of disposal. The IPC lists acceptable discharge points:

The code prohibits discharging storm water into a sanitary sewer unless a combined sewer is used. It also prohibits discharging storm water into a private sewage disposal system (septic system).

Business-level responsibility: As the licensed master plumber, you must verify the availability of a public storm sewer before designing a system. If none exists, you must design an on-site disposal system (e.g., dry well) that meets local groundwater and soil conditions. The dry well must be sized based on the roof area and soil percolation rate, though the IPC does not provide a specific dry well sizing table — this is typically left to local code or engineering judgment.

1.9 Code Navigation: Where to Find It

For the open-book exam, efficient navigation is critical. Here is your storm drainage roadmap in the 2018 IPC:

Memory aid: Chapter 11 is the storm chapter. The numbering follows a logical pattern: 1101 (general), 1102 (roof drainage), 1103 (materials?), 1104 (trapeze hangers?), 1105 (tests?), 1106 (sizing), 1107 (testing), 1108 (overflow). The sizing tables are 1106.2 and 1106.3 — remember “1106” for sizing.

1.10 Common Exam Traps and Field Pitfalls

1.11 Practical Field Application for the Master Plumber

On site, the master plumber is the responsible licensed person. Your duties include:

Business note: Storm drainage failures are a common source of liability. A clogged primary drain with an undersized or missing overflow can cause structural damage. Your professional judgment in sizing and installing the overflow system is a key part of your duty of care.

1.12 Summary

Storm and roof drainage is a distinct discipline within the IPC. The key to mastery is understanding the difference between vertical conductors and horizontal drains, using the correct sizing tables, and never neglecting the emergency overflow requirement. In the open-book exam, your ability to quickly locate Table 1106.2 and Table 1106.3 will save you time and prevent errors.

Remember: the code is your reference. Know the structure of Chapter 11, and you will be able to answer any storm drainage question with confidence.