Traps, Interceptors & Storm Drainage

Learning Objectives

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

Identify the minimum requirements for trap design, materials, and installation.

Explain the purpose of venting in relation to trap seal protection.

Distinguish between traps, interceptors, separators, and backwater valves.

Apply code requirements for interceptor sizing and maintenance access.

Navigate the IPC sections governing storm drainage systems, including roof drainage and conductor sizing.

Recognize common field violations and inspection points.

1.1 Fundamentals of Trap Seal Protection

A trap is a fitting or device that maintains a water seal against sewer gas entry. The trap seal is the vertical distance between the trap's crown weir and the dip (bottom of the internal curve). The IPC requires that every trap have a minimum seal of 2 inches and a maximum seal of 4 inches (except where a deeper seal is specifically required for a particular fixture). Traps with seals deeper than 4 inches are prohibited because they increase the risk of self-siphonage and clogging.

Key points:

Every fixture directly connected to the drainage system must have a trap.

A trap must be placed within 24 inches of the fixture outlet (measured along the fixture drain) for most fixtures. For sinks, the distance is measured from the outlet of the fixture to the inlet of the trap.

No fixture may be double-trapped. A double trap creates an air pocket between the two seals, which can prevent drainage and cause pressure differentials.

Traps must be protected by venting to prevent siphonage and blowout.

Trap types commonly seen in the field:

P-trap – most common; used for sinks, lavatories, and bathtubs.

Drum trap – allowed only for bathtubs in existing installations; prohibited in new work unless specifically approved.

House trap – generally prohibited on the building sewer or drain, as it is a source of blockage and requires a separate vent.

Prohibited traps include: traps with moving parts, traps that depend on interior partitions, and bell traps (often found in old floor drains).

1.2 Trap Materials and Joints

Traps must be made of materials listed for use in drainage systems: cast iron, galvanized steel, copper or copper alloy, brass, PVC, ABS, or other approved materials. The material must match the piping system to which it is connected, or be joined with approved transition fittings.

Inspection point: Check that the trap is not chrome-plated brass in a concealed location unless approved. Chrome traps are only for exposed applications.

Slip joints (mechanical joints) are permitted only on the trap inlet and outlet connections for fixtures such as sinks and lavatories. They are not allowed in concealed locations. A slip joint nut must be accessible.

1.3 Interceptors and Separators

Interceptors and separators are devices installed to prevent harmful, hazardous, or undesirable materials from entering the public sewer or drainage system. The IPC requires their use where the nature of the discharge warrants it.

Common types:

Grease interceptor – required for commercial kitchens, restaurants, and food processing areas. Sizing is based on the fixture load and flow rate, not on the volume of the interceptor alone.

Oil and sand separator – used in garages, workshops, and service stations.

Hair interceptor – required for beauty salons and barber shops.

Lint interceptor – for laundries and commercial washing facilities.

Plaster interceptor – for plaster traps in medical or dental offices.

Acid neutralizer – for laboratories or battery rooms.

Installation requirements:

Interceptors must be accessible for cleaning and maintenance.

They must be vented to prevent siphonage.

They must be installed so that the flow enters through the inlet and exits through the outlet without short-circuiting.

No interceptor may be installed in a way that allows the discharge of a water closet or urinal to pass through it, unless the interceptor is specifically designed for that purpose.

Sizing note: The code does not provide a single sizing table for grease interceptors. Instead, sizing is based on the drainage fixture unit (dfu) load and the flow rate of the fixtures served. The interceptor must be sized to handle the peak flow without allowing grease to escape. In practice, many jurisdictions require a minimum capacity (e.g., 20 gallons per minute) for commercial food service.

1.4 Backwater Valves

A backwater valve is a device installed on the building drain or sewer to prevent reverse flow (backflow) of sewage from the public sewer into the building. The IPC requires backwater valves where the lowest fixture is below the elevation of the next upstream manhole or sewer cover.

Key requirements:

The valve must be of a type that provides a positive mechanical seal.

It must be accessible for inspection and maintenance.

It must be installed on a separate branch or on the building drain, upstream of the lowest fixture.

The valve must be vented to prevent pressure buildup.

Inspection point: Verify the valve is not installed in a location where it can be accidentally covered by concrete or flooring without an access panel.

1.5 Storm Drainage: Scope and Definitions

Storm drainage systems collect and convey rainwater, surface water, and groundwater away from the building. The IPC covers:

Roof drains and gutters

Conductors (vertical pipes) and leaders

Building storm drains (underground)

Storm sewers (off-site)

Important definitions:

Roof drain – an outlet installed on the roof to collect rainwater.

Conductor – a vertical pipe that conveys storm water from the roof drain to the storm drain or sewer.

Leader – a pipe that conveys storm water from a gutter or downspout to the drainage system.

Building storm drain – a horizontal pipe that conveys storm water to the storm sewer or other point of disposal.

Prohibition: Storm water must not be discharged into a sanitary sewer unless the jurisdiction specifically allows combined sewers. In most cases, storm and sanitary systems must be separate.

1.6 Roof Drainage and Sizing

Roof drains must be placed at the lowest points of the roof, and the roof must be sloped to drain. The IPC provides a sizing table (Table 1106.2) based on the rainfall rate (inches per hour) and the roof area (square feet). The table gives the required diameter of the roof drain and conductor for a given flow.

Sizing steps:

73.Determine the design rainfall rate for the locality (often 1 to 4 inches per hour, but can be higher in storm-prone areas).

74.Calculate the roof area served by each drain.

75.Use the table to select the minimum pipe size.

Example: For a 2-inch per hour rainfall rate, a 4-inch roof drain can serve approximately 1,800 square feet of roof area. For a 4-inch per hour rate, the same drain serves only about 900 square feet.

Inspection point: Check that the roof drain has a strainer or dome that prevents debris entry. The strainer must have an open area at least equal to the pipe area.

1.7 Conductors and Building Storm Drains

Vertical conductors must be sized according to the same table as roof drains. Horizontal storm drains must be sized based on the slope and the dfu load or the hydraulic flow. The IPC provides a separate table for horizontal storm drain sizing (Table 1106.3) based on slope.

Minimum slopes for horizontal storm drains:

1/8 inch per foot for pipes 3 inches and larger

1/4 inch per foot for pipes smaller than 3 inches

Cleanouts are required on storm drains at intervals not exceeding 100 feet, and at changes in direction greater than 45 degrees.

Venting: Storm drains are not required to be vented unless they contain traps (e.g., a floor drain in a parking garage that connects to the storm system). If a trap is used, it must be vented.

1.8 Combined Sanitary and Storm Systems

In some older municipalities, a combined sewer carries both sanitary and storm water. The IPC allows this only if the jurisdiction permits it. In new construction, separate systems are the standard. If a combined system is allowed, the sanitary and storm loads must be added together for sizing.

1.9 Subsurface and Groundwater Drainage

The IPC also covers foundation drainage and sump pumps:

A sump pump is required when a building drain or fixture is below the sewer elevation.

The sump must be vented, and the pump discharge must be connected to the building sewer or a dedicated storm line.

The sump pit must be covered and have a gasketed lid to prevent sewer gas escape.

The pump must have a capacity sufficient to handle the peak inflow.

Inspection point: Verify that the sump pump discharge has a check valve to prevent backflow into the pit.

1.10 Code Navigation: Where to Find It

For an open-book exam, knowing where to look is half the battle. Use this quick reference:

Topic	IPC Section
Trap seal requirements (2–4 inches)	Section 1002.1
Trap distance from fixture (24 inches)	Section 1002.2
Prohibited traps	Section 1002.3
Trap materials and joints	Section 1002.4
Interceptors and separators (general)	Section 1003.1
Grease interceptor sizing	Section 1003.3
Backwater valves	Section 1102.5
Storm drainage scope	Chapter 11 (general)
Roof drain sizing table	Table 1106.2
Horizontal storm drain sizing	Table 1106.3
Cleanouts on storm drains	Section 1105.1
Sump pumps and ejectors	Section 1110
Rainfall rate data	Appendix B (informative)

Note: Appendix B is not part of the code text but is often used for design rainfall rates. Check if your jurisdiction adopts it.

1.11 Practical Inspection Points

When performing a code inspection on traps, interceptors, and storm drainage, the inspector typically verifies the following:

105.Trap seal depth – Is the trap a standard 2-inch seal type? No deep-seal traps unless specified.

106.Accessibility – Can the trap be reached for cleaning? No trap may be embedded in a wall or floor.

107.Venting – Is the trap vented within the required distance? A trap without a vent will siphon.

108.Interceptor access – Is there a cleanout or access door for grease interceptor cleaning?

109.Slope – Is the horizontal storm drain sloped correctly? Use a level to check.

110.Roof drain strainers – Are they present and not blocked?

111.Backwater valve location – Is it accessible and not upstream of a fixture that would be unprotected?

112.Sump pump discharge – Does it have a check valve and an anti-siphon device?

1.12 Common Code Violations to Avoid

Installing a trap more than 24 inches from the fixture outlet.

Using a bell trap or a trap with moving parts.

Failing to vent a trap, allowing siphonage.

Connecting a grease interceptor without a vent.

Sizing a roof drain based on roof area without considering rainfall rate.

Discharging storm water into a sanitary sewer.

Installing a backwater valve without access.

Using a slip joint in a concealed location.

Summary

This chapter covered the core requirements for traps, interceptors, and storm drainage systems. In the exam, you will be asked to apply these rules to practical scenarios. Remember that the IPC is a performance-based code: it sets minimum standards, and you must be able to find the exact section quickly. Practice navigating the code by looking up each section listed in the Code Navigation table above. On exam day, your code book is your best tool—use it early and often.

Final tip: For any question about sizing, always check the tables first. The IPC tables are the definitive source for pipe sizes, slopes, and capacities. Do not rely on memory for numbers; rely on your ability to find them.

Traps, Interceptors & Storm Drainage