Upon completing this chapter, you will be able to:
4.Identify the code requirements for trap types, minimum and maximum trap seal depths, and trap sizing based on fixture discharge rates.
5.Distinguish between traps that are permitted and those that are prohibited, including the specific conditions for bottle traps and drum traps.
6.Apply the requirements for trap venting, including the distance limitations from the trap weir to the vent connection.
7.Evaluate when interceptors are required, how to size them, and the specific materials and venting requirements for grease, oil, and solids interceptors.
8.Determine when backwater valves are required, how to select the correct type, and the access and maintenance obligations of the property owner.
9.Navigate the 2018 IPC and Maryland State Plumbing Code to locate the exact sections governing these components during an open-book exam.
1.1 The Purpose and Function of Traps
A trap is a fitting or device that maintains a water seal to prevent sewer gas and vermin from entering the building through the drainage system. The code requires that every fixture directly connected to the drainage system be equipped with a trap. The trap seal is the vertical distance between the weir (the overflow point of the trap) and the dip (the bottom of the internal partition). The minimum seal depth is 2 inches, and the maximum is 4 inches, per IPC Section 1002.2. Traps with a seal depth greater than 4 inches are prohibited because they create a self-siphoning condition that is difficult to vent properly.
The code also requires that each trap have a cleanout at the trap seal level. This cleanout must be accessible and allow for rodding of the fixture drain without removing the trap. For a master plumber, this means verifying that the cleanout plug is not buried behind finished walls or under fixtures.
1.2 Trap Types: Permitted and Prohibited
Permitted Traps
The IPC allows the following trap types:
P-traps – The standard trap for most fixtures. The P-trap must be the same size as the fixture drain, and the trap arm (the horizontal pipe from the trap weir to the vent) must not have a slope greater than one pipe diameter.
S-traps – Prohibited unless absolutely necessary and only where the vent cannot be installed otherwise. In practice, S-traps are almost never acceptable under the IPC because they are prone to siphoning. The exam will test your knowledge that S-traps are prohibited for new installations.
Bell traps – Prohibited. These were historically used in floor drains but do not maintain a reliable seal.
Crown-vented traps – Prohibited. A crown vent is a vent connection made at the trap weir itself, which causes siphoning of the seal.
Drum traps – Permitted only for bathtubs and only where the trap is not less than 4 inches in diameter and the installation is accessible. The drum trap must have a seal of 2 to 4 inches. If a drum trap is used, it must be provided with a cleanout and must be located where it can be accessed for service.
Bottle traps – Permitted only for lavatories and sinks where the trap is exposed and the fixture is not required to be vented by other means. Bottle traps are not permitted for kitchen sinks or other fixtures that discharge significant amounts of waste.
Prohibited Traps
The following are prohibited under IPC Section 1002.3:
Traps with moving parts (e.g., mechanical traps that rely on flaps or floats).
Traps that depend on the action of moving water to maintain the seal.
Traps that are not self-cleaning.
Traps with interior partitions that are not removable.
The rationale for these prohibitions is that moving parts fail, and non-self-cleaning traps accumulate debris that blocks the flow and breaks the seal.
1.3 Trap Sizing and Fixture Unit Loading
Trap size is determined by the fixture discharge rate, not by the size of the fixture outlet. The IPC Table 1002.1 provides the maximum fixture unit load for each trap size. For example:
A 1¼-inch trap may serve a lavatory or a bidet.
A 1½-inch trap may serve a kitchen sink, a laundry tub, or a bathtub.
A 2-inch trap is required for a shower stall or a floor drain.
A 3-inch trap is required for a water closet (though water closets have integral traps).
The key exam point is that you cannot simply match the trap to the fixture outlet size. You must verify the fixture unit value from Chapter 29 (Table 2902.2) and ensure the trap is sized to handle that load. For example, a kitchen sink with a garbage disposal has a fixture unit value of 2, which requires a 1½-inch trap. A commercial dishwasher may require a 2-inch trap.
1.4 Trap Venting and Distance Limitations
Every trap must be protected by a vent. The vent connection must be made on the trap arm, not on the trap itself. The distance from the trap weir to the vent connection is limited by the diameter of the trap arm and the slope of the pipe. IPC Table 1002.4 specifies the maximum distance:
For a 1¼-inch trap arm: maximum distance is 2 feet 6 inches.
For a 1½-inch trap arm: maximum distance is 3 feet 6 inches.
For a 2-inch trap arm: maximum distance is 5 feet.
For a 3-inch trap arm: maximum distance is 8 feet.
For a 4-inch trap arm: maximum distance is 10 feet.
The trap arm must not have a slope exceeding one pipe diameter (i.e., the fall cannot exceed the pipe diameter over the length of the trap arm). This is to prevent the water seal from being siphoned out by the velocity of flow. A common field error is installing a trap arm with excessive slope to gain vent clearance, which violates the code and causes seal loss.
1.5 Interceptors: When Required and How to Size
Interceptors are devices installed in the drainage system to separate and retain harmful or hazardous materials before they enter the public sewer. The IPC requires interceptors for:
Grease – For commercial kitchens, restaurants, and food processing facilities. The interceptor must be sized based on the flow rate and the retention time. The code requires a minimum of 2 minutes of retention time and a minimum capacity of 500 gallons for a single interceptor, unless the local authority approves a smaller unit. The interceptor must be vented and must have a cleanout at each end.
Oil and flammable liquids – For garages, service stations, and industrial facilities. These interceptors must be designed to prevent the passage of oil and flammable vapors. They must be located outside the building where possible, or in a ventilated area if indoors.
Solids and heavy particles – For laundries, sand traps in car washes, and similar applications. These interceptors must be sized to allow settling of solids.
Sizing Grease Interceptors
The sizing formula is based on the number of meals served, the flow rate, and the retention time. The code references the manufacturer's sizing data and the Plumbing and Drainage Institute (PDI) standards. A common exam question involves calculating the required capacity based on the number of seats or meals per hour. The formula is:
Capacity (gallons) = (Number of meals per hour) × (Waste flow rate per meal) × (Retention time in hours)
For a typical restaurant, the waste flow rate is approximately 6 gallons per meal, and the retention time is 2 minutes (0.033 hours). Therefore, a restaurant serving 100 meals per hour requires:
100 × 6 × 0.033 = 19.8 gallons per minute flow rate, which translates to a minimum interceptor capacity of approximately 1,200 gallons when accounting for the required 2-minute retention.
The master plumber must verify that the interceptor is accessible for cleaning and that the venting is adequate. Grease interceptors must be vented independently to prevent the buildup of explosive gases.
1.6 Backwater Valves
A backwater valve is a device installed on the building drain or sewer to prevent sewage from flowing back into the building during a municipal sewer backup. The IPC requires backwater valves where the building drain is below the elevation of the first manhole or where the public sewer is subject to backflow.
Types of Backwater Valves
Swing check valves – These are the most common and are permitted for use on building drains. They must be installed with the cover accessible for cleaning.
Gate valves – Not permitted as backwater valves because they do not automatically close.
Combination valves – These include a gate valve and a check valve and are used where manual shutoff is also required.
The code requires that backwater valves be installed in an accessible location and that they be provided with a cleanout. The valve must be sized to match the building drain. The valve must be of a type that does not obstruct the flow of sewage when open.
Installation Requirements
Backwater valves must be installed on the building drain upstream of the building sewer connection. If the building has multiple fixtures below the backwater elevation, each fixture must be protected individually, or a single valve may be installed on the main drain. The valve must be accessible for inspection and maintenance. The property owner is responsible for maintaining the valve, and the master plumber should provide clear documentation of the valve location and maintenance schedule.
A common field point: backwater valves are not a substitute for a properly graded drain. If the drain has inadequate slope, the valve will not function correctly. The valve must be installed level, and the flapper must be able to close fully under backflow conditions.
1.7 Special Conditions for Floor Drains and Indirect Waste
Floor drains are required to have a trap with a minimum 2-inch seal. The trap must be protected by a vent, and the floor drain must be of a type that allows the trap to be cleaned. In areas subject to flooding, the floor drain must be equipped with a backwater valve or be connected to a sump pump system.
Indirect waste connections (e.g., for dishwashers, ice machines, and commercial food preparation sinks) must discharge into a receptor through an air gap. The air gap must be at least 2 inches above the flood rim of the receptor, or 1 inch if the receptor is a floor drain. This prevents cross-contamination and ensures that a backup in the drainage system does not siphon waste back into the fixture.
1.8 Code Navigation: Where to Find It
For the open-book exam, you must be able to locate these sections quickly:
Concept
Code Location
Trap seal depth and requirements
IPC Section 1002.2
Prohibited traps
IPC Section 1002.3
Trap sizing and fixture unit loads
IPC Table 1002.1
Trap arm distance to vent
IPC Table 1002.4
Interceptor requirements
IPC Section 1003.1 – 1003.8
Grease interceptor sizing
IPC Section 1003.3 and manufacturer data
Backwater valve requirements
IPC Section 1102.4
Backwater valve installation
IPC Section 1102.4.1 – 1102.4.3
Indirect waste and air gaps
IPC Section 802.1 – 802.3
Maryland amendments
COMAR 05.02.02 (State Plumbing Code)
The Maryland State Plumbing Code adopts the IPC with amendments. The amendments are minimal but may affect specific sections. Always check the Maryland amendment page first if you are unsure whether a local requirement differs from the IPC baseline.
1.9 Practical Field Points for the Master Plumber
85.Trap seal verification – On new installations, fill the trap and verify the seal depth with a ruler. A seal of less than 2 inches will allow sewer gas to enter the building.
86.Cleanout placement – Every trap must have a cleanout. For P-traps under sinks, the cleanout is the plug at the bottom of the trap curve. For floor drains, the cleanout is the trap primer access.
87.Trap primers – Floor drains in mechanical rooms and public restrooms must be equipped with trap primers to prevent the seal from evaporating. The primer must be connected to the cold water supply and must be accessible.
88.Grease interceptor maintenance – The owner must have a maintenance log showing the interceptor is cleaned at least every 90 days, or more frequently based on usage. The master plumber should verify this during inspections.
89.Backwater valve labeling – The valve should be labeled with the manufacturer's name, the size, and the direction of flow. This is critical for emergency responders and maintenance personnel.
1.10 Common Exam Traps
S-trap prohibition – The exam will present a scenario where an S-trap is installed under a sink. The correct answer is that it is prohibited, even if it functions.
Trap seal depth – A trap with a 5-inch seal is prohibited. The maximum is 4 inches.
Trap arm slope – A trap arm with a slope of 1 inch per foot is acceptable for a 2-inch pipe only if the total fall does not exceed the pipe diameter. For a 2-inch pipe, the maximum fall over a 5-foot trap arm is 2 inches.
Grease interceptor venting – Grease interceptors must be vented, but the vent must not be connected to the main vent stack without a relief vent.
Backwater valve location – The valve must be installed on the building drain, not on the building sewer. The building sewer is outside the building and is not required to have a backwater valve.
Fixture unit vs. trap size – A 2-inch trap is required for a shower, even if the shower drain outlet is 1½ inches. The trap size is based on the fixture unit load, not the outlet size.
1.11 Summary
The master plumber must understand that traps, interceptors, and backwater valves are the first line of defense between the building occupants and the public sewer system. The code requirements are specific and must be followed precisely. In the open-book exam, your ability to navigate to the correct section quickly is as important as your knowledge of the content. Practice locating these sections in your code book before the exam, and pay attention to the Maryland amendments that may modify the IPC baseline.
Remember that as the responsible licensed professional, you are accountable for the design and installation of these systems. A failure in a trap seal, an undersized grease interceptor, or a missing backwater valve can result in property damage, health hazards, and legal liability. The code is your minimum standard; good engineering practice often exceeds it.
