IPC Sanitary Drainage Systems 3 — Questions and Answers
Question 1: According to the IPC, what is the minimum size for a building sewer serving a building with 150 DFU?
- 3 inches
- 4 inches (Correct answer)
- 5 inches
- 6 inches
Correct answer: 4 inches
A 4-inch building sewer at 1/8 inch per foot slope can handle up to approximately 180 DFU under the IPC building sewer sizing table.
The IPC provides building sewer sizing tables that correlate pipe diameter with maximum DFU capacity at various slopes. A 4-inch building sewer installed at the minimum 1/8 inch per foot slope can accommodate approximately 180 DFU, making it adequate for a building with 150 DFU. A 3-inch building sewer would only handle about 42 DFU at the same slope, which is insufficient. Building sewers must never be smaller than the largest branch or stack they serve, and the minimum building sewer size is typically 3 inches. Proper sizing prevents surcharging, backups, and premature system failure due to overloading.
Question 2: What does the IPC specify regarding the use of solvent cement joints on underground drainage piping?
- Solvent cement joints are prohibited underground
- They are permitted for ABS and PVC drainage pipe (Correct answer)
- Only mechanical joints are permitted underground
- Solvent cement is only for above-ground use
Correct answer: They are permitted for ABS and PVC drainage pipe
The IPC permits solvent cement joints for ABS and PVC drainage piping in both above-ground and underground installations when properly made with approved cement and primer.
The IPC permits solvent cement joints for ABS and PVC drainage piping in all approved installation locations, including underground. For PVC pipe, the joint must be made using approved primer and solvent cement conforming to ASTM D2564 (for PVC) or appropriate standards. The primer prepares the surface by softening the material, and the solvent cement creates a chemical weld that is actually stronger than the pipe itself. For ABS pipe, solvent cement conforming to ASTM D2235 is used. Proper application technique — including surface preparation, adequate cement application, and joint assembly within the working time — is essential for a sound joint. Underground joints must also be protected from backfill damage.
Question 3: Under the IPC, what protection is required where drainage piping passes under a building foundation?
- No special protection is needed
- A sleeve or casing two pipe sizes larger must be used (Correct answer)
- Only cast iron pipe is permitted
- The pipe must be encased in concrete
Correct answer: A sleeve or casing two pipe sizes larger must be used
The IPC requires that drainage piping passing under or through building foundations be protected with a sleeve or casing to allow movement and prevent damage from structural loads.
The IPC requires that drainage piping passing through or under a building foundation be protected from structural loads and differential settlement. A sleeve or casing pipe, typically two pipe sizes larger than the carrier pipe, is installed to provide this protection. The annular space between the carrier pipe and the sleeve is typically sealed with a flexible sealant that allows some movement while preventing water infiltration and soil gas entry. This protection is critical because foundations can exert significant loads on piping and differential settlement between the building and surrounding soil can cause shearing forces on pipes. Without proper protection, pipes can crack, separate at joints, or become misaligned.
Question 4: According to the IPC, what is the minimum diameter for a drainage stack serving water closets?
- 2 inches
- 3 inches (Correct answer)
- 4 inches
- 6 inches
Correct answer: 3 inches
The IPC requires that any drainage stack receiving discharge from water closets be a minimum of 3 inches in diameter.
The IPC specifies that drainage stacks serving water closets must be a minimum of 3 inches in diameter. This requirement is based on the discharge characteristics of water closets, which produce a high-volume, high-velocity surge of water and solids. A 3-inch minimum ensures adequate capacity to handle this discharge without surcharging or creating excessive pneumatic pressure in the stack. However, in practice, most installations use 4-inch stacks for water closet connections to provide greater capacity and reduce the risk of blockages. The 3-inch minimum applies to stack vents as well when they serve as the vent for a water closet group.
Question 5: What does the IPC require regarding the grade of underground horizontal drainage piping?
- A uniform grade throughout the entire run (Correct answer)
- Variable grade is acceptable if the minimum is maintained
- No grade requirements apply underground
- Reverse grade is acceptable for short distances
Correct answer: A uniform grade throughout the entire run
The IPC requires a uniform grade throughout the entire run of underground horizontal drainage piping to maintain consistent flow velocity.
The IPC requires that underground horizontal drainage piping maintain a uniform grade throughout its entire run. A uniform grade ensures consistent flow velocity, which is essential for transporting waste solids through the pipe. Variations in grade — including dips, bellies, or reverse slopes — create points where flow velocity changes, leading to solid deposition and eventual blockages. Even if the minimum slope is technically maintained at every point, an inconsistent grade creates low points where solids accumulate. Proper bedding and backfill are essential to maintaining uniform grade after installation, as settlement of poorly compacted backfill material can cause pipe bellies over time.
Question 6: Under the IPC, when is an oil separator required on a drainage system?
- In all commercial buildings
- When flammable liquids or oils may be discharged into the system (Correct answer)
- Only in buildings with emergency generators
- Oil separators are never required by the IPC
Correct answer: When flammable liquids or oils may be discharged into the system
The IPC requires oil and flammable liquid interceptors when there is a possibility that such substances will be discharged into the drainage system.
The IPC requires the installation of oil and flammable liquid separators (interceptors) when the drainage system may receive discharges containing oil, grease, sand, flammable liquids, or other substances that could obstruct or damage the drainage system or public sewer. Common applications include auto repair garages, commercial kitchens (grease interceptors), laundries, and industrial facilities. The separator allows the lighter-than-water oils and flammable liquids to float and be retained while allowing the treated water to pass through to the drainage system. These devices must be properly sized, accessible for cleaning, and maintained regularly to function effectively.
IPC Sanitary Drainage Systems 3
According to the IPC, what is the minimum size for a building sewer serving a building with 150 DFU?