For venting offsets more than 45 degrees in drain stacks, when must vents be provided?

• A. Vents must be installed at the top of the stack above the offset
• B. Vents are not required for offsets
• C. Vents must be placed at the base of the offset
• D. Vents serving offsets more than 45 degrees shall be vented where 5 or more branch intervals are located above the offset

Answer: (D)

When a drain stack has an offset greater than 45 degrees, venting is required only if there are five or more branch intervals located above the offset. The reason is that with several fixtures placed above the offset, draining water can create pressure changes that threaten the traps downstream of the bend. Providing a vent at a point where five or more branch intervals are above the offset allows air to enter and equalize pressure, preventing siphonage of the traps and protecting the drainage system’s function. If there are fewer than five branch intervals above the offset, this venting requirement doesn’t apply. That’s why simply venting at the very top or at the base isn’t the mandated solution in that scenario, and why the specified condition (five or more branch intervals above the offset) is the deciding factor.