Size of Pipe
The size of the waste water pipe determines the maximum distance the vent can be located from the plumbing fixture. Small pipes, pipes 1 1/4 inches in diameter, require the vent pipe be within 30 inches of the plumbing fixture. A pipe of this size supports a single fixture such as a sink or tub. On the other end of the spectrum, a 4-inch waste water pipe can be placed up to 10 feet from the fixture. This size of pipe is used in multiple dwelling or commercial buildings and can support up to 216 fixtures.
Measure From the Trap
The horizontal distance is measured from the vertical vent pipe to the closest portion of the P-trap. The trap commonly sits directly under the plumbing fixture or drain and may be further from the vent pipe than the edge of the plumbing fixture.
The small, 1 1/4 inch, waste water pipes require the same diameter of vent pipe. The larger waste water pipes can use slightly smaller vent pipes. A 4-inch waste water pipe can be vented with a 3-inch pipe. The vent pipe must extend at least 6 inches, but no more than 12 inches, above the roof deck.
Drain vents do occasionally become plugged. This can cause gurgling sounds from the plumbing fixtures and slow drainage of water. The most common cause of plugged vents is ice accumulation during cold winter weather. The air rising through the vent is warm and moist. If it begins to freeze before it exits the vent stack the ice crystals within the vent pipe can close the opening.