Pipe Size for Plumbing Ventilation

Purpose of Ventilation Pipes

• The easiest way to understand the importance of a ventilation pipe is to take a bottle of soda or water and turn it upside down with the lid off. The first thing that happens is the liquid inside glugs out of the opening in bursts, rather than pouring out in a smooth, uniform stream. But if you puncture a hole in the bottom of the bottle and then turn it over, the liquid pours out smoothly. This is air pressure in action -- and is the reason for ventilation pipes.

Drainpipes and Sizes

• While the overall size of the vent pipe is important, it is also defined by the size of the other drainpipes into which it is connected. Drainpipes for tubs and showers go up to a maximum of 2 inches, while drainpipes for washing machines are 3 inches. If the drainpipes are too big, the water will not be forced to flow through the pipes, and will instead linger in the bottom of the pipes because the air pressure from the vents isn’t enough to push it through pipes that are too large.

Vent Pipe Size

• The basic rule for pipe vents is that they must be half the size of the soil or waste stack at the lowest point of connection within the overall structure. As a general rule this is between 2 and 4 inches, but it depends on how many connections are attached to the drains, as well as the overall height of the pipe, which is dependent upon how many levels the house has. Only a licensed plumber can help you determine the exact specifications based on your home’s pipes.

Local Codes

• One of the most important things to remember is that no two houses are ever the same, and what applies on a federal level may be different on state, county and city levels. In order to know the exact specifics for your home, you should always work with a local plumber so he can help guide you through any plumbing processes. If you attempt to install your own vent pipe without his help, you could be violating local codes, which voids insurance and warranties on your home.