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Proper Sizing Of Plumbing Vents

Plumbing systems are complicated yet understanding how they work is not, let’s take a closer look at how the sanitary plumbing system works. When water goes down the drain it goes to the sewer which smells bad this is where the p-traps come in eliminating the smells to the living space. Vents keep water in the traps without the proper venting water would just get sucked down the drain that’s why we add vents.

The International Residential Code states every trap requires a vent this applies to washing machines and mud room sinks. The vent system consists of a main vent that extends far enough beyond the roof to prevent obstructions such as snow and debris in most cases 10-12 inches of exposed vent pipe above the roof will suffice (always check with your local building code department). A vent system also needs atmospheric pressure to operate properly, air needs to get in just as sewer gases need to escape, vents need to be half the diameter of the main drain pipe they serve; for instance if your main drain pipe is 3 inches than the vents need to be 1.5 inches. In some instances you can run all the vents into the attic and tie all of them together into the 3 inch main drain pipe and then vent the 3 inch pipe out through the roof, this method is a lot cleaner look from the outside and also eliminates roof or pipe flashings to be installed that could lead to potential leaks in the years to come. Using this method is great if most of the plumbing and vents are in a central location. Also a good idea to add clean outs below every p-trap, a little bit of insurance for the later years (just in case you ever get a clogged drain.



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