Tying in to a Plumbing Vent Stack
Vent covers keep the vent stack free of obstructions.
Venting is required in a plumbing drain system to maintain atmospheric pressure in the pipes. Without proper venting, flowing water creates negative pressure that can slow water flow and empty P-traps. Every fixture must have its own vent that must terminate in open air, but in most houses, only one vent pipe goes through the roof. The fixture vents all tie into this pipe, which is called the main vent stack.
The Main Vent Stack
The main vent is stack is often, but not always, a continuation of the soil stack, which is a 3- or 4-inch waste pipe that extends vertically down from the uppermost bathroom to the sewer. Plumbing codes are not the same in every state, but most require the cross-sectional area of the vent pipe to equal at least half of that of the lowest portion of the soil stack. It’s common practice in many states to reduce the soil stack to 2 inches, and extend that 2-inch pipe through the roof. It must extend at least 6 inches above the roofline.
Individual Fixture Vents
The vent that services a fixture must have a diameter at least half as large as the drain pipe, but the minimum pipe diameter allowable is 1 1/4 inches. It’s common to use a 1 1/2-inch vent for a 2-inch drain because that ensures that the drain will get enough air. That size vent on a 2-inch drain must be no more than five feet from the fixture P-trap — the distance varies according to drain and vent diameters — and it must rise vertically until it is above the overflow line of the fixture. It can then run horizontally to tie into another vent, as long as it maintains a 1/4-inch-per-foot slope toward the drain.
Tying Into a Vent Stack
Individual vents must rise above the overflow line of the topmost plumbing fixture in the house before you can tie it to the main vent stack. This is a precaution that prevents the vent from becoming a drain if that fixture’s drain gets blocked. If the vent slopes up to the main stack, you tie it in with a Y-fitting. If it rises to the level of the connection and proceeds horizontally from there, you use a vent tee, which doesn’t have a sweep. You can usually tie up to eight fixtures to a single stack, but local codes differ, so it’s a good idea to check.
The vent opening must be far enough above the roof and away from doors and windows to prevent the sewer gases from drifting into the house. The code requires a 10-foot horizontal distance from any door or window, and if the roof serves any function other than being a roof, such as being a balcony, the vent must rise 7 feet above it. Blocked vents can create drainage and sanitation problems, so protecting the vent opening with a grate or cover is a prudent move. Increasing the vent opening to three inches just before it goes through the roof in frost-prone areas is another precaution that prevents blockages.