Ensure proper venting when installing new drainpipes
By Tim Carter
Tribune Media Services
Q. My husband refuses to help me install a new basement laundry sink. To make it easy, I have found a location immediately adjacent to a vertical plastic plumbing stack. Surely it can’t be that hard to cut into the pipe and install the necessary fittings. What is involved, and what do I need to do to make sure the work meets the plumbing code? — Mary C., Edmond, Okla.
A. What a shame your husband is not a team player. I wonder if he is intimidated by the project. There are indeed some challenges you will face, but it isn’t hard to cut into the plastic polyvinyl chloride (PVC) plumbing drainpipe. The hard parts are knowing where to make the cuts and making sure the new drainage branch you install is vented properly.
What is particularly vexing to many is the need for vent pipes. When water flows from a plumbing fixture into a drainpipe, it displaces the air that moments before was in the drainage pipe system. This air must be replaced, and vent pipes that extend through the roofs of houses are usually the entry point for this needed air.
If a vent pipe becomes clogged or if vent pipes are improperly installed or sized wrong, the plumbing system may get the needed air by sucking it through the drain of one or more fixtures in a home. If you have ever heard a tub or sink drain gurgle or sputter when you flush a toilet, you are experiencing this siphon phenomenon firsthand. When water gets sucked from a fixture trap, sewer gas or vermin in the piping system can easily enter your home.
The first thing you must do is calculate the rough-in height off the basement floor for the center of the horizontal drainpipe that will service this new sink. You do this by temporarily setting the new sink in place and connecting the P-trap to the tailpiece drainpipe that extends from the bottom of the sink. Slide the trap up the tailpiece of the drain as high as possible so that the rough-in height of the trap outlet is as high off the floor as possible. Believe me, you will need every inch you can spare.
You will need three fittings to create the new branch drainage line: a wye, a 45-degree fitting and a tee fitting. A wye is a plumbing fitting that allows a new drain line to branch from a stack at a 45-degree angle. The size of the wye fitting must match the diameter of your existing stack.
I am confident it is either a 3- or 4-inch PVC stack. Let’s assume it is a 4-inch-diameter plumbing stack. In this case, you will need a 4-by-2 wye, as I suggest a new 2-inch diameter stack for this new fixture. In addition, you will need a 2-inch street 45 fitting and a 2-by-1.5-by-1.5 tee fitting.
The street 45 fitting has a male end and a female end. The male end is inserted into the socket of the wye to create a smaller vertical secondary stack next to your existing stack. The tee fitting sits on top of the 45-degree fitting and creates the entry point for the drainpipe that extends from this new smaller vertical stack to the outlet of the trap under the sink.
The top of the tee fitting is where the vent for this drain begins. A vertical 1.5-inch pipe extends up above the flood level of the sink and continues upward until it connects to another vent pipe. If you can’t find a nearby vent pipe, you may have to extend this pipe all the way up to the roof. In some parts of the nation, certain plumbing codes do allow mechanical vent pipes that eliminate the need for traditional vent pipes.
Before you get started, I urge you to check with your local plumbing inspector. First, you need to know that some cities and states do not allow homeowners to alter the plumbing systems inside their homes. Make sure you are allowed to do this job by yourself, and find out about the necessary inspections. It is vitally important that you use all approved fittings, the correct clamps, the proper plastic primers and welding cements and the proper pipe sizes. The plumbing inspector can assist you with this in many instances.
