Plumbing your own basement is a large project, but it’s very manageable with the right tools and information. What could cost you up to $2000, you could do it for as little as $250, knowledge and some hard work. Adding a bathroom to your finished basement not only increases convenience for you and your family while you enjoy your home, but if you ever decide to sell, your basement bathroom will increase the value of your home. This ‘how-to’ will guide you through the steps you need to take to install a “DWV” in your bathroom; a drain, waste, and vent.
What you will need to do first is find your drain line. To do this, locate the pipe that runs from the ceiling to the floor called the “main stack” (Point A) and then locate a cleanout plug which should be near the wall closest to the street (Point B). Because the drain line usually goes straight from Point A to Point B, you should be able to dig up the concrete and check the slope to make sure it is allows for enough room for the pipes you will be putting in. If you own a home with its own septic tank system and don’t know where your drain is, make an educated guess or two. This method will only cost you the price of concrete, but calling a plumber to do the same job could cost you anywhere from $75-$200.
To calculate the proper slope for your project, measure the depth where the ‘T-fitting’ joins three pipes together (‘x’), and measure again at the horizontal pipe’s depth under the drain (‘y’). The formula is (x-y) x 4 = the maximum length measured in feet from the horizontal pipe under the drain to the main pipe. If you don’t have enough room under the main, you will need to set up a sewage ejection pump or locate your fixtures closer to the line.
Make a blueprint on your floor of where you want everything to go in pencil (the walls, tub/shower, sink, toilet, etc.), make changes if you need to and then spray paint the lines where the pipes will go. Consider assembling all of the parts on your floor first to be sure this is where you want them. If you have cast-iron pipes, rent a ‘snapper’ from your local hardware store, if you don’t want to do that, or have plastic pipes, use a reciprocating saw.
The floor will need to be trenched using a sledgehammer along your lines. The hole needs to be wide enough to fit your spade and start digging based on your slope calculations. Separate large concrete chunks from the rest of the dirt, which you will use to fill the hole later.
To begin building your drain system you need to cut into the main line and splice in a ‘y-fitting.’ You can use cast-iron, PVC or ABS piping, whichever is your preference. The hardest part of this whole project will be maintaining the proper slope, so if you’re having trouble, consider purchasing a torpedo level. Buy more fittings than you think you need; it’s a lot easier to return parts afterwards than run back and forth to the hardware store in the middle of things. Pack dirt under completed sections of piping so they don’t shift while you work. Make sure you leave enough room for framing and drywall before you put in the drain for the toilet.
Without a proper venting system, air pressure will develop, which means either noisy pipes, or worse, sewer gasses backing up into your home. Your toilet will need a vent, but there is a built in trap. If you want to do horizontal vent pipes, they need to be 6 inches above where water would overflow your sink, tub, or toilet. The maximum horizontal distance for 2 inch pipes is 30 inches. Running the vents under the floor joists is easier than running them through and while it has no chance of damaging the structure of your home, it does decrease the height of your basement bathroom’s ceiling. Most of the time you should be able to connect your bathroom vents to the vent from your laundry sink. Plumbing codes vary based on minimum vent sizing and choice of fittings, so be sure to check.
After you have framed the walls of your bathroom, install your vent pipes. If you are connecting to an existing vent, you can glue short sections of plastic pipe into a T- or Y-fitting. Make connections with rubber couplers after cutting out a section of the existing vent pipe. You can also install metal strapping to ensure stability while the cutting is being done, and leave it in tact afterwards for extra support.