You will complete the plumbing in three phases:
- Install the drain and supply pipe below floor level after placing the concrete formwork.
- Install above-ground sections of the drain, supply, and vent pipe after roof sheathing has been installed.
- Install the fixtures after the drywall and cabinets have been installed.
Step 1
Before beginning the plumbing work, consult with your local water and wastewater utility (if applicable) to understand the requirements for connecting to the sewer and water supply pipe at the street.
Start plumbing drain installation at the sewer or septic tank and work up to the individual drains in the house. Install 4” SDR 21 sewer pipe from the sewer line at the street to the house, making sure to bury it deep enough to comply with local code (typically 18 inches) and to protect the pipe from collapsing under the weight of any vehicle traffic (if applicable). Note that you can use more expensive “Schedule 40” pipe for additional strength. Dig the trench for the pipe using a shovel (or Mini Excavator). Plan the slope of the drain so that there is at least a 1/8” vertical drop per horizontal foot and make the slope as consistent as possible (avoid any big drops). Make straight connections by placing the regular end of one section of pipe into the bell end of the downhill section of pipe. Other connections (e.g., 45-degree, 22.5-degree) will require cutting off the bell end of the downhill pipe section so that the fittings can be attached. You can cut the pipe quickly and easily with a Hackzall. It is important to get the cut nearly straight and remove any large burrs. Weld the PVC using primer and PVC cement, according to the instructions on each container.
It is a good practice to “dry fit” pipe and fittings before welding them together, to ensure everything fits well. All turns in the drainpipe should be made gradually if possible. Be sure to use long sweeping elbows where needed, rather than the sharp 90 degree elbows sold for vent and supply pipe.
Typically, the water utility will charge you a tap fee and a new customer fee to install and connect a water meter to their system at the front of your lot. Run a 1-inch polyethylene pipe from the valve next to the water meter to a central location in the wall between the bathrooms and put a cap or valve on the end of the pipe so that you can verify that there are no leaks before you pour the concrete. After the house is framed, you can connect this to the water supply piping you will install in the walls of the house. Make sure that the polyethylene pipe extends vertically above the concrete at least 3 feet, and that the pipe is buried beneath the frost depth in your area (you can find this online), or at least 12 inches deep. It is good practice to separate the water supply line and sewer line by 10 feet horizontally where possible, and by at least 2 feet vertically when they most cross each other.
Step 2
Install plumbing drain, vent, and supply pipes, using PVC for drain and vent pipes and PEX for water supply pipes. PEX pipe can be cut with a utility knife if you are careful to keep the cuts straight. The fittings can be crimped with a PEX crimp tool. As with PVC, it is best to install the pipe and fittings without crimping them first, to ensure a good fit. For simplicity, use copper stub outs for the sinks and toilets. If you can’t attach them to a stud, use a small piece of 1×4 spanning two studs to locate the stub out in between them.
Step 3
Install fixtures after drywall and cabinets have been installed, according to the fixture instructions. I recommend using prefabricated tub and shower surrounds for simplicity and ease of cleaning. However, if you want to save money and get fancy, Mr. Money Mustache describes a simple and elegant method for making your own custom shower, which I used once and found to be very durable and leakproof.
Figure 8. Plumbing Detail