Modern residential plumbing works surprisingly like ancient plumbing. We may use electric pumps to send water up to water towers, but oftentimes, the water then proceeds to our homes simply under the power of gravity. The continued importance of gravity to plumbing means that elevation is crucially important. When installing plumbing fixtures in the lowest point in the house—particularly when installing basement bathrooms—make sure to account for the following factors:

Gravity: The Foundation of Good Drainage

While gravity sends water from upstairs drainpipes plunging quickly down to the main sewer line, basement drainpipes are on the same level as the main sewer line and therefore must be properly graded to encourage flow.

Installing a basement drainpipe

To install a basement drainpipe that will service a toilet, sink, and shower, the concrete floor surrounding the main sewer line must be broken up and removed so a trench can be dug. The new pipe must be installed with a downward slope of one-quarter inch per foot, which should be easy as long as the main sewer line connection is far enough below the floor.

If there’s not enough room for a new drainpipe beneath the concrete floor, or if the main sewer line is actually above the floor in the first place, homeowners hoping to install a basement bathroom have other options. One solution is to build up the basement floor, creating a false floor within which the new drainpipe will run. This allows you to avoid tearing up the concrete floor, but it will leave your basement toilet elevated on a slab—a setup most homeowners dislike.

Freestanding sewage-ejector systems

If any concrete construction is out of the question, there are toilets and pump systems that send wastewater straight up. So-called upflushing toilets sit entirely above the floor and include a pump that sends wastewater to a higher access point in the main sewer line. Freestanding sewage-ejector systems perform essentially the same function and can be concealed behind a “wet wall” along with ventilation piping and water lines for the sink and shower. Upflushing toilets and sewage-ejector systems can even help drain basement sinks and showers; by attaching the fixtures to either kind of system, the mechanism will send the sink and shower wastewater up along with that from the toilet.

Space: You’ll Need Room to Set the Trap

drain trap under a sink

The short, U-shaped pipe beneath sink and shower drains is called the drain trap. It prevents hair, sediment, and fumbled valuables from proceeding down the drain and forming inaccessible clogs. However, traps perform another, more important service: the water remaining in the trap’s low point provides a barrier against sewer gases farther down the line. In addition to smelling bad, sewer gases can also be noxious or combustible, which is why all plumbing fixtures are required to have traps.

Adding traps to your basement plumbing

Toilets include built-in traps; you can sometimes see the trap’s signature curve molded into the contours of the porcelain. Sink traps are located well above the floor and should present no special problems when being installed in a basement. Shower drains, however, have traps located just beneath the floor, so homeowners should be aware that it will take a little extra digging to accommodate the pipe.

Preventing sewer gases

A more persistent problem arises if the fixture you’re installing won’t be frequently used: over time, the water in the trap will evaporate, allowing sewer gases to leak into the basement. This problem is common among rarely used floor and shower drains, but basement toilets are susceptible as well. To prevent this problem, you could replenish the trap by periodically pouring water down the drain.

Also adding mineral oil, which won’t evaporate, creates an effective, longer-lasting seal against sewer gases. If you have trouble remembering to water your plants, let alone your idle drains, you can install a trap primer, a simple connection between the house’s clean water supply and the drain; as other plumbing fixtures throughout the house are used, the changing pressure will inject a small amount of water into the drain to maintain the trap.

Ventilation: The Key to Free-Flowing, Good-Smelling Plumbing

plumbing pipe air valves

It may surprise you to learn that your plumbing system includes vents as well as drains. The vents act as outlets for sewer gases and allow water to drain smoothly without creating a gurgling vacuum. Like all plumbing fixtures, basement toilets, showers, and sinks must be vented.

Local plumbing codes dictate how far away a fixture can be from the main conduit of the drain-waste-vent system, sometimes called the soil stack. If the soil stack is inaccessible, some codes allow drains to be fitted with air admittance valves—one-way vents that provide enough air for water to drain smoothly without allowing sewer gases to escape. When considering the many configurations of ventilation piping as well as attachments like air admittance valves, you will want the expertise of a professional plumber; the building codes must be followed precisely for safety and if the house will someday pass inspection.

The difficulties of installing a basement bathroom aren’t insurmountable; they can be overcome with a little bit of knowledge and lot of conscientious planning. The final product—a fully functional bathroom that’s both a great investment and a jumping-off point for finishing the rest of the basement—will make the plumbing challenges worth it.