· by Michael Blair, Nashville's Remodeling
Every new bathroom needs three things brought to it: hot and cold supply, a drain, and a vent. Supply lines are small, flexible and pressurized, so they go almost anywhere. The drain has to fall continuously to the main stack or the sewer line, at a set pitch, with a vent connected in the right place. Whether that can be done without opening floors and ceilings is what separates a straightforward bathroom from an expensive one.
The drain is the whole problem
A toilet needs a three-inch drain falling at a quarter inch per foot toward the stack. Over a twenty-foot run that is five inches of fall, which has to be found inside the floor structure or below it. In a house over a crawl space that is usually easy: the new drain hangs under the floor and joins the existing line. On a slab it means cutting and trenching concrete along the whole route. On a second floor it means finding a path between joists that run the right way, or opening the ceiling below.
Where the drain joins the existing system matters too. Tying into a stack near an existing bathroom is simple. Running a new line across the house to reach the sewer connection is a different job, and if the new bathroom is lower than the sewer, a basement bathroom for instance, gravity does not work at all.
Venting
Every fixture drain needs a vent, a pipe that lets air into the drain so water flows and traps keep their seal. Vents run up and out through the roof, or join an existing vent in the attic. A bathroom near an existing one can usually tie into its vent. A bathroom on the far side of the house may need a new vent stack through the roof, and the roof penetration and flashing are part of the cost.
Air admittance valves, the one-way valves that let air in without a pipe to the roof, are permitted in some situations and not others, and Metro Codes has its own position on where they are acceptable. They solve some venting problems and are not a general substitute for a vent.
- Drain: three-inch for a toilet, continuous fall to the stack
- Vent: tie into an existing vent or add a roof penetration
- Supply: half-inch hot and cold, routed almost anywhere
- All three have to be inspected before the walls close
Where a bathroom goes easily
Back to back with an existing bathroom, sharing the wet wall. Directly above or below one, sharing the stack. Next to the kitchen or laundry, where there is a drain and vent already. Over a crawl space, on the side of the house closest to the sewer connection. Those locations put the new fixtures a few feet from the existing system and the plumbing becomes a modest part of the job.
A closet next to a bathroom is the classic conversion. A corner of a large bedroom on the same wall as the hall bath is another. When the plan puts the new bathroom at the opposite corner of the house from every existing fixture, the plumbing route is the first thing to price, before any fixtures are chosen.
When gravity will not do it
A bathroom in a basement below the sewer line, or in a location with no route for a sloped drain, can use a macerating toilet or a sewage ejector pump. A macerator sits behind the toilet, grinds waste and pumps it through a small-diameter line up to the drain. An ejector pit is a sealed basin in the floor with a pump that lifts everything from that bathroom to the main line.
Both work and both are common in Nashville basements. Both are mechanical, need power, make some noise, and eventually need service, and a macerator restricts what fixtures and what toilet can be used. They are the right answer where gravity is not available, and the wrong one where a little more demolition would have given a gravity drain.