Bathroom Plumbing Basics

Bathroom Plumbing ProjectWhether renovating an existing bathroom or adding one on, you need to understand the basics of home plumbing, even if you are just planning the overall project rather than doing the actual work. Avoiding problems later on, like a bathtub that takes forever to fill or a toilet that won’t flush, makes some self education in the subject worthwhile.

The systems, plumbing, heating, ventilation and electrical, which modern residential bathrooms are built on, are hidden behind walls, floor, and ceilings, like the roots of a tree, but without them, it would be just another room in the house. Obviously, the most important mechanical system in a bathroom is the plumbing.

Plumbing System Functions

The function of a home’s plumbing system is to bring water into the house from the local water system, route it and deliver it to where it is needed, and also to remove it from the house and return it to the local water system. Residential plumbing basically is made up of two systems; the Water Supply system, and the Drain/Waste/Vent system (DWV).

The water supply system relies on sufficient water pressure in order to enable water flow at each fixture, and needs to be able to maintain proper flow when multiple fixtures are in simultaneous use. Since DWV systems do not rely on pressurized water to enable removal of liquids, larger diameter pipes are used in them; these allow gravity and atmospheric pressure to do all the work.

Water Supply System

Water comes in through the house main from either the municipal grid or a well system. Municipal supply is pressurized at between 45 and 75 pounds per square inch (psi), and well supplied water is usually somewhat lower. Your house’s plumbing system should ideally be in the 30-80 psi range. Systems less than 30 psi will require larger diameter pipes than standard sizes in order to maintain a useful flow.

Common pipe diameters are as follows:

  • Main House Supply Line: 1 inch
  • Branch Supply Lines: ¾ inch
  • Toilet Supply Line: ½ inch
  • Shower Supply Line: ½ inch
  • Sink Supply Line: 3/8 inch

In general, as you can see, the closer the pipe gets to the destination fixture, the smaller the diameter. In a few cases, where large volume of water is required, like large whirlpool tubs or multiple showerhead fixtures, a ¾ inch diameter pipe is run straight from the main and water heater line.

Pipe Materials and Water Pressure

Besides the diameter of a pipe, material and age will also have an effect on the pressure and performance of the supply system, as will the chemical properties of the water itself. Older galvanized steel pipe is prone to scaling and corrosion, which both can reduce the effective inside diameter of a pipe.

Plumbers know that a ¾ inch galvanized steel supply line can sometimes have a real inside diameter the size of a cigarette, which drastically cuts down the water pressure and volume available at the end fixture. In areas where the water supply has high levels of mineral content, such as calcium and magnesium, referred to as “hard water”, the problem is especially prevalent.

For the above reason, among others, copper is now the preferred pipe material for water supply lines. It is more resistant to scaling and corrosion and more durable. Pipe material options are as follows.

    Copper Pipe

    Used in both supply and DVW lines, copper is lightweight, easy to install and widely available, as well as being corrosion resistant. On the minus side, it is relatively expensive, and can be damaged during freeze and thaw cycles, leading to burst pipes.

    Flexible Copper Tubing

    Copper flex tubing can be installed bent around corners and angles, so it is used in cramped spaces where access is difficult. Though it less prone to damage from freezing, at joints it can cause damage from “water hammer” problems. Used in water supply lines only.

    Galvanized Steel

    Although quite strong and resistant to water hammer damage, it is heavy, requiring better support, and installation is labor intensive time and consuming. As mentioned above, it is prone to scaling and corrosion. Used in water supply lines only.

    Chlorinated Polyvinyl Chloride (CPVC)

    Used in water supply lines only, but can be used with both hot and cold water. CPVC is lightweight and does require additional structural support. It is easy to work with, cutting and installation are quick, and the material is resistant to freezing damage.

    Cross Lined Polyethylene (PEX)

    PEX is flexible, even more resistant to corrosion and scaling than copper, and is lightweight and easily installed. On the down side, it may not yet be approved in all building codes, being relatively new, and requires tools and fittings proprietary to different manufacturers. See our page on Plumbing with PEX. Used in water supply lines only.

    Cast Iron

    Used in Drain Waste Vent lines, cast iron is long lasting, resistant to chemicals and quiet. Its quietness is useful for toilet drain lines to dampen flushing noise. Relative to copper DVW pipe, it is heavy, expensive and labor intensive to cut and install.

    ABS and PVC

    Acrylonitrile Butadiene Styrene (ABS) and Polyvinyl Chloride (PVC) plastic pipe is used in Drain Waste Vent lines. ABS is black or dark grey and PVC is white colored, if you ever need to tell one from the other. These are easy to work with and inexpensive.

Drain Waste and Vent System

The DVW lines funnel water from the house into either your septic system or the municipal sewer system, using gravity to move water through them rather than water pressure. The DVW system also includes vent lines to give exit routes (usually a pipe exiting through the roof) for sewer gases. Vent lines also function to equalize pressure in the DVW system. Without them, water would not be able to flow freely.

The DVW system is also comprised of traps at each fixture location in order to block sewer gas from entering the house through the drain line when it is empty. A short U-shaped section of pipe traps enough water in it to seal off the drain.

Because drain waste vent systems depend on the force of gravity, the lines need to be sloped down, the minimum slope should be 1/8 inch of vertical drop per horizontal foot; ¼ inch vertical drop per foot is optimal, and any slope more than ½ inch per foot will make the liquid waste separate from the solid, which leaves lines prone to clogging.

The two most common materials for DWV lines are cast iron and plastic. Depending on local plumbing building codes, copper can also be used, and sometimes pipe materials can be mixed in the same system.

More Details

There is much more to residential plumbing than this general guide; the details are covered elsewhere. Here are a few links that may be helpful:

    Local Plumbing Codes: detailed information on building codes for all 50 states, major cities, and some counties.
    Hard Water: Vermont Dept. of Health page on hardness in drinking water
    Water Hammer The Natural Handyman talks about how to get rid of water hammer, and what causes it.

Photo: Winni3, Creative Commons License