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New Fixture Know-how

Replacing fixtures is a great way to change the look of a room and to enhance the usability of the area. But a fixture is more than a design element. This project will walk you through the areas you need to consider when replacing a fixture. 

Courtesy of Better Homes and Gardens®

About this Project

Working with fixtures means that you need to have an understanding of your home’s supply, drain, and vent systems. While some fixture projects can simply be add-ons, where you hook new fittings or fixtures to existing pipes, other projects require new lines. Running new lines takes coping with the constraints of your situation and an understanding of how supply, drain, and vent pipes work.

New supply lines are the easiest to plan. They require no slope or venting, just the correct pipe size (see Measurement Tip). You can run them wherever you need them.

Far trickier are the drain-waste-vent (DWV) lines that carry away water, waste, and gases. The illustrations below and on Venting Possibilites show straightforward ways to tap in for a new fixture. Your situation may not be this simple and may require the skill of a professional.

For any new installation, even a minor one, you'll probably need to apply to your local building department for a permit and arrange with them to have the work inspected before you cover up any new pipes.

New Fixture Know-how001

Installing new fixtures requires
careful planning.

Planning Drain Lines

The first step in planning an extension of your plumbing system is to map out exactly where existing lines run. Your home probably has a drainage arrangement similar to the one shown. Notice that some of the fixtures (toilet, double sink) cluster near a wet wall containing the main stack. A wet wall is usually a few inches thicker than other walls to accommodate the 3- or 4-inch-diameter stack that runs up through the roof. (A way to find a wet wall is to note the location of the stack on the roof.)

The fixtures drain directly into the main stack or into horizontal runs that slope downward at a pitch of at least 1/4 inch per running foot. Fixtures more than a few feet from the stack (like the bathtub and bathroom sink shown) must be vented with a loop that goes up and back to the stack. Called a revent or a circuit vent, this branch can be concealed inside walls and floors of normal thickness. Fixtures even farther away (like the shower and utility sink shown) may require a separate new stack. Requirements vary on revents and new vents, so check local codes.

Position the new fixtures as close as possible to an existing stack to minimize wall damage.

New Fixture Know-how2

Venting Possibilities

Think of a main stack as a two-way chimney: Water and wastes go down; gases go up. Just as you wouldn't install a fireplace without a chimney, neither should you consider adding a fixture without properly venting it. Strangle the air supply of a drain, and you risk creating a siphoning effect that can suck water out of traps. This in turn breaks the seal that provides protection from gas backup -- and often retards the flow of wastes as well.

Codes are specific about how you must vent fixtures. These requirements differ from one locality to another, so check your community's regulations for details about the systems shown here.

With unit venting -- sometimes referred to as common venting -- two similar fixtures share the same stack fitting. This method allows you to put a new fixture back to back with one that already exists. The fixtures are installed on opposite sides of the wet wall. To install a unit vent, open up the wall, replace the existing sanitary tee with a sanitary cross, and connect both traps to it. The drains of unit-vented fixtures must be at the same height.

Wet venting uses a section of one fixture's drain line to double as the vent for another. Not all codes permit wet venting. Those that do often specify that the vertical drain be at least one pipe size larger than the upper fixture drain. In no case can it be smaller than the lower drain.

Regardless of how you vent a fixture, codes limit the distance between the trap outlet and the vent. These distances depend on the size of the drain line you're running. For 1-1/4-, 1-1/2-, and 2-inch drain lines -- the sizes you'll most likely be working with -- 2, 3, and 5 feet, respectively, are typical distances. (For help adding plastic drain line see Adding Plastic Drain Lines, Related Projects. For how to tap into a cast-iron drain, see Tapping into Cast-iron Drain Lines, Related Projects.)

Often the best way to install a new fixture is with a revent, or circuit vent. Clear this with your local building department first. They may not allow you to do this with heavy-use items such as toilets or showers.

Sometimes, the only solution is to install a separate vent running up through the roof (see Adding New Vents, Related Projects). In some situations -- especially if the fixture is on the top floor -- it may be relatively simple.

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Vent system 

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