Soil stack and plumbing

Extending upward from the house drain are the stacks, the pipes into which waste and soil branches empty. Most homes are equipped with only one, but if the house is large or has numerous or widely separated fixtures, others may be necessary. When correctly assembled, soil-stack inlet hubs and threaded connections should face up and out.

Factors to consider

Installing soil stacks is a problem because branches from them must connect to fixtures, because soil-stack hubs are large, and because they must be constructed so that they will not touch the partitions between which they run. The last practice is necessary to prevent walls and other partitions from cracking as they settle because no leeway was left for them. That calls for careful pre-planning by both builder and plumber. The soil stack is usually placed before walls are ready for finishing, and after due consideration of such construction details as the placement of windows, doors, built-in cabinets, and other permanent additions.

The minimum diameter for a stack serving a water closet is 3", but an internal diameter of 4" is most commonly used. That means that either 4" copper tubing or 6 1/2" outside diameter cast iron pipe will be used, as both have the same internal diameter. Both kinds are strong and will stand most corrosive action. Copper is more susceptible to temperature changes, but cast iron is harder to install.

Some of the connections to the soil stack will cause more trouble than others. For example, the location of the Y or T-Y fitting for the connection of the water closet bend presents a most difficult layout problem. Since you must run your pipe out horizontally from the branch of the T-Y fitting, beware lest your horizontal run protrudes below the ceiling in the room under the bathroom.

Such possibilities must be considered beforehand, at the time you are selecting the location for plumbing pipes and fixtures. How to conceal the pipes for a new installation being added to an old system also must be decided. It is usually easier for an amateur plumber to leave pipes exposed and then conceal them by boxing them in, especially if they run along a ceiling or are in a conspicuous corner.

Also remember the need for vent piping. In running waste from two or more fixtures to a soil stack, install additional piping to act as a vent to prevent siphonage of trap seals and also to prevent back pressure from building up in the plumbing system.


In the actual installation of the soil stack, one of the initial considerations should be fastening the hub to floor beams of the first floor. This is accomplished by means of a pipe rest set in notches which are cut into the joist or beams, or by means of wooden cleats. This hub should be firmly placed as it is the foundation on which the stack is built up to the roof.

If your home has only one story, with a ground-floor bathroom, it will be necessary to use a T-Y fitting instead",of a hub, as this will receive the discharge from the water closets all other fiixtures on the floor. In that case, the entire support of the stack will depend on this fitting, which must be located properly to assure the proper pitch of the piping.

In the typical two-story installation, sections of the soil pipe are caulked to the correct height for the placement of the T-Y fitting. Set this assembly temporarily into the stack so that the T-Y opening for the water closet faces in the direction of the water closet. Get your roughingin measurement for the water closet connection-usually 12" to 14" from the finished wall. This distance to the T-Y branch opening will be the length of the closet bend.

If you are using a cast iron closet bend, it will be necessary to cut off the outlet end of the bend so that the upright or inlet end of the bend will come "dead center" to the roughed-in measurement of the water closet. Cast iron bends are manufactured with rings on each end so that they may be cut off easily to any length desired. They are cut in exactly the same manner as cast iron soil pipe.

Once the necessary measuring and fitting is completed, remove the assembly from the stack and caulk the water closet bend into the T-Y fitting; then drop the entire assembly of cast iron pipe with T-Y fitting and attached bend down into the hub coming from the floor below.

Fasten the closet bend and T-Y securely into position so that they cannot shift or move while you are caulking the end of the pipe into the hub of the lower pipe. When properly fitted and caulked, the completed installation should show the soil stack running through a single opening between the floor and ceiling with a T-Y fitting in the proper location for attaching the fixture.

The soil stack should now be up to the bathroom floor. Caulk a piece of cast iron pipe to the top of the fitting. This piece of pipe should long enough so that its top is above the overflow level of the highest fixtures in the house and at least above the finished floor. A 4"x2" tapped-in T for the vent line from the fixtures is added on top of the stack.

From the top of the T, continue the stack up to within 8" or roof. Cut a hole in the roof above the soil stack, slip 4" pipe through the hole of about 10" above the caulk the bottom into the hub of the soil stack. Add a lead roof-flashing over the top and fasten it to the as directed by the manufacturer, so that no rain water can get in under it. Then beat or press the flashing inside and around the rim of the pipe and place another short piece of cast iron pipe tightly in the hub to bold the flashing and keep it watertight.

The top of the stack, running up from the top T, is actually part of the venting system. This, and any other vent stack, should extend at least one foot above the roof at the point where it comes through the roof. Its top should also be at least three feet above any window, door, or other opening in the house that is within ten feet of the stack, so gases can be freely dispersed into the air, not into the house.

One danger to the venting system in some places is that the top of the stack may be clogged or closed by frost. Dangerous frost-clogging may be prevented by insulating the stack above the roof, or by using an increaser, which is a section of pipe that provides a larger opening at the top than that of the stack itself. It should start from the stack about a foot below the roof, and its upper opening should have at least a 4" diameter.

Turns, connections, supports

As in all drainage-system piping, changes in direction must not be abrupt. A sharp turn, no matter what the diameter of the pipe involved, causes solid waste to accumulate and may eventually result in stoppage. If it occurs in an inaccessible section of the stack, the accumulation will be hard to remove. Therefore, if turns are necessary, make as few as possible, and be sure to use fittings with a long, sweeping curve.

Good and airtight connections not only forestall leaks, they also prevent foul-smelling sewer gases from escaping. Take special care to inspect all fittings and junctions and to test the system thoroughly before making final attachments to house and drain fixtures.

No matter how well the soil stack is installed, its efficiency and durability will depend on how it is supported. Vibration, expansion and contraction, and house settlement all have their effect on the stability of the soil stack, so some means of counteracting them must be used. Plumbing supply houses can furnish a variety of hangers and supports.

Good judgment will show you the best places for them. Vertical runs should have a minimum of one rest or clamp on every floor level, and horizontal runs should have hangers or clamps every five to ten feet, according to the size, kind, and weight of the pipe you are hanging. Supports and hangers should be placed as close to caulked joints as possible, since they are the points most likely to give under strain.

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Installing lavatory
Water closet install

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Soil stack
Vent piping

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