Drainpipes always slant downward or run vertically, and bends are gentle to make the wastes flow smoothly. The crucial element in the drain system, comparable to the main service line of the supply system, is the large vertical drain called the main soil stack. Most toilets drain directly into the main stack; most other fixtures and appliances empty into slanting branch drains, which connect to the main stack. It, in turn, empties into a slanting main drain?generally laid just beneath the basement floor?that carries the wastes to a public sewer or a septic tank.

At several points in this system are elements intended to keep air passing through the sewer and drains but not into the house. The free passage of air is essential to maintain atmospheric pressure at every point, otherwise a vacuum will develop that will stop waste water from flowing down. This air should not enter living space, of course, for it contains sewage gases.

To keep the sewage gases out of the rooms of a house, every fixture and some drain lines empty into traps'U-shaped passages, permanently filled with water that serves as a barrier to the gases. Sink and lavatory traps are generally accessible and easy to clear or, if necessary, to replace; bathtubs and shower stalls, which empty their wastes through the floor, can be cleared, but replacing the traps alone is difficult.

While traps seal air inside the drains, it passes through them freely. When waste water flows, air goes downward with it; when no waste flows, air passage reverses and rises in the pipes to escape from the house. This two-way movement is made possible by a venting system'a network of pipes linking many branch drains to the main soil stack, which has a chimney-like pipe rising through the roof to let air in and out.

Plumbing alterations of any kind are controlled by local regulations.

There is no nationwide code for the United States or Canada?the so-called National Plumbing Code of the United States sets minimum standards only for federally built structures and may not apply to your home. Local codes have the force of law, and must be observed, but none of them prevents you from working on your own plumbing system, so long as you follow the provisions in the codes. Because they are established to meet the special problems of the particular area in which you live, observing them not only is necessary, but is also a wise precaution for your own health and safety.

Aside from local building codes, national environmental standards are affecting plumbing more and more. While many of these laws are directed at community waste treatment and water supply facilities, one part of the federal Safe Drinking Water Act of 1 986 directly affects the home plumber: it is no longer legal for any solder containing lead to join the parts of a plumbing system containing potable water. Instead of the old standby 50-50 solder, which contains half tin and half lead, home milwaukee plumbers must now use solders containing mixtures of copper, tin, silver or other metals. Although some of these are more difficult to use, at least one safe-water solder looks just like 50-50 and is used in the same way. Others will soon be developed.

Also because of environmental concerns, many communities today require water-saving toilets, showerheads and faucet sets in all new construction and in existing homes when replacements are made.

Local codes also typically take into account the hardness or softness of the water?that is, whether it is rich or poor in dissolved minerals. And the nature of your water affects the kind of pipe that you should use. Almost everywhere in the United States and Canada, for example, copper pipe is acceptable?but not quite everywhere. In some areas of hard water, minerals build deposits on the inner walls of copper tubing so quickly that supply lines clog. In some soft-water areas, on the other hand, copper tubing is vulnerable to acids in the water. In parts of both areas, copper pipe is prohibited: plumbers must use plastic or galvanized steel.