DEAR TIM: I have two pipes which project through the roof on the front of my house. I have been told that they are plumbing pipes. They are very objectionable. I am getting ready to install a new roof on my house. The pipes are clearly visible in the attic. Are they really necessary? Is it possible to have them moved to the back side of the roof? D. T.

DEAR D. T.: Plumbing vent pipe locations have angered many homeowners. In almost all cases, the frustration and disappointment that homeowners felt could have been avoided.

Plumbing vent pipes are a necessity. They allow your plumbing drainage system to breathe. Their primary function is to equalize pressure in the drainage system every time you run water in a sink, drain a bathtub, or flush a toilet.

In many homes, all of the plumbing fixtures are interconnected to the same drainage system. The system looks very similar to a tree. In fact, certain parts of a plumbing system are called branches. These branches can drain one or more fixtures.

If a drainage system is constructed without vents, strange things begin to happen. For example, you may flush a toilet on the second floor of your house and the kitchen sink drain might begin to make strange sounds. The drain will sound like someone with a giant straw is sucking the water from the trap beneath the sink. In fact, that is exactly what is occurring.

This happens for a very simple reason. As the water from the toilet drains down the pipe going to the basement or sewer, it is pushing ahead of it the air that was in the pipe just before you tripped the toilet handle. This creates a vacuum just behind the water. Because the system is unvented, it searches for the easiest place to replace this air. The vacuum can actually be strong enough to suction the water from a fixture trap. It is extremely important for the water to remain in the traps below the fixtures. This water seal stops offensive odors and vermin from entering your house.

Plumbing fixture vents solve this problem. Plumbing vents form another type of tree within your house. For the most part, each plumbing fixture has a vent pipe associated with it. These vent pipes also interconnect in many instances. The termination point for the vent system is the roof of your house. The roof vent pipes are the point where the replacement air enters your plumbing system as a fixture drains water. These vent pipes have certain requirements with regards to their size and length. They also must be installed in such a manner so as to continually slope toward the drainage lines. Water or debris must not be allowed to collect in vent pipes. Blocked vent pipes can create a scenario as described above. If one of your existing drains bubbles or gurgles when another fixture is draining, you may have a blocked vent pipe.

Vent pipes, in many instances, can be installed so that they exit the roof where you would find them to be the least objectionable. The plumber merely has to avoid as many sharp bends as possible. Based upon your description, your vents pipes can be easily relocated.

Planning comes into play as well. Architects and designers should consult with plumbers if they intend to place plumbing fixtures on the front half of a house. Many plumbers will gladly discuss what they can and can't do with the vent pipes. This simple step will go a long way in avoiding homeowner disappointment and frustration.