Ever wonder what those pipes sticking out of the roof of houses is for? Why would someone put an open pipe through the roof of their house? The answer lies in modern day plumbing.
Sometimes the pipes are a means of exhaust for a gas fired appliance so it can safely release the carbon monoxide gases created during the combustion process. Other times the pipes are used to vent the drainage lines in a home. These are the ones we are discussing. When used in this application it is called a soil pipe. Soil pipes are typically made of either PVC, ABS, cast iron, or galvanized steel. The soil pipes are an integral part of the drainage system and are used to vent the drain lines to allow them to flow properly.
To better understand this concept, think of a water bottle filled with water. If you flip the water bottle upside down, the water will drop out in gulps as it tries to replace the lost water with air in the bottle. It does not flow smoothly and may seem like it is taking forever. After doing this you may have discovered that by holding the water bottle upside down at an angle will allow the water to flow faster. This is because air is more easily able to access the bottle to replace the water. The same principle can be applied as water attempts to flow through the drain lines of a home. Without a vent of some sort the water being drained will not be able to flow smoothly as there is limited air to replace the space the water was taking up. Suppose you had a tub full of water and wanted to drain it. When you pull the drain plug you would see air bubbles come out of the drain as the water replaces the air in the drain line. To correct this soil vent pipes are used. Instead of the water having to replace the air, the water easily pushes the air out the vent pipe.
The picture above gives a visual as to how soil pipe vents are plumbed. They are tied directly into the sewage lines of the home and depending on the layout of the home there may be more than one soil vent stack through the roof. The next time you see these pipes extending through the roof of a home you will now know their purpose.