Vent stacks are part of the plumbing system, designed to allow adequate air circulation in all parts of the sanitary drainage system and provide a means for sewer gas to vent harmlessly to the atmosphere. All homes should have at least one vent stack.
If a house does not have any vent stacks, it is usually and indication that the plumbing system is not properly vented – almost always a violation of the plumbing code. During a roof inspection, look for a pipe that projects through tre roof and terminates about eight inches above the roofline. The vent stack is usually quite obvious, especially in newer homes. In some older ones, however, it might be missing or not visible when looking at the roof. In some cases, vent stacks terminate inside the attic, but this situation is also a violation of the plumbing code. The vent stack must be extended above the roofline so that the exiting gases can discharge to the outside. If you come across this condition, record it on your worksheet so vent pipe extensions shall be installed.
Moreover, in some older homes, when bathrooms or kitchens are renovated, vent stacks are often run up along the outside of the building to a point above the roofline. However, with homes located in the northern part of the U.S., this type of installation is undesirable. Because of the moisture in the escaping gas, it could freeze in cold weather and eventually block the pipe. Also undesirable are vent stacks that terminate near a window, because the discharging sewer gas might get inside the house when the window is open.
With properly installed vent stacks, i.e. that terminate above the roof, often there is a black ring at the base of the stack adjacent to the flashing. This black ring is asphalt cement, used to seal the joint. This joint should be regularly inspected because it is vulnerable to leakage.
Sewer Vent Stack Filters – Eliminate Foul Odors from Sewer Gases
If you notice sewer or septic smells inside a home or business but you are not near a plumbing fixture, the odors are most likely coming from the sewer vent stacks on the roof. Unless they are fitted with a filter, all plumbing vent stacks produce foul odors, including that “rotten egg” smell produced by hydrogen sulfide gas. One of the most common plumbing vent problems is the sewer gas (H2S) drawn into the fresh air intakes on the roof from the air conditioning systems. Industrial buildings code also require fresh air intake for building air balance against all the stale air exhausted through roof vents. Basic smell of odor when on the roof or around the perimeter of the building will help you identify this potential problem. However, a professional can use a H2S measuring device to detect and measure sewer gas concentration omitted from certain roof vent stacks. Most homes and buildings have vents omitting gas because this is the way they are designed. 
Raising roof vent stacks will not solve the problem, it will just displace it. On a non-windy day, sewer gas is heavier than air and will settle on the roof, causing intermittent sewer odors in the home or building. In a windy day, it will blow over the exterior wall to open windows, patio, fireplace, garden, pool area, driveway and so on.
One of the odor filters available on the market is charcoal vent stack filters, but it is less effective when introduced to moisture – it will solidify or cake up. Sewer gas from the vent stacks is extremely high with moisture, as the system’s main function is to drain water in a sealed system underground, away from the building.
Vent stack filters that use Zeocarbon may be a better odor eliminator. Zeocarbon is a mixture of carbon and zeolite rock, which has been proved to work up to seven times longer than regular carbon alone. Vent stack filters based on this technology are unique because they actually regenerate themselves for up to five years, using sunlight. The interior, exterior and zeocarbon replaceable bags are impregnated with titanium dioxide, which produces ozone when exposed to direct sunlight, destroying odors and regenerating the zeocarbon.