A recent reader letter triggered a reminder on the importance of addressing venting and odor issues. A homeowner from Wisconsin wrote complaining about sewage odors both inside and outside the house. He explained they live on top of a hill, which is always windy, indicated by the lack of mosquitoes in the summer. (Only people in upper Michigan, Wisconsin and Minnesota understand the full significance of this.) He also explained their onsite system includes a septic tank and drainfield trenches fed by gravity — so a conventional system.

In a typical setup like this, the drainfield and septic tank vent back through the roof stack of the house. One common cause of odors outside the house is that the roof vent does not extend far enough above the roofline to carry away gases and odors. Before moving directly to the roof vent, though, the service provider should check the septic tank and soil treatment area to make sure the outside odor is not caused by sewage coming to the surface in the drainfield. In addition, all manhole covers and inspection ports on the septic tank should be checked to ensure a tight fit.

Search for Leaks

If the openings to the septic tank are not watertight and airtight, wind blowing across the tank will draw the air and odors from the tank into the yard. If the manhole covers are buried 6 inches under the soil this is not a problem. However, many states now require manhole risers and covers be brought to the surface for easier maintenance. So, it is important the covers (lids) be on tight.

We have seen this more on plastic lids and risers when all of the screws are not tightened down. This is a concern for releasing odors, but it’s worse from a safety standpoint. Every year there are reports of children falling into septic tanks because lids are not being properly secured. When you leave the site as an installer or service provider, make sure the covers are secure.

When concrete covers seem to be part of the odor problem, weather stripping can usually be installed around the lid. However, in no case should a sealant or glue be used on the lids. This defeats the purpose of allowing access for maintaining the system. Unfortunately, we still see this when we visit some sites. We realize that homeowners can be very upset with odors, but if it takes replacing the risers and lids to provide a good seal, that should be the solution.

Venting Issues

If you eliminate the tank and drainfield as the source of odors, turn your attention to the roof vent. Look at other houses in the area and you may see vents extending only a foot to 18 inches above the roof and there are no apparent odor problems. Tall trees around the house or a hill behind the house can cause air currents to swirl, creating downdrafts that send odors down to the ground around the house. Homeowners may say things like, “I only get the odors when the wind shifts to the north.” Wind direction and speed can impact the presence of odors.

For a typical ranch-style-home roof with a 4 to 12 pitch (4 inches vertical in 12 inches) the house sewer vent should extend 6 feet 2 inches above the roof or 2 feet above the roof ridge. The 4 to 12 pitch is a very common slope and you see very few 6-foot vent pipe extensions. For roofs with steeper pitches, the pipe extension will need to be even longer to meet the 2 feet above roof ridge criteria. So even though the home is situated on a hill, if the vent pipe does not extend above the roofline, sewer odors can be directed downward under certain conditions.

Moving Inside

In terms of odors in the house, first make sure they are not coming from outside through an open window or door. Odors due to effluent surfacing or from the roof vent can be drawn into the house. One of the main causes we have seen for indoor odors is when a plumbing trap has dried out or been emptied for some reason. The empty trap allows sewer gases to be vented back through the house plumbing.

A common reason for a dry trap is in older houses where either a floor drain or drain for a washing machine is no longer used, and either the owner has forgotten about it or they bought the house and were not aware of the drain. We have seen floor drains covered over during basement remodeling and then drying out over time, creating odor problems. The solution, of course, is to locate the drain and either properly close it off so it is airtight, or to periodically add water to the trap. Homeowners who must tear out walls or flooring to get at the offending drain are usually not happy!

Investigating further, make sure sewer is airtight to the inside and properly vented through the roof vent. Depending on age and service, the lids may not fit tight or the vent piping may not be properly glued, allowing sewer gas and odors to enter the basement. We hope readers share their own take on sewer odors and problems they have seen.