Septic Tanks & Perc. Tests

One of the first things you learn when you go to evaluate your lot for building, is whether or not there is public sewer available to tap into. In many cases, available lots are located in more rural locations where public sewer is not available so you have to turn to the dreaded septic system to dispose of your waste water. There's a ton of confusion, misinformation and horror stories around this aspect of the homebuilding process. But have no fear - we're here to walk you through the 'need to know' aspects of this stage of your home building journey.

The 'Key Take-Aways':

  1. Land should only be purchased with an offer contingent on a successful perc test and permit.

  2. A failed perc test is not the end of the road. There are a variety of higher-cost but effective alternative septic systems.

  3. Septic systems are tricky to layout. Familiarize yourself with the lot characteristics and regulations to ensure that you can place your Home Plan in the perfect spot without affecting the functionality of your septic system.

  4. A local installer will help guild you through the process and will be very knowledgeable about your local jurisdiction's rules, regulations, and costs.

Septic Tank Diagram

Background:

A septic system typically consists of an underground pipe containing all the wastewater from your house that feeds into an underground 'septic tank' about 10-15ft from your home. This tank is sized according to the number of occupants your house has in order to allow gravity and bacteria to separate out the solids to the bottom of the tank and allow the remaining effluent to flow out the other side into an underground zig-zag perforated pipe called the 'drain field' (it is often referred to as a 'leach field' as well). The length of the perforated drain field pipe is also sized according to the number of occupants as well as the soil conditions.

A Note on Regulations: The testing, design, permitting and installation of a septic system is typically governed by your local health department. The first step in to contact them and understand their location specific requirements are. We recommend retaining a local installer who will be very knowledgeable in this domain. Sometime the rules are not explicitly written down or made available so having an expert to help navigate this aspect of homebuilding is important.

Placement Notes:

Septic Tank:

Typically located 10-15 feet from the edge of the house and in an area unencumbered by landscaping or hardscaping. The tank will have several manholes that need to be accessible for regular maintenance. Plan to keep these and the area around them clear and accessible to a maintenance truck.

Drain Field:

Locating the drain field is the most complicated part of this process. It's best to meet the following criteria:

  1. Lower Elevation than the tank. Because the water flows via gravity, it's best to choose a drain field that's lower in elevation than the output of the tank. If this is not possible then your septic installer will have to consider a pumped solution which adds cost and maintenance complexity.

  2. Clear of Vegetation and Future Development Needs. A drain field will have to be excavated so it's best to choose an already clear and level site. It also needs to be kept clear so choose a placement that you will not need to build upon down the road.

  3. Optimal Soils & Drainage. Ideally, undisturbed sandy soils provide the best bed for your drain field. Clay soils will be much less absorbent and require a larger drain field to meet the absorption requirements. In some cases, they may not suffice at all.

  4. Potable Water Buffer. Usually installing a septic system also means that you are going to have to install a well for your water supply. It's important to locate your drain field away from your (and your neighbor's) well heads and associated underground water piping. The typical buffer regulation is 50-200ft.

  5. Wetland Buffer. Most jurisdictions prohibit placing a septic tank or drain field within a buffer zone to surface water like lakes, streams, rivers, wetlands etc.

  6. Ground Water Levels. Most jurisdictions will specify a minimum vertical distance (around 5ft typically) between the drain field trench and the highest annual level of the groundwater table during the peak wet season to minimize direct contamination of groundwater. For this reason, many jurisdictions only permit percolation testing in the spring.

The Perc(olation) Test:

Now that we understand the background criteria for locating all the components of your septic system we can proceed with the perc test. Note that a failed perc test makes it much harder (and in some cases impossible) to build. Your offer to purchase land should always be contingent on a successful perc test and permit. Note also that perc tests often expire after a few years so it's important to ensure that they will be valid when you go to pull permits.

The methodology for performing your perc test will be governed by your local jurisdiction. It involves a professional digging one or more test holes in your proposed drain field (under the supervision of your local inspector) filling it with water and measuring the rate of infiltration. Sometimes the local jurisdiction will require a deep test hole to be dug 8-10ft deep with a backhoe.

The rate of infiltration will be input into your drain field design to determine the length and spacing of the perforated pipe.

Cost:

A perc test tends to cost between $750-1750 to perform. The local jurisdiction may also charge a fee.

Failed Perc Test:

With much of the buildable land being bought up, this happens with increasing frequency. It's not the end of the road. If a conventional system won't work then there's a variety of alternative septic systems that you can explore depending on you local jurisdiction's rules.

Alternative Septic Systems:

Types of Septic Systems Compared
  1. Chamber System - A drain field comprised of a series of interconnected plastic chambers that do not require gravel to function (unlike a conventional system).

  2. Recirculating Sand Filter - An above or below ground system that actively pumps effluent through a tank filled with sand to pre-filter it before heading to the drain field.

  3. Evapotranspiration - This is a type of watertight drain field where the effluent actually evaporated instead of infiltrating. This kind of system only works in hot and dry environments.

  4. Aerobic Systems - these are basically small scale sewage treatment systems. They can drastically reduce the amount of effluent and the required size of the drain field.

  5. Mound/Drip Systems - these are a type of engineered drain field for when the natural soils are too permeable or impermeable poor for proper infiltration or when bedrock or groundwater levels are too high. It consists of an artificial mound placed above the existing soils to pre-filter the effluent before it drains to the natural soils. A key feature of these systems is a tank chamber that doses the effluent at a regular pace to avoid overwhelming the drain field.

  6. Holding Tanks - If all else fails you can always install an underground holding tank which will need to be regularly pumped every 6-8 weeks.

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