The most important aspect of an OSSF design is an accurate site evaluation of the property by a professional site evaluator. Our job as a site evaluator is to assess site conditions, drainage, and soil types; and determine the best type of OSSF for each particular property, home or building size.

We first have to determine where the most suitable site location for the proposed OSSF would be. Considerations would be drainage, setbacks from property lines, water wells, easements, recharge features, slope, lot size and flood hazards.

Once the location is determined, a minimum of two soil profile holes shall be excavated to a minimum depth of 48 inches towards each end of the designated area. The best method for profile hole excavations is with a backhoe. This method provides the most accurate visual profile of the soils. By visual inspection of the profile hole, the evaluator will look for signs of restrictive horizons such as rock or dense clay strata or ground water that would hinder the movement of water downward and through the soils. If there are no restrictive horizons present, a soil analysis is then performed on the most restrictive soil present to determine soil texture and classification.

The percentage of sand, silt and clay determine the classification of soil. Soils are classified as 1b (sand and loamy sand), II (sandy loam and loam), III (sandy clay) and IV (soils which contains more for than 40% clay). Class Ib, II and III are considered suitable soils. Soil absorption OSSFs can be utilized in these types of soils with sufficient soil depth. A class IV soil is considered unsuitable and these soils cannot be considered for soil absorption OSSF; therefore, an alternative type OSSF would be utilized.

Once the soils are properly classified, the proper type OSSF can be determined. Once the type of OSSF is established, then sizing the OSSF according to the number of bedroom and square footage of the home and number occupants can be determined. For a commercial facility the number of employees and building usage will determine the sizing of the OSSF.