According to the International Residential Code, what is the minimum depth of the filter material beneath the perforated line in a gray water trench?

• A. 12 inches
• B. 3 inches
• C. 8 inches
• D. 6 inches

Answer: (D)

The concept being tested is ensuring there is enough filtration media to allow proper drainage and prevent clogging in a gray water trench. The minimum depth of filter material beneath the perforated line is six inches. This amount creates a stable, porous layer that lets the gray water spread from the pipe and percolate through the media into the surrounding soil, reducing velocity and filtering solids before they reach the soil. If there were less media, fine particles could migrate into the soil and clog pores, hurting performance. More depth isn’t required by the code and would add unnecessary material cost. So six inches is the required minimum.