Subsoil drains shall be open-jointed, horizontally spill or perforated pipe conforming to one of the standards listed in Table 18-29-1102.5. Such drains shall not be less than 4 inches (100 mm) in diameter. Where the building is subject to backwater, the subsoil drain shall be protected by an accessibly located backwater valve. Subsoil drains shall discharge to a trapped sump or approved location above ground. The subsoil sump shall not be required to have either a gastight cover or a vent. Access for the purpose of cleaning and removing obstructions in subsoil drains shall be provided at every change of direction.