A sub-slab depressurization system consists of a fan/blower which draws air from the soil beneath a building and discharges it to the atmosphere through a discharge pipe. Low Pressure/High Flow systems generally use 4 inch diameter piping which is installed in an extraction trench. The piping is connected to a fan/blower, which is mounted outside of the building. Exhaust piping is run so that the discharge is above the roof line.