Almost all the homes in Oak Park were built with gravity sewer systems. This means the house sewer lines, which typically are only four-to-six inches in diameter, run directly into the main sewer in the street. Many homes also have all internal drain lines from toilets, sinks and other fixtures, as well as gutter downspouts, connected to their sewer system, which leads to the main system in the street. When the main sewer line in the street is full, the flow of house sewer lines into the main sewer line is greatly diminished. As rainfall levels exceed the municipal system capacity, the sewer lines cannot handle the additional loads from downspouts and water use inside the home, sometimes causing them to back up into the house.