A hydrostatic test is the process of temporarily plugging the sanitary drain piping under a house, filling the system with water and monitoring the water level to see if it drops. A falling water level indicates a leak on the drain piping beneath the residence. Hydrostatic testing should be performed when leaks are suspected on the drain piping under a residence. Hydrostatic testing can also be performed as part of a pre-purchase home inspection whenever the budget allows, particularly when purchasing a property at high risk for underslab leaks. These high-risk properties include homes constructed using cast-iron sanitary piping (built prior to 1980) and homes which have had prior foundation repair (foundation repair often damages the underslab drain piping).
The consequences resulting from the underslab sanitary piping failing a hydrostatic test are both complex and controversial. In addition to the fact that leaks on the underslab sanitary piping are very costly to repair, they are also the number one blamed cause of foundation damage by representatives of the foundation repair industry. Many of the leaks discovered during hydrostatic testing do not allow drainage to escape the piping under normal use conditions, have no potential to cause foundation damage, and therefore their repair could be considered unnecessary. However, once leaks are discovered and documented, buyers and sellers will have to reach an agreement in dealing with the consequences.
A hydrostatic test begins with locating the sewer cleanout or installing one if required. Prior to the early seventies, cast-iron piping was used for the sanitary drainage system under residences. Many of these older homes had multiple sanitary systems exiting from beneath the foundation in different locations and joining together in the yard before connecting to the city sewer main. Each of these independent sanitary systems must be tested separately, requiring time to locate and possibly excavate each exit from beneath the foundation. Homes constructed after the early 1970’s generally use plastic piping (ABS and PVC) for their sanitary drainage, have only one exit from beneath the foundation, and typically have a readily accessible main cleanout. In general, a four- or five-bathroom home constructed using plastic sanitary piping can be tested more quickly and less expensively than a one-bathroom home constructed using cast-iron sanitary piping.
After a cleanout has been located or installed a testball is inserted into the piping and inflated near the perimeter of the foundation. The piping is then filled with water, and the water level is monitored at a first floor shower or commode flange. A falling water level indicates a leak on the system. Additional tests (isolation and flow) are necessary to determine the location of the leak(s) and whether or not the leak(s) allows drainage to escape the piping under normal service conditions.
The cost of a hydrostatic test through an existing cleanout averages between $250 and $400. The variance in the basic hydrostatic test price depends on the following factors: How difficult was it to locate the cleanout? Was in necessary to remove and reset a commode to observe the water level during testing? How extensive was the video pipe inspection? What type of documentation was requested (written report, photographs, drawings, copy of video inspection)? On older homes with multiple independent cast-iron sanitary systems, many of which will require locating and excavating, the hydrostatic testing of all the independent system may easily double or even triple the average price. In general regarding time requirements, houses constructed since the early 1970’s typically require about two hours for hydrostatic testing. Houses constructed prior to the early 1970’s may take anywhere from two hours to an entire day, depending on the number of independent sanitary systems and the need for excavations, cleanout installations, and removing and resetting commodes.
This definition of hydrostatic testing is from Vortex Technologies.