Understanding Hydrostatic Tests

What is a hydrostatic test?

A hydrostatic test is performed to determine if there is a leak in the sewer lines underneath a home’s slab foundation. 

First, a rubber inflatable ball is used to block the main sewer line at the clean-out.  Next, the sewer lines are filled with clean water, typically by removing a toilet inside the house as an observation point and running water from a faucet to fill the entire sewer system up to slab level. Then the water level is monitored for about 20-30 minutes.  If the water level drops while the system is clogged at the clean-out, this would indicate there is a leak. 

How do you know if you need a hydrostatic test?

If you’re buying a new home, you’ll definitely want the home inspected.  During inspections, the general home inspector and/or a structural engineer might discover that portions of the home’s foundation might possibly be sagging after taking measurements with a Ziplevel. This could be a sign that the under-slab sewer lines are leaking, as the leaking moisture would cause the soil under the foundation to erode, thereby causing it to sag in certain spots. (This is more common in older homes with cast iron sewer lines.)

In instances like these, a hydrostatic test would be needed to determine whether or not a leak might be present.

Can a hydrostatic test damage the plumbing system?

The hydrostatic test in and of itself wouldn’t be the cause if something is damaged during the test. If damage occurs, it is only because something was already broken, i.e. already cracked, and when the system was filled with water the crack was exposed.

Let’s remember, during a residential hydrostatic test the water isn’t pressurized. You’re just filling it up with normal running water from a faucet, which is something the sewer lines, by design, should be able to handle. Therefore if something breaks or busts, it was already broken before the test.

An example from inhouseplumbing.com further illustrates this point.  Imagine filling a glass of water to the very top. Also imagine that the bottom of the glass represents our “blockage”, like the rubber ball in the clean-out during a hydrostatic test. If the integrity of the glass is intact, nothing will happen. But if the glass were to leak or burst, there was something wrong with the glass in the first place.