Slab leaks are caused by small holes that form in the pipes under your concrete foundation (if they form above the concrete they are called pinhole leaks, and most often occur in the wall or ceiling).  There are a few things that can cause these holes, including:

– The foundation shifting

– Too high of water pressure

– The chemicals in the water reacting with the pipes

– The chemicals in the soil reacting with the pipes

– Low quality work/materials/supplies

– Something puncturing the pipes, such as a nail from another project

– An improperly wrapped pipe that comes into contact with something else and wears away over time

– Movement of the pipes.  Since pipes can move a bit when water flows through them, and hot water pipes may expand and contract as the water changes temperature, friction can eventually cause a hole to form.

Some experts recommend that you re-pipe your home if you have experienced more than two previous slab or pinhole leaks.