The foundation for the bath, as a rule, is made of strip. In this case, the depth of its occurrence must be calculated from how much the earth freezes in the region. Usually used a coefficient of 0.5-0.7 from the planned depth of freezing. The quality of the drain is of great importance. Experts say that the service life of the bath depends on how the drain is made and made. If you know that no more than 4 people will wash in the bathhouse at a time, then you can make a simple drainage pit under the floor of the bathhouse. If the number of washing ones will be larger, then in addition to the drainage pit, you will have to dig a well, to which the pipe from the drain will be directed.