Except when erected upon solid rock or otherwise protected from frost, foundations, walls, piers and other permanent supports of structures shall extend thirty (30) inches below finished grade, and spread footings of adequate size shall be provided when necessary to properly distribute the load within the allowable bearing value of the soil, or such structures shall be supported on piles when solid earth to rock is not available. Footings shall not be founded on frozen soils unless such frozen condition is of a permanent character.