Replacing an underlayment under a wall is necessary when the underlayment is no longer structurally sound. The subfloor is the solid foundation under the floor covering that it depends on for the stability of the floor and walls. It is not the main support system, it is the work of beams, posts and beams, but it does its part in the work.rnThe subfloor can rot with water, usually the case with the walls behind the shower controls or the walls under the malfunctioning windows. In some cases...