Abstract: A water-proof wall trim system for an interior building wall includes a baseboard trim element which is affixed directly to the wall studs and which abuts the floor underlayment. A tongue extends from the top of the baseboard trim an abutment with the bottom edge of wallboard. The depth of the tongue is equal to the depth of the wallboard so that a flush joint between the wallboard and trim is formed. A wallboard covering element, such as tile or paneling may then be laid over the front face of the tongue and the wallboard which are co-planar. This covers over the flush joint to ensure a water-tight seal, preventing any wash-water from reaching the joint between the wallboard and top edge of the baseboard trim. The baseboard trim further includes a sloping incline portion at the top and a front face which extends from the bottom of the incline to the floor covering.