Undertile refers to materials, membranes, or systems designed to be installed directly beneath a tiled surface. In roofing, this usually means the breathable membrane or felt that acts as a secondary weather barrier. In flooring or bathrooms, it refers to waterproofing layers or radiant heating mats laid over the subfloor before the finish tiles are set.
Heard on the job
Roll out the undertile membrane and overlap the seams before we start carrying the slates up.