Some sub-floors are inset below the top surface level of surrounding flooring's joists and such subfloors and a normal height joist are joined to make a plywood box both molding and containing at least of concrete (A mud floor" in builders' parlance). Alternatively, only a slightly inset floor topped by a fibrous mesh and concrete building composite floor cladding is used for smaller high quality tile floors these "concrete" subfloors have a good thermal match with ceramic tiles and so are popular with builders constructing kitchen, laundry and especially both common and high end bathrooms and any other room where large expanses of well supported ceramic tile will be used as a finished floor. Floors using small ( and smaller) ceramic tiles generally use only an additional layer of plywood (if that) and substitute adhesive and substrate materials making do with both a flexible joints and semi-flexible mounting compounds and so are designed to withstand the greater flexing which large tiles cannot tolerate without breaking.