Thinset is a tile adhesive composed of Portland cement, sand, water retention agents and if desired, latex polymers that increase strength and extend drying time. It is usually applied 3/8" thick or less thus the name "thinset". It is usually used to attach tile to a cementious backing and it is recommended for residential floor installations and wall installations that are subject to heavy contact with water such as shower walls and floors. It is slow setting and subject to sagging so plastic tile spacers are recommended during installation.