Bulletin #18

Revised May 3, 2020

Large and Heavy Tile Mortar Definition

Large and Heavy Tile "thin-set" mortars or what have formerly been classified as Medium Bed Mortar, have been around the tile industry for many years. They initially were developed to compensate for the variations in the thickness of natural stone tile during a thinset tile installation. Today, Large and Heavy Tile mortars continue to grow in popularity and are used for a variety of stone tiles, to compensate for the slight irregularities in tile like warpage and for today’s larger and heavier tile.

Unfortunately, the tile industry has seen an increase in failures due to the inappropriate use of “normal” thinset built up too thick in large format tile installations. A normal thinset is intended to go from a minimum of 3/32” to a maximum of ¼” after the trowel notches have been beat in. When a normal thinset is used too thick, shrinkage of the mortar can occur, as a result of the lower ratio of sand to cement mix in the normal thinset. This then can cause bond loss, depressions, lippage, irregular grout joints and cracking in the tile installation. Due to the increase in failures, thinset thickness had become an industry subject of concern. MMSA members quickly realized the industries lack of definition and recommended thickness for normal as well as Large and Heavy Tile Mortar thin-set. Large and Heavy Tile dry-set mortars or Large and Heavy Tile latex-Portland cement mortars must be used in cementitious bond coats from 3/32" to 1/2" thick after the tile is embedded.

Large and Heavy Tile Mortar industry standard has become part of ANSI A118.1, ANSI A118.4 and ANSI A118.15 standard. Test methods are descripted for the Large and Heavy Tile Mortar as an example test for shrinkage and lippage with heavy tile. Those that pass that part of the mortar standards, receive the H designation. An LHT mortar could be identified as ANSI A118.4H. This designation is used like the other ANSI A118.1, ANSI A118.4 and ANSI A118.15 mortar designations passed in 2012: F for Fast Setting, E for Extended Open Time or T for Non-Sag. The benefit, for a Gauged Porcelain Tile project some characteristics that would be beneficial are a large and heavy tile mortar with extended open time and non-sag, so look for a A118.15EHT mortar.

The intention of the ANSI A108 changes is to provide guidelines on mortar characteristics to the design professional, specifier and builder of the likely need for Large and Heavy Tile mortars. These mortars are not designed to be used for the trueing of substrates out of tolerance. Proper surface preparation should be done before the installation of large format tile/stone. The tile industry has struggled with change orders and trying to get paid when dealing with the increased need for flatter and truer substrates due to large format tiles. These potential changes in the tile industry standards may require education to the designer but provide the builder, tile installer and the owner with a more biddable, durable and successfully installed tile project.