Is Thinset Waterproof?

By Samuel N •  Updated: 07/03/23 • 

Thinset or mortar is a product to install flooring and wall tiles. It is made from a mixture of cement, sand, and some water retention agents and comes in a dry mix or pre-mixed variants. Thinset is thinner than mortar, which enables it to spread evenly across a surface for an even surface.

Thinset is only used to bond tiles over a floor or walls. Although some thinset is water resistant, like modified thinset, they are not waterproof. This means water can flow over the thinset and will not penetrate. However, when standing water is left to sit on a thinset, it penetrates and goes under the tiles.

Applying the Thinset Using the Notched Edge of the Trowel

Applying the Thinset Using the Notched Edge of the Trowel

Modified vs. Unmodified Thinset

When choosing the type of thinset to use, there are modified and unmodified thinset options. Unmodified thinset has been around for a long time and is the most popular option.

An unmodified thin-set is made from three ingredients: cement, sand, and water-retention additives. Modified thinset is similar to unmodified thinset but includes additional latex or acrylic polymers. Depending on how you’re installing the tile or the size of the tile, among other reasons, you can choose to use a modified or unmodified thinset.

Applying Porcelain Tile Polymer Modified Thinset Mortar

Applying Porcelain Tile Polymer Modified Thinset Mortar

The additives used in the modified thinset make it retain more water and cure slowly. However, it also makes the product more durable, stronger, and flexible once fully cured. These additives can come pre-mixed or in liquid form that you mix with these instead of using water.

Latex-modified thinset is more resistant to water compared to unmodified thinset. However, this does not make it waterproof because it can still draw moisture from the joints.

Thinset vs. Grout

Grout comes in different colors and is a paste-like product used to fill the gaps between tiles. Unlike thinset, which is used to lay tiles down, the main function of grout is to fill the spaces between your tiles. It also adds rigidity and strength to your overall tile installation.

There are different types of grout, from epoxy, cementitious, sanded, and unsanded. Of all these grout options, epoxy is the one that does not require to be sealed and is the most durable option. However, other types of grout will need to be sealed because they are naturally porous. Sealing helps protect the grout from staining and also reduces water penetration.

Samuel N

Samuel N is the founder of Improve Floor and has been in the flooring industry since 2005. Since then, his mission has been to make flooring easier for everyone. He helps countless people with flooring installation, finishing, maintenance, and repairs each year.