The tile adhesive is the bonding agent that is going to allow your tile to adhere to the top layer of your built up shower pan. The adhesive is important in providing further strength and bonding to the shower floor during normal shower usage. There are varying types of tile adhesives which have specific uses. You need a good understanding of these different types and where each can be used as a bonding agent in order to have an effective tile installation.
Dry Mixed Adhesive
A typical complete tile installation in a built up shower pan over a shower floor will use a thin-set mortar mix over the top layer substrate. There are two specific tile adhesives, first is the mix on site version which is sold as a dry mix and intended to be mixed with water just before applying it to your top layer. This thin-set contains a portland cement base, which has a grainy but not runny consistency when mixed, this is easy to spread using a trowel. Depending upon manufacturer, the thin-set mix will also contain specific inorganic aggregates and chemicals to ensure that a proper bond is made between the top layer and tile.
The second is a pre-mixed version which is sold in a plastic container and most commonly will use the words “pre-mixed thin-set mortar” or “adhesive” in it’s name or description. It is important to understand that some pre-mixed versions contain mastic as their primary bonding agent, these manufacturers will identify their product with the word “adhesive”. Mastic is considered an organic adhesive and upon submersion in water, it begins to deteriorate, therefore lessening the integrity of your tile installation. The manufacturer will identify the locations where their product is not advised to be used, most often the shower floor, where their product will come into contact with the most amount of moisture, and even be submerged. Given it’s organic nature, it is not considered to be an effective bonding agent between the top layer and tile.
Modified or Un-Modified
A modified thin-set mortar is considered to be any mix which contains polymers formulated into the dry mix that increase its performance and strength. These polymers allow for the cement to retain strength, while also containing a sufficient amount of flexibility, therefore giving the tile installation over it a solid bond that allows for movement.
An un-modified thin-set mortar does not contain these polymers. If an un-modified thin-set mortar mix is used as a bonding agent, in order to increase its strength and flexibility, a liquid polymer would have to added in place of water, or per manufacturer specification, during the mixing process.
Mixed or Pre-mixed
A mixed on-site or pre-mixed thin-set mortar mix each have their uses inside your shower pan. It is a standard for complete tile installations on a shower floor to use a mixed on-site thin-set mortar mix, first for the benefit of strength and flexibly, and second for the justification of purchasing a single or multiple dry mix bags. The pre-mixed thin-set mortar mix, if the primary bonding agent is organic in nature, is best to be used for maintenance purposes, such as where a single tile needs to be replaced.
The application of your tile adhesive to the top layer of your shower floor and to the tile itself is just as important as your choice of mix and modification types. The tile adhesive should be applied with a trowel for better manipulation across the top layer. Where the top layer appears to have dips or the pitch is not consistent to the shower drain, you can apply the necessary amount of thin-set needed to level the specific area of the top layer. Where you intend to install a tile piece, apply a sufficient amount over that immediate area, then using the trowels teeth, scrape to leave peaks and valleys. This will ensure that a vacuum seal is created when a tile piece is set over it. The tile adhesive is also applied to the back of the tile piece, referred to as buttering, covering the entire surface and removing excess at the edges. This will give you ample thin-set to ensure a solid bond is achieved with the top layer and also enough to allow for the tile to be manipulated and aligned correctly for a clean finished installation. It is also important to not use too much tile adhesive on the top layer and the tile itself, since excess would be manipulated outward, and if left to cure, there would not be available area for grout to enter in between each tile piece.