Before installing ceramic or stone tile, most subfloors must be properly prepared with an underlayment or membrane to prevent transmission of movement that can result in cracked tiles or grout. This underlayment can be a preferred bonding material like cement backerboard or an elastomeric membrane. However, an uncoupling mat is another option, particularly if you choose to install over an un-bonded system.
What is Uncoupling?
For centuries, tile has been installed on reinforced mortar beds that have been free to move independently of the subfloor. This has been accomplished with a loose or weakly bonded plane placed between the reinforced mortar bed and the subfloor. If the subfloor moves and a crack opens, it will not be transmitted to the underlayment and through to the tiled surface. This method of installation is still used today and appears in method F111 of the Tile Council of North America (TCNA) Handbook. The un-bonded mortar bed method has a long history of success, providing that the floor will accommodate the addition of an underlayment that is 1-1/2″ – 2″ thick.
Most new construction and remodels today are not designed for this added floor height and weight. These projects rely on the use of the thin-set method to install tile. To minimize the thickness of the floor covering, a thin-set mortar can be used to bond tile directly to the existing flooring, resulting in a total increase in floor height less than 1/2″. Bonded membranes and flexible mortars have been developed to prevent the migration of crack movement in the subfloor to the tiled floor and are detailed in F125 of the TCNA Handbook.
In some situations, a crack isolation membrane or mortar may not be appropriate. For instance, most manufacturers of tile installation products require that concrete slabs cure 28 days before bonding tile directly to the surface. During the first 28 days after pouring, a concrete slab undergoes constant dimensional changes as the concrete cures and shrinks. If tile is bonded to the surface too soon, the stresses developed as the concrete cures can be transmitted to the tile installation and then cause the tile to come loose or create cracks in the tile and grout.
One solution to this is the proven method detailed in F111 for un-bonded, reinforced mortar beds. Another solution is installing uncoupling membranes or mats to provide this function in a less intrusive manner. The tiles are bonded to the top surface of the mat, while the loosely bonded, bottom fleece is bonded to the concrete subfloor. The uncoupling mat absorbs stress from substrate movement and preserves the surface and integrity of the tile and grout.
Installations that Call for Uncoupling
Installation method F128 in the TCNA Handbook describes using this uncoupling membrane over “young concrete.” Young or green concrete is described as concrete that has cured sufficiently to support tile installation traffic. While tile should not be installed the day after the concrete is placed, it may be installed 14 or 21 days after placement.
Custom Building Products has developed Installing An® Uncoupling Mat for these installations. The fleece on Installing An Uncoupling Mat is bonded to the young concrete and then tile is bonded to the geometrically configured surface of the mat. Any movement in the concrete will cause the fleece to loosen and protect the tile installation, while the reinforcing will hold the mortar and tiles securely on the surface.
Installing An Uncoupling Mat is also suitable for installing tiles over single sheet plywood subfloors supported by joists spaced 19.2″ or less o.c. This is detailed in TCNA method F148 and is useful when there is a need to further minimize the height gain of the floor. It is important that an ANSI A118.11 polymer-modified mortar is always used to bond the mat to the plywood surface; an ANSI A118.1 mortar will not develop a bond to the wood surface.
As an added benefit, Installing An Uncoupling Mat creates a waterproof barrier and provides moisture vapor control when correctly installed with Installing An Seam Tape. The mat is made from 70% recycled materials by weight and contributes to green building initiatives.
Use a Mortar Recommended by the Tile Manufacturer
Some of the uncoupling membranes available today recommend the use of an ANSI A118.1 non-modified thin-set mortar to bond the ceramic or stone tile to the plastic surface of the membrane.
However, most manufacturers of porcelain or natural stone tile caution against using a non-modified mortar for installation, instead recommending a polymer-modified mortar meeting the requirements of ANSI A118.4 or A118.15. A dilemma is created if you need to bond a tile requiring an ANSI A118.4 or A118.15 modified mortar to an uncoupling membrane that recommends using an ANSI A118.1 non-modified mortar.
Installing An Uncoupling Mat meets the installation recommendations of many porcelain and natural stone tile manufacturers for use of a polymer-modified mortar meeting ANSI A118.4 or A118.15.
Large Format Tile requires Specific Mortars
The vast majority of tile sold today qualifies as large format tile, which is any tile with a side 15″ or longer. To properly install these tiles, special mortars have been developed to compensate for irregularities in the flatness of the tile and floor. Because an uncoupling mat will follow the contours of the floor, it becomes necessary to compensate for irregularities with tile bonding mortar. Thin-set mortars are suitable for correcting irregularities up to 1/4″. However, when installing large format tiles where both the floor and tile are not completely flat, it may be necessary to correct larger than 1/4″ variations.
In these cases, it is best to use a mortar that has been designed for use with large and heavy tile. These mortars can be applied up to 3/4″ thick. Thicker mortar beds are more easily damaged by movement in the floor. For this reason, many manufacturers of large format tile (LFT) mortars add a polymer modification.
While there are no ANSI specifications for medium bed mortars, most of the available mortars can also be used for thin-set applications and meet the ANSI A118.4 or A118.15 performance specifications. Custom Building Products has developed many mortars to specifically address the needs of large format tiles that work in conjunction with Installing An Uncoupling Mat. ProLite® Large Format Tile & Stone Mortar and Complete ContactTM-LFT Premium Rapid Setting Large Format Tile Mortar are just two of these mortars for setting large and heavy tile.
Improved Bond with Flowable Mortar
The mortar is designed to flow under normal installation pressure to completely fill in the valleys between the ridges left by the notched trowel. This improves coverage of the mortar to the back side of the tile, assuring maximum bond strength. Mortar coverage of up to 95% can be critical with the large format porcelain tile common today.
The unique nature of LFT allows it to support the weight of large format tile, reducing the possibility of lippage between tiles. Its rapid setting formula cures quickly to allow grouting in as little as 2 hours and light traffic in as little as 4 hours. Complete Contact-LFT is polymer-modified and exceeds the requirements of ANSI A118.4F and A118.15F, which is recommended by many manufacturers of porcelain tile and natural stone tile. Complete Contact- LFT also exceeds the requirements of ANSI A118.11, making it ideal for bonding the Installing An Uncoupling Mat to plywood substrates.
Installing An Uncoupling Mat Solves the Problem
While most ceramic and stone tile installation projects will benefit from a fully bonded system, there are instances that demand the unique properties of an uncoupling system. While there are several uncoupling products and methods available to the installer, the use of an uncoupling mat minimizes the floor height gain from additional tile installation materials. This translates into less work in surrounding areas to maintain level floors when transitioning between different floor coverings.
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