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Cleavage Membrane

What is thinset mortar, dryset mortar, or drybond mortar?

Thinset mortar is a blend of cement, very finely graded sand, and a water retention compound that allows the cement to properly hydrate. Tile set by the thinset method is adhered to the substrate with a thin layer of "thinset" cement. The terms thinset cement, thinset mortar, dryset mortar, and drybond mortar are synonymous. This type of cement is designed to adhere well in a thin layer - typically not greater than 3/16th thick. For example, a 3/8" notch trowel will produce a 3/16th inch thick coating after the tiles are pressed in to the cement. While very minor adjustments in height can be made, this method is not appropriate for adjusting the level or flatness of a surface - rather the tile will follow the plane of the substrate.

The American National Standards Institute (ANSI) defines the properties of thinset mortar in the A118.1 specification.

What is a thick-bed installation, mortar bed installation, or thick-set installation?

Thick-bed installations are based on the traditional method of packing a mortar bed over a surface before installing the tile. The tile is adhered to the mortar bed either while the mortar bed is green (just beginning to dry) or after the mortar bed has cured. The mortar bed may be reinforced with wire and either set over a cleavage membrane (that allows the mortar bed to "float" free of the substrate) or bonded to the substrate; hence, the use of the terms "floating mortar bed" or "bonded mortar bed". For wall applications, metal lath is mechanically anchored to the substrate, and the mortar locks into the metal lath as it cures. The terms thick-bed installation, mortar bed installation, and thick-set installation are synonymous.

What are the advantages of a mortar bed?

In the case of the floating mortar bed, the tile layer is unaffected by minor cracking and movement in the substrate. This can be very important in applications over concrete where cracking in the concrete could result in cracking in the tile. It is also very important in tile installations over structural slabs (not on grade) or other structures where vibration and deflection can be expected (as occurs in some exterior walls).

Mortar beds also allow for the following:

  1. Level out unevenness in the substrate.
  2. Create an ideal surface to which the tile can be bonded.
  3. Incorporate slope in the tile layer if needed (e.g., slope to a drain).
  4. Reinforcement of the substrate (usually relevant in wood framing applications).
  5. Allow radiant hydronic tubing to be installed.
  6. Protect metal, PVC, or CPE waterproof pans.

As bonded mortar beds do not float over the substrate, they do not offer protection from cracking or movement in the substrate. However, they do offer the same advantages listed above and in many cases can be safely installed in a thinner layer than a floating mortar bed.

Note: Anti-fracture membranes used in thinset or bonded mortar bed applications can also protect the tile layer from most kinds of cracks in the substrate.

What are the advantages of thinset installations?

Thinset applications are less expensive and typically faster to install than mortar bed applications. However, as the tile is bonded directly to the substrate, any variation or movement in the substrate can affect the tile. Also, there are many types of polymer-modified thinsets on the market allowing the specifier the opportunity to match the thinset properties with the project requirements. In many mortar bed installations, the mortar bed will be allowed to cure and then polymer-modified thinset will be used to bond the tile. 

What is latex-modified thinset?

Thinset cement, to which polymers have been added, is commonly called latex-Portland cement mortar. In fact, this term is a bit of a misnomer. The original polymers used to modify thinset were based on latex and the term originates from their use. Today, there are over 10,000 polymers considered by cement chemists when formulating their products. Polymers such as EVA, PVA, SBR, and others are all commonly used in the industry. Many of these polymers are acrylics and not latex chemicals. 

The use of these polymers allows specific properties to be imparted to the cement; commonly, freeze/thaw resistance, improved flexibility, and improved adhesion. There are also polymers used to make the cement waterproof or sufficiently elastic so that it acts like an anti-fracture membrane.

The American National Standards Institute (ANSI) set minimum performance levels for latex-Portland cement mortars in the A118.4 and the A118.11 specifications.

What is the desired compressive strength for a mortar bed?

There has been quite a bit of debate regarding this subject in the tile industry. Originally, mortar beds were intended as a leveling and load dispersing layer over a subfloor that met the standard L/360 criteria. As such, compressive strengths could be very low (less than 1000 lbs. and high sand to cement mixes were commonplace). Also, this method allowed the floor to packed relatively quickly with short curing times.

Over the years, grout and mortar manufacturers have developed richer mixes with greater compressive strengths. Typically, these mixes also require more liquid to develop their maximum strength. In some applications, these stronger mixes are used over insulation and provide structural rigidity.

However, longer curing times and mortar bed curling (from uneven drying) have also resulted. The debate in the industry stems from differing opinions on whether the greater compressive strength is desirable over slab on grade construction or whether it provides no benefit (and could be detrimental due to longer cure times, increased shrinkage, and the possibility of curling).

There is also debate regarding the merits of greater compressive strength when using mortar beds over various suspended slab systems.

What is a cleavage membrane or slip sheet?

These terms are synonymous and refer to the sheet used between the mortar bed and the substrate. This sheet prevents the mortar bed from bonding to the substrate and allows it to "slip" if there should be movement in the substrate.