Mortar Bed

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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 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 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.

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