What lurks beneath: Preventing failures due to insufficient floor preparation

Figure 7: Maintaining a wet edge between successive self-leveling cementitious underlayment (SLU) pours and gauge raking before exceeding the products’ working time, ensure a smooth surface for all types of flooring.
Figure 7: Maintaining a wet edge between successive self-leveling cementitious underlayment (SLU) pours and gauge raking before exceeding the products’ working time, ensure a smooth surface for all types of flooring.

Additional items to evaluate

Moisture vapor: A moisture vapor emission rate (MVER) in excess of 1.4 to 2.5 kg/93 m2 (3 to 5 lb/1000 sf)/24 hours or 75 to 80 percent relative humidity (RH) could exceed the flooring manufacturer’s guidelines or restrictions for their product. Ceramic and stone tiles as well as cement-based SLUs are more compatible with excess moisture vapor. However, the effects of the vapor, such as production of alkaline deposits on the substrate, efflorescence formation on the flooring surface, staining, mold growth, and warping, may plague the finished floor, particularly luxury vinyl tile (LVT), carpet, and wood. MVER or RH readings that are not compatible with the flooring product must be remediated with 100 percent solids, two-part epoxy, moisture mitigation membrane.

Construction joints and cracks: Movement joints and cracks that may have developed after the concrete’s installation require specific methods when installing underlayments and ceramic tile and stone. The TCNA Handbook provides several useful details such as:

  • EJ171, Movement Joint Guidelines for Ceramic, Glass, and Stone – Expansion joints in the concrete must brought up through the tile assembly, including SLU, in the same plane;
  • F205 (on-ground concrete) and F205A (above-ground concrete) – Both flooring details include a cementitious self-leveling underlayment, and are recommended for concrete slab construction where no bending stresses occur, and Ff is critical (F205 and F205A are similar, but the latter requires a bonding mortar that is approved by the manufacturer for above-ground use; and
  • F125 – Partial – Allows for the relocation of a static joint or crack through the tile assembly by incorporating a partial crack isolation membrane in the assembly, applicable to F205 and F205A flooring installations.

Additionally, full slab depth joints and cracks require filling to prevent the self-leveling underlayment from flowing through the slab into the room below or soil subgrade. If SLU seeps through the concrete slab after the products’ heal time, ≥ 10 minutes in accordance with ANSI A118.16, a permanent depression can form in the SLU’s surface directly over the crack. This depression can reflect through resilient flooring.

Substrate priming: Concrete is a porous material. If not correctly primed so as to not absorb SLU’s mix water, the SLU’s workability, flow and heal rates, adhesion, strength, and surface flatness are compromised. A properly applied primer penetrates the surface, forming a uniform film, free of voids and pinholes. Non-porous substrates, such as hard-troweled concrete may also be a problem and could require mechanical preparation before a primer is applied.

Figure 8: A flattened, and otherwise sound substrate, ensures success for all components of a tile or stone assembly.
Figure 8: A flattened, and otherwise sound substrate, ensures success for all components of a tile or stone assembly.

ASTM F3191, Standard Practice for Field Determination of Substrate Water Absorption, is a simple test for assessing the substrate’s porosity. Apply drops of water onto the concrete, particularly on suspected problem areas. If the water is readily absorbed, creating a darkened spot, the concrete is porous. If the water drops bead up on the surface, and no dark damp spot is visible, the concrete is non-absorbent. Extremes in porosity are equally problematic and require special attention. The primer will need multiple applications on porous concrete, whereas repellent surfaces may require mechanical preparation or a special primer. Also, primer applications over gypsum-based or OSB underlayments, cutback adhesive residues, epoxy and cement terrazzo, metal, ceramic and stone tile, and well adhered resilient flooring may require special consideration before priming.

Going with the flow

Self-leveling cement underlayments can be mixed and applied using automated mixing and pumping equipment or manually mixed onsite with drill-powered ‘eggbeater’ paddle and poured from the mixing barrels. Both methods are suitable for commercial and residential installations.

A wet edge must be maintained between successive pours of SLU. Fresh material poured adjacent to product that has already started to harden can form a ridge or bump along the unintended cold joint, which could create a visible ridge in resilient flooring. The window of time for mixing to final finishing is approximately 20 minutes for standard set SLUs. Temperature, humidity, and air movement must be factored into the installation so that the SLU’s working time and flow and heal rates are not compromised.

Cold temperatures are also a potential problem. The ideal ambient, substrate, and product temperature is 10 C (50 F) to 32 C (90 F). Manufactures provide rapid and delayed setting formulations when installing at temperature extremes. Strategic material and equipment staging, and adequate labor must be managed for a successful installation.

As material placement proceeds, another contractor will be pushing and pulling the flowable product to the required thickness with a gauge rake. The gauge rake has adjustable feet so that the blade is set and maintained at the correct elevation above the substrate. Gauge raking is followed by lightly pulling a smoother across the SLU (Figure 7).

The smoother breaks the surface tension of the SLU. This tooling contributes to product flow and distribution across the concrete, allows entrapped air to escape and refines surface smoothness. Gauge raking and smoothing must be completed before the product’s working, flow, and healing times are exceeded. Any attempts to work the SLU once it loses workability will leave permanent depressions on the surface.

Generally, non-moisture sensitive ceramic and stone tiles can be installed after the SLU has cured for four hours, and other floorcoverings after 16 hours. SLU drying time and subsequent flooring installation can be impacted by temperature, humidity, and material thickness. It is advisable to confirm moisture limitations of flooring and adhesives before installing over a self-leveler (Figure 8).


What is under the tiled floor should not be a mystery or reveal itself through failure. If specified and correctly executed, there are many methods, materials, standards, and guidelines that will minimize the possibilities of failure due to insufficient floor preparation or any other stage of an installation.

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