#MAPEITechTip: What is the difference between a self-leveling topping and a self-leveling underlayment? And how to select the right primer?

Concrete toppings are formulated as 'wear surface floors' with high-compressive strengths to handle residential to commercial applications, eg: mall stores, restaurants, hotels, residential units...including industrial warehouse floors subject to continual vehicular traffic. These decorative toppings can be stained, carved and polished, making them a great alternative to traditional flooring.

Underlayments are formulated for leveling, smoothing and providing a flat surface prior to the installation of tile, stone, VCT, wood flooring, resilient and other floor coverings, and must be covered.  Although some specialized cementitious underlayments can be used while the building 'core & shell' is still open to exterior conditions, underlayments are meant for interior use only, and are available in both cementitious and gypsum based formulations.
 
Since both toppings and underlayments are highly engineered self-levelers designed for specific applications, always consult the most current technical data sheet for proper use and limitations.

What about the primer, does it really matter which one I use?  In a word yes, for both toppings and underlayments, primers improve adhesion, bond strength and reduce surface defects (pinholes) in the finished surface.  When selecting a primer the following factors must be considered:  1.) Type of substrate—porous and non-porous concrete, gypsum-based, plywood, existing tile, adhesive residue.  2.) The ‘service conditions’ that the topping or underlayment will be exposed to during its ‘service life’, from a warehouse to residential flooring.  3.) Primers can range in chemical make-up from water-based acrylic to two-component 100% solids epoxy and are formulated for various substrates and porosities —to match the primer to the specific application.

It is important to note that Toppings or high-compressive strength Underlayments subject to dynamic loading (heavy wear & tear and vehicular traffic) require a 100% solids epoxy primer such as MAPEI Planibond EBA or Primer E with a sand broadcast. For proper primer selection, consult "Primers for SelfLeveling Materials Product Selection Guide"