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Underlayments Introduction

Factory-prepared, powdered underlayments can be trowelable or pourable/flowable, and usually fall into one of two categories: 1. Gypsum based 2. Cement based latex underlayments Gypsum based underlayments. (trowelable) Gypsum based patching compounds are polymer-fortified materials, composed of gypsum and other chemicals and are typi cally fast setting but not fast drying. They can be used for floors and or walls. They may be mixed with water or a latex admix ture, (for greater strength), but are to be used only in dry areas since gypsum-based materials are more sensitive to moisture than cement-based materials. When exposed to water, these products become soft and may swell therefore the use of them in wet areas is highly cautioned. These materials are normally used in the resilient flooring industry for patching small holes, cracks or for correction of thickness variations of adjacent flooring materials, up to 1/2 in. (12mm). However, larger areas may be leveled covered with products that can go from 1/8 in. (3mm) up to 2 in. (50mm) in most cases, with a pourable material. Prior to installing ceramic or porcelain tile over a gypsum-based underlayment, it is recommended that the underlayment be primed. Priming the surface before applying the mortar will act as a “sealer” between the mortar and underlayment, preventing premature drying of the mortar or adverse chemical reactions between a Gypsum product and a Portland cement product. Waterproof membranes can also be used to protect the underlayment from top-down moisture and give you a better bonding surface for the mortar. 2. Cement based underlayments, (trowelable) Cement based underlayments are formulated from a blend of Portland and hydraulic cements and are very fast setting and drying. They can be used for floors or walls. While typically un-sanded, some cement based underlayments have a very fine sand aggregate in them for thicker applications. Cement based underlayments are usually mixed with water but can also be mixed with a latex or an acrylic additive, for strength, flexibility, and better bonding. Cement-based underlayments can be installed over a variety of substrates. They can be installed directly over concrete or wood without a primer and can be installed over a non-porous substrate with the use of a primer. Texture is not needed for bonding purposes, but the surface must be porous. If the surface is not porous, a primer must be used to bond to the substrate. 3. Flowable hydraulic cement underlayments, (aka pourable/flowable self-leveling compounds) are pourable/flowable and are composed of Portland cement, fine aggregates and a variety of select polymers, that increase flowability, setting and strength. Substrate priming is required when using a flowable hydraulic cement underlayment for purposes of bonding and for greater working time. Wood substrates are usually primed with an acrylic or latex primer. Concrete substrates are usu 1.

ally primed with an acrylic, latex or epoxy primer. Non-porous substrates are usually primed with an epoxy or latex primer. Always check with the manu facturer for priming recommendations. Most flow able hydraulic cement underlayments are mixed with water, but some may be mixed with a latex admixture for better bond strength and flexibility. Flowable hydraulic cement underlayments can be applied from 1/8 in. (3mm) up to 2 in. (50mm) neat or can be extended with the addition of aggregate. Check with manufacturer. Although not as vulner able to moisture as gypsum-based underlayments, it is recommended a waterproof membrane be applied over the cement-based underlayment prior to the installation of tile in wet areas.

Photo courtesy of Custom Building Products

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NTCA Reference Manual | 2024 / 2025

Chapter 3 | Backerboards and Underlayments