NTCA Reference Manual: exploring underlayments (April 2015)

NTCA_RMThe NTCA Reference Manual, an essential industry document, explores the subject of underlayments in Chapter 3. This publication is free as part of NTCA membership or can be obtained through the NTCA website at www.tile-assn.com.

The NTCA Reference Manual gives an overview of the types of underlayments one is likely to need or encounter on a tile or stone setting project, as follows:

Factory-prepared powdered underlayments usually fall into one of three categories:

1. Gypsum based
2. Cement-based latex underlayments
3. Cement-based self-leveling underlayments

1-underlayment_article1. Gypsum-based underlayments are predominantly composed of various grades of gypsum, chemicals to control set time, and may be sanded or unsanded. They may be mixed with water or a latex admixture, but are to be used only in dry areas, since gypsum-based materials are highly sensitive to moisture. These materials are normally used by the resilient flooring mechanic for patching small holes, cracks or for correction of thickness variations of adjacent flooring materials. However, larger areas may be leveled with products that require 3/4” minimum thickness over wood and 1/2” over concrete substrates. Gypsum-based underlayments are not recommended for use under ceramic tile or stone.

2. Cement-based latex underlayments are composed of cement, aggregate and are mixed with a latex additive. Most instructions recommend the application of a slurry coat to the substrate made from the powder and the latex additive. This slurry is only allowed to dry to a tacky condition before application of the normal mix. It normally requires sanding after curing to remove trowel marks and for further leveling.

3. Self-leveling underlayments are composed of cement, aggregate and chemical modifiers that increase flowability and strength. Substrates are normally primed with a latex material that serves as a bonding agent and a sealer. Most self-leveling materials may be mixed with water or with latex admixtures.

The NTCA Reference Manual also presents a table of Problem- Cause-Cure parameters for some common problems that arise when using underlayments. Following are the categories of underlayment woes and the problems that contribute to the difficulty.

unapproved_latexesLoss of Underlayment
Bond To Substrate

  • Improper preparation of substrate. Applications of material over dust, dirt, curing compounds, old adhesives, spalled or soft concrete, etc.
  • Deflection of substrate.
  • Failure to prime the substrate according to directions on the product.
  • Diluting latex additives with water.


  • Mixing product with too much water or latex.
  • Bridging expansion joints, control joints, or slab cracks.
  • Application of material exceeding thickness restrictions.
  • Over-troweling/overworking surface.
  • Exposure to excessive wind or direct sunlight during initial curing stage.

powdery_underlaymentSoft or Powdery

  • Mixing underlayments with too much water or latex.
  • Mixing with high-speed drill.
  • Diluting latex additives with water.
  • Using gypsum-based materials in areas subject to moisture.
  • Using cememt-based underlayment over gypsum underlayment.
  • Mixing with foreign products or substituting one product for another
  • Moisture penetration followed by freeze/thaw cycles

Poured gypsum underlayments

Also included in the NTCA Reference Manual exploration of underlayments is a section on poured gypsum underlayments. They have distinctive properties, characteristics, and capabilities that are presented as follows:

poured_underlaymentPoured gypsum underlayments can provide a satisfactory surface to receive ceramic tile installation systems. These floors are available in compressive strengths of 1,000 to over 8,000 PSI. It is recommended that the tile installer verify that the poured floors meet a minimum compressive strength of 2,000 PSI and a minimum density of 115 lbs. per cu. ft. when tested in accordance with ASTM

C472. Poured gypsum underlayments are suitable for interior substrates only, above grade, and in areas not subject to constant water exposure or immersion.

There are currently four approved methods the TCNA Handbook uses for poured gypsum underlayments in tile installations.

F200 – Poured Gypsum over Concrete

F180 – Poured Gypsum Underlayment over Plywood

RH111 – Poured Gypsum over Concrete with Hydronic Heat

RH122 – Poured Gypsum over Wood with Hydronic Heat

Drying of the gypsum underlayment

Poured gypsum floors are made with a job site mixture of powder and water and require time to dry before they can be primed/sealed and tiled. Verification that the gypsum underlayment is dry can be determined in accordance with ASTM D4263: Plastic Sheet Method. This test process shall be performed by the gypsum installer, prior to the application of any primers/sealers. Do not proceed with the tile installation until the poured gypsum is deemed dry and has been primed/sealed.

As a general guideline, the following drying times should be observed prior to testing the surface for dryness.

Thickness of poured gypsum and dry time before testing:

1/4”      48 hours

1/2”      72 hours

3/4”      5 days

1”      7 days

2”      2 weeks

Preparation of the poured gypsum surface

In general, ceramic tile is not bonded directly to gypsum underlayments. While each manufacturer of these materials has their own specific requirements, the use of a primer/sealer or a primer and membrane is required. Any exceptions to this recommendation are proprietary in nature and suitability rests solely with the gypsum manufacturer.


Some gypsum manufacturers recommend the use of a primer/sealer over the surface of the dry gypsum before installing any membrane or setting material directly. Also referred to by some as a “sealer” or “overspray,” the use of these primers is intended to prevent the gypsum from absorbing water from the setting material, which can result in poor adhesion. Please note that while you may not have installed the poured gypsum, you must verify that the primer/sealer was applied in accordance with the manufacturer’s recommendations, after the gypsum floor was deemed dry.


The Tile Council of North America and some gypsum manufacturers recommend the use of membranes in addition to the primer prior to installing the tile over their poured underlayments. For the purposes of these applications, membrane is as defined by ANSI A118.12 for crack isolation or ANSI A118.10. Check with the manufacturer for their individual requirements.

NOTE: The tile contractor shall obtain written documentation verifying that the poured gypsum floors have met or exceeded the minimum compressive strength of 2,000 psi and minimum density requirements of 115 lbs. per cu. ft. per ASTM C472, has been tested per ASTM D4263 and deemed to be dry, and has been primed/sealed in accordance with the manufacturer’s recommendation. This information shall be provided to the tile contractor by the general contractor, owner, builder or certified poured gypsum installer.

NOTE: The requirements of this document exclude patching compounds.