2024-25_NTCA_Reference_Manual_dig_dwnld

Soft or Powdery Underlayment

Problem

Cause

Cure

Non-uniform cured matrix. Sand at the bottom and thin cement cream at the surface.

Excessive water or latex additive causes the underlayment to segregate with the sand sinking to the bottom and the fine particles, cement binder to float to the surface. Using the wrong mixing speeds can de velop air in the mix, once cured the air bubbles dissipate leaving a void in the mix. These air pockets significantly decrease the strength of underlayments. Diluting latex additives with water, when not called for in the directions, can cause rapid evaporation of water from the applied material. This greatly reduces cement hydration or curing resulting in a very weak underlayment. Gypsum based materials are very sensi tive to water. Even latex modified gypsum based materials, after curing, will turn powdery or soft and mushy when exposed to enough moisture, is powdery or soft and mushy. Gypsum based products are sensitive to water. When cement based products containing latex additive or water are ap plied over gypsum, the water softens the gypsum. Then, when the cement product goes through its normal shrinkage while curing, it pulls the weakened gypsum product apart. Many products may appear to be the same, especially latex additives and primers, however, various raw materials may be contained in the same basic type of latex, which changes its performance charac teristics.

Always gauge the liquid and powder ac cording to manufacturer directions.

Improper mixing.

Always mix following the manufacturer’s instructions.

Diluting latex additives with water.

Always use proper proportions according to manufacturer directions.

Using gypsum based materials in areas subject to moisture.

Use a cementitious underlayment or consult gypsum based underlayment manufacturer for directions.

Using cement based underlayment over gypsum underlayment.

Use only cement based products manufac turer recommended for use over gypsum products.

Mixing with foreign products or substitut ing one product for another.

Consult underlayment manufacturer for approval before making any substitutions

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NTCA Reference Manual | 2024 / 2025

Chapter 3 | Backerboards and Underlayments