2024-25_NTCA_Reference_Manual_dig_dwnld

Loss of Bond Dry-set mortar adheres to tile but not to substrate

Problem

Cause

Cure

Dust or dirt embedded in dry-set and/or on substrate.

Dust or dirt on substrate.

Properly clean substrate prior to applica tion of dry-set mortar.

Dry-set mortar releases from the sub strate completely where curing com pounds or release agents are present. Water will not penetrate into slab or, if compound or agent has transferred to the dry-set, water will not penetrate into dry-set mortar. Visible evidence rarely ap pears on substrate. Portions of cement plaster may be embed ded in dry-set mortar. If plaster mix is too rich, excess shrinkage results, causing en tire sections of tile to bow away from wall. Failure usually occurs at interface of dry set mortar and gypsum or lime-based substrate. Particles of the substrate are usually found on back of dry-set mortar or on surface of substrate. In some cas es, where structural movement has taken place, fractures may occur deep within these substrates, indicating the inher ent weakness of this type of substrate, instead of deterioration caused by water.

Curing compounds or release agents on surface of concrete.

Properly test for curing compounds or release agents prior to installation. Refer to "Curing Compounds, Release Agents, and Sweeping Compounds" (page 58) for proper testing and removal methods. Precast, preformed, or prestressed con crete should always be tested for curing compounds or release agents. Use correct proportions of materials and amount of water. Apply and cure in ac cordance with manufacturer recommen dations. Direct bond dry-set mortar installations over gypsum-based substrates are not an accepted practice of the industry due to the potential formation of ettringite. Gypsum and lime putty are very sensitive to moisture. Even the amount of water contained in dry-set mortar during ap plication is enough to cause deteriora tion of the substrate. Check with setting material manufacturers for primer and/or membrane recommendations for gypsum substrates. Metal studs shall conform to ASTM, C–645. Refer to TCNA Handbook W241. Stud spac ing not to exceed 16 in. (406 mm) on cen ter. Minimum stud width 3-5/8 in. (92 mm). Studs shall be 20 gauge or heavier and properly anchored. Substrate shall conform to ANSI deflection requirements. Horizontal bridging (crossbracing) 4 ft. (1.2 m) on center may be needed. First, key the dry-set mortar into the sub strate with flat side of trowel. Add mortar as required to achieve required thick ness then comb with the notched side of the trowel in one direction. Comb only an amount where application of tile can take place before skinning occurs. See page 206 for additional information.

Weak Substrates – Improperly mixed Portland cement plaster.

Inappropriate or Unacceptable Sub strates – Gypsum plaster, lime putty coat, light-weight concrete, gypsum underlayment, or gypsum taping com pound.

Unacceptable Substrates – Lath and plaster applied to metal studs that are too thin, improperly spaced or anchored or improperly crossbraced.

Failure occurs soon after installation. Bond failure can occur at any point, from interface of dry-set to rupture of tile bisque.

Dry-set mortar separates from the sub strate, showing evidence that the mor tar was not forced into the substrate surface.

Setting material not properly applied to achieve mechanical bond to the sub strate.

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NTCA Reference Manual | 2024 / 2025

Chapter 8 | Thin-Bed Method Installations