2024-25_NTCA_Reference_Manual_dig_dwnld

Loss of Bond

Dry-set mortar adheres to substrate but not to tile

Problem

Cause

Cure

Improper trowel used, causing insuffi cient material to be applied.

ANSI A108.5 requires 80 percent cover age of dry-set on back of tile except ex terior and wet areas, where 95 percent coverage is required. Refer to “Trowel Guidelines” on page 161. First, apply dry-set mortar with flat side of trowel. Then, notch only an amount where application of tile can take place before skinning occurs. Refer to “Proper Bedding Procedures” on page 190 for ad ditional information. Dry-set mortars that are not designed as large and heavy mortars are limited to 1/4 in. (6.4 mm) in thickness between the back of tile and the substrate. Tiles exhibiting powdery substance on back of tile must be washed with clean water and a brush to remove free release agent prior to setting tile. Properly mixed and slaked dry-set mor tar should always hold ridges of trowel after application. Correct consistency is indicated when properly prepared dry set mortar will cling to the bottom side of a margin trowel and will not run off. In hot weather conditions, mix dry-set mortar with cool water. Slaking is very important. Dampen substrate prior to application. First, apply dry-set with flat side of trowel. Then, notch just before tile is applied. Give special attention to storage of tile and other materials in cool conditions. Use extreme caution when installing tile when temperature is over 90° F (32°C). When temperatures exceed 90°F (32°C) use specially formulated high temperature mortar. Dry-set mortar must be mixed and ap plied according to directions. The chemi cals in dry-set mortar do not function un less properly mixed prior to application.

Tile comes off clean or has small residue of dry-set indicating little contact with dry-set mortar.

Contamination on back of tile.

Dry-set mortar allowed to skin over prior to application of tile. Proper bedding did not rupture the skin.

Trowel ridges are clearly visible and/or imprint of tile shows in mortar. Tile can come off completely clean or show trac es of dry-set where ridges contacted tile, but skinning prohibited bonding. Dry-set mortar separates from the sub strate; can exhibit cracking within the dry-set. May leave dry-set residue on substrate. Tile comes off clean. White residue from release agent used during manufactur ing present on back of tile or embedded in dry-set. 1. Mortar is weak and powdery. 2. Mortar shows signs of sagging, slump ing, or running.

Dry-set mortar applied in excess of thickness limitations inducing excessive shrinkage during curing.

Excessive release agent on back of tile.

Too much water used to mix dry-set mor tar.

Weather, substrate, water, or tiles too hot causing dry-set mortar to flash set.

Dry-set mortar releases from tile, leaving residue on back of tile. Dry-set may be weak and brittle.

Dry-set mortar used as dry sprinkle over mortar bed, instead of Portland cement.

Tiles come off mortar bed clean or with light residue of dry-set mortar.

Contamination on back of tile.

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NTCA Reference Manual | 2024 / 2025

Chapter 8 | Thin-Bed Method Installations