2024-25_NTCA_Reference_Manual_dig_dwnld

Proper Bedding Procedures

GENERAL The use of proper bedding procedures is an integral part of suc cessfully setting tile or stone and achieving the required mini mum mortar coverage. Coverage is defined as a continuous dry mortar thickness ≥3/32 in. (2.38 mm) between the substrate to the tile underside. Acceptable coverage results can only be accomplished with proper surface preparation; using the correct size trowel; trow eling technique. Placement of tile into the mortar, moving it in a back-and-forth motion perpendicular to the trowel ridges. This movement collapses the trowel ridges into the valleys leaving very few if any voids in the mortar which supports the entire back of the tile. Different tile types, shapes and sizes require adjustments in the procedure to exceed the 80% minimum cov erage for interior dry areas and the 95% minimum coverage rate for wet and exterior applications. The bedding process involves four major requirements: (1) cre ating a bond to the substrate; (2) a bond between the tile and substrate; (3) a bond to the underside of the tile; and (4) as suring adequate support for tile edges. An excessive amount of voids as produced with spot bonding (see Spot Bonding pg. 303), leads to tile damage or bond loss due to deflection, im pact, point loading, thermal and moisture changes. For stone installations, substrate and tile flatness affects the bedding procedure. ANSI A108.02 requirements for tiles <15 in. (38 mm) on any side must meet a tolerance at ≤1/4 in. in 10 ft. (6 mm in 3 m) and no variation ≤1/16 in. in 12 in. (2 mm in 305 mm). For natural stone or large and heavy tiles (formerly referred to as large format tiles), the tolerance is ≤1/8in. in 10ft. (3 mm in 3 m) and no variation ≤1/16 in. in 24 in.(3 mm in 610 mm). For Gauged Porcelain Tiles and Gauged Porcelain Tile Panels/ slabs substrate and installations requirements, refer to ANSI A108.19 for interior applications and A108.20 for exterior appli cations. Tiles and substrates must be cleaned of contaminates prior to applying mortar. Tiles may arrive or become covered with con struction dust; quarry dust, laitance, mold, kiln dust or pressing residues. Natural stone may have dried slurry from the fabrica tion process. Wiping the concrete, plywood or backerboard sur face with a slightly damp sponge not only helps remove small particulates but can lower dryness and temperature thus im proving mortar adhesion and open time. MORTAR APPLICATION Best results are obtained when mortar is firmly “keyed” into the substrate with the flat side of the trowel held at an angle. This step helps the mortar to create a bond to the substrate by filling in the substrate micro-pores. Right away, additional mortar is to be applied using the notched side of the trowel also at an an

gle to form ridges of similar height. The notched ridges should be combed uniformly in one direction. To assist with collapsing ridges and removing air, comb the notches perpendicular to the long edges of rectangular tiles. (See page 206 Bonding Large Sized Tiles) Tile is then forcefully moved side to side perpendic ularly across the ridges to collapse them fully and evenly. Based on the tile and trowel dimensions, the side to side movement will vary, typically 1/8 in. to 3/4 in. (3 mm to 19 mm). Tile can be made flat to each other using a beating block and/or rubber mal let if necessary, especially when wet-setting into a mortar bed. Remove a tile periodically to assure you are achieving proper coverage and edges are supported. Using the appropriate trowel (See page 197 Trowel Guide lines) may achieve adequate coverage but note that tile un dersides of all sizes may have cavities or a rough profile; or be warped to a degree that will prevent adequate cov erage. If the trowel does not account for this extra space, the space will require pre-filling or flat back-troweling (See page 202 Flat Back-troweling and Notched Back-troweling [formerly Backbuttering]) with mortar just prior to being set into the troweled mortar on the substrate. LARGE-AND-HEAVY-TILE COVERAGE Due to the large areas covered by some individual tiles, it is more difficult to handle and place these formats. Flat back-troweling large manufactured and natural stone tiles is recommended to improve bonding and achieve minimum recommended cover age. NOTE: The mortar coverage standard at this time is 80% for interior dry applications, but there are an increasing number of specifications calling for 90-95% coverage due to the high per centage of allowed space. Example: A 12 in. x24 in. (30 cm x 60 cm) tile is 288 sq. in. (1858 sq. cm) with 80% coverage allows for 57 sq. in. (314 sq. cm) voids. QUARRY, PAVERS, AND NATURAL STONE Quarry tile and pavers can present unique challenges to achieve proper embedment. They are often set with racks and this pro cess can make it more difficult to slide the tiles adequately and ensure a proper collapse of the thin-set ridges. When using a rack to slide the tiles back and forth, a lot of force will be neces sary. Always follow proper bedding procedures when installing quarry or pavers, even when using a beating block or tapping the individual tiles with a rubber mallet. Natural stone comes in many different types, shapes, thick nesses and sizes. Keep the following key items in mind for proper embedment of thin-set mortar. Make sure to thorough ly clean the back of the stone tiles to remove any quarry dust or contaminants. Keying the thin-set mortar into the substrate and the backside of each piece is a very important step to en sure proper adhesion. The surface profile on the backside of the SPECIFIC TILE BEDDING METHODS

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NTCA Reference Manual | 2024 / 2025

Chapter 8 | Thin-Bed Method Installations