2024-25_NTCA_Reference_Manual_dig_dwnld

General Contractor's Guide for Ceramic and Stone Tile Installations Over Engineered Wood Structural Floor Assemblies

MEMBER DESIGN Truss or joist members are often designed or specified using predetermined span tables or computer programs that may utilize dead load criteria (normally 10 psf) that are not representative of the actual weight of modern tile floor assemblies. In some cases the framing, subfloor, and underlayment material are near the 10 psf assumption even before the weight of the tile installation is considered. It is critical that the design for strength and deflection (stiffness) of all structural components in a floor be based on realistic uniform dead loads (weight of materials) for a tile installation as well as anticipated concentrated loads whenever possible. Failure to evaluate all structural components to ensure that they will adequately resist the higher dead loads found in ceramic tile and stone installations can lead to potential problems with the installation due to excessive deflection and curvature. Dead load should include the weight of all materials that make up the tile installation, including the trusses or joists, subfloor, underlayment products, mortar bed, and the tile or stone. This is especially important for those tile installations incorporating thick mortar beds, heavy dimension stone slabs, or underlayment products. Concentrated loads that should be considered include permanent equipment or fixtures, such as a kitchen island, as well as heavy semi-permanent furnishings such as pianos and pool tables. Designing for deflection has been confusing in the past, but in recent years the ANSI A108 committee on tile and the TCNA Handbook has adopted the following language to help clarify the requirements: “Floor systems, including the framing system and subfloor panels, over which tile will be installed shall be in conformance with the IRC for residential applications, the IBC for commercial applications, or applicable building codes.” Keep in mind that these are minimum requirements and it is good practice to consult the material manufacturers for any more stringent criteria that they may recommend for use with their products. TRUSS OR JOIST SPACING The spacing of trusses or joists not only impacts the deflection of the floor parallel to (the span of) those members, but also impacts the deflection of the sub-floor spanning across the joints in the perpendicular direction. Deflection or curvature of the subfloor panels under loading (plywood or OSB) between trusses or joists is perhaps the biggest structural cause of tile failures since the typical code-minimum subfloor panel thicknesses are not sufficiently stiff to support concentrated loads without excessive deflection or curvature. As truss or joist spacing increases, the problem is magnified.

Many TCNA Handbook methods limit spacing to 16 in. (406 mm) on center for these reasons; however, options for wider on center spacings are available. TCNA Handbook method F149 requires the subfloor panels to be 23/32 in. (18.3 mm) exterior glue tongue-and-groove panels meeting PS 1 or PS 2 criteria and the underlayment to be 19/32 in. (15.1 mm) exterior-glue plywood for joists spaced 24 in. (610 mm) on center. There are additional TCNA Handbook methods for 19.2 in. (488 mm) and 24 in. (610 mm) on center spacings with less stringent requirements for subfloor and underlayment panel thicknesses that incorporate crack suppression or uncoupling membranes to address the problems caused by excessive curvature. While these methods will help minimize the undesirable effects of increased curvature, the tile remains susceptible to cracking if the loads exceed the tile’s flexural bending or breaking strength. SUBFLOOR AND UNDERLAYMENT PANELS Subfloor panels must be installed with the face grain or strength axis perpendicular to the support members (long dimension across supports) to ensure proper strength and maximum stiffness. **Subfloor panels must be glued/adhered and attached with the specified mechanical fasteners. A 1/8 in. (3 mm) space must be provided at time of panel installation between the panel edges and anything they abut such as perimeter walls, pipes, etc. Underlayment panel ends should be staggered by at least one joist spacing from the ends of the subfloor panels and offset from the floor joist below by two inches so that underlayment end fasteners do not penetrate the joist. This helps to minimize the potential for “pops.” Underlayment panel edges should be offset from the edges of subfloor panels at least two inches to minimize the potential for cracking over joints. The face grain of the underlayment panels should be oriented perpendicular to the supports. Underlayment panels must be installed with a 1/8 in. space between the panel edges and anything they abut such as perimeter walls, pipes, etc. Some underlayment panels may be considered structural and their contribution may add to the stiffness of the floor-panel system. For example, the use of plywood as an underlayment panel will add stiffness to the floor when properly installed and is considered a suitable substrate for tile installation. However, other proprietary underlayments, while providing a suitable substrate for tile, may not be considered structural components and may not add any stiffness to the floor assembly. To be considered a structural component, the underlayment panel must be tested in accordance with industry standards, and in all cases, floor assemblies should be designed to conform to building code and the applicable details in the TCNA Handbook .

**Refer to project documents and wood and tile industry standards for specific installation instructions for subfloor and underlayment panels, for fastener type, diameter, length and spacing, and for the type of glue/adhesive required.

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NTCA Reference Manual | 2024 / 2025

Chapter 2 | Substrates