2024-25_NTCA_Reference_Manual_dig_dwnld

Summary of Subfloor (Plywood or OSB) Design and Construction Guidelines for Tile Installation

• Direct adhesion of tile to OSB has traditionally been prohibited by the tile industry. This is due to the potential for swelling during or after installation, as well as the lack of recovery upon drying, both of which have been proven to adversely affect tile adhesion. As with plywood, it is important that tile be installed only over those floors that are not subject to unusual wetting or humidity conditions. • The traditional and proven wood subfloor thickness for tile installations on joists spaced 16 in. (406 mm) is two layers of 5/8 in. (15.9 mm) or 19/32 in. (15.1 mm) thick plywood. The deflection of this assembly over 16 in. (406 mm) joist spacing with uniform loads prescribed by building codes (10 psf [48.8 kg/m 2 ] dead load and 40 psf [195 kg/m 2 ] live load) is in the range of L/1000. The deflection in accordance with TCNA Handbook methods under a single 300-pound concentrated load in the center of one span is approximately L/400, assuming average quality plywood. • Stiffness calculations show that two layers of 3/4 in. (19 mm) or 23/32 in. (18.3 mm) thick plywood over joists spaced 19.2 in (488 mm) on center will perform equivalently to the two layers of 5/8 in. (15.9 mm) or 19/32 in. (15.1 mm) thick plywood over joists spaced 16 in. (406 mm) on center. With joists spaced 24 in. (610 mm) on center, it takes two layers of 7/8 in. (22.2 mm) plywood or one layer of 1-1/8 in. (28.5 mm) plywood to provide equivalent performance. These calculations, however, do not take into consid eration stiffening techniques such as lamination/attachment between layers of sub-floor and underlayment panels. Therefore, it is acceptable to comply with TCNA Handbook methods for sub-floor and underlayment recommendations for 19.2 in. (488 mm) and 24 in. (610 mm) I-Joist spacing. . • Other generic or proprietary underlayments, including thicker proprietary plywood products manufactured for tile assemblies, or mortar beds and cement backer boards, may be substituted to achieve required equivalent stiffness, but the substitution under layment or subfloor must be tested in accordance with Industry Standards. • Subfloor panels (plywood and OSB) must be installed with the face grain or strength axis perpendicular (long dimension across) to the I-Joists to ensure proper strength. Subfloor panels must be both glued and attached with minimum 6d ring shank nails or screw fasteners. A 1/8 in. (3 mm) space must be provided between subfloor panels at both panel end and edge joints. • The ends of the top layer of plywood (underlayment layer) should be installed with the joints over a support and offset at least one joist spacing from the end joint of the subfloor panels below. Underlayment joints located between supports will not provide maximum structural stiffness to the floor system.

The traditional and proven wood subfloor thickness for tile installations on joists spaced 16 in. (406 mm) is two layers of 5/8 in. or 19/32 in. (15.1 mm) thick plywood.

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NTCA Reference Manual | 2024 / 2025

Chapter 2 | Substrates