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General Contractor’s Guide for Ceramic and Stone Tile Installations over Engineered Wood Structural Floor Assemblies

REDUCING JOIST SPACING FOR TILE IN LIMITED AREAS

STORAGE All wood products must be properly stored (covered, off the ground) in accordance with manufacturer’s instructions to prevent excessive moisture absorption. Avoid excessive concentrated loading of wood framing after installation, especially if exposed to prolonged periods of rain. I-JOIST CONSTRUCTION DETAILS Follow the manufacturer’s required construction details for all floor-framing conditions to ensure specified performance (examples: solid blocking over support beams, load bearing web stiffeners where required and blocking near perimeter support walls). OSB 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. OSB should only be incorporated as the subfloor to receive some type of underlayment over it, provided the underlayment panel manufacturer will allow OSB to be used as a subfloor. In wood construction, it is important that tile be installed only over those floors that are not subject to unusual wetting or humidity conditions, such as improperly protected crawl spaces and basements.

If tile is to be installed in limited areas only (kitchen, bath, foyer), an increase in stiffness for the entire residence may not be necessary or economically feasible. It may be more practical to decrease truss or joist spacing to 16 in. (406 mm) or 12 in. (305 mm) on center under those areas if possible. As required by building code, use tongue-and-groove subfloor panels or utilize solid blocking between members to both support subfloor panel edges and stiffen the subfloor panel either as a preventative or corrective measure. DEFLECTION Wood framed floor assemblies designed to receive a tile floor finish should have a maximum deflection as prescribed by building code and industry standards – including manufacturers’ literature. When installing stone tile, consult with The Natural Stone Institute (NSI) for recommendations on deflection. DIFFERENTIAL DEFLECTION Joists located adjacent to and parallel to an end wall or internal load-bearing wall have significantly different curvatures under load than those located over the continuous support of the rim joists or bearing wall. Differential deflection, especially at longer spans, may result in excessive panel curvature due to the difference in deflection over a relatively short distance, adjacent to the perimeter support or internal support. It is advisable to compensate for the differential deflection in these areas with framing details (stiffer joists, closer joist spacing, outriggers, or blocking).

Beams with joists.

NTCA Reference Manual | 2024 / 2025

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Chapter 2 | Substrates