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Interior Horizontal Installation of Tile over Exterior Glue Plywood (EGP)

Introduction

The following is intended to offer possibilities for causes, characteristics of these causes, and prevention of certain problems related to interior installation of tile over exterior grade plywood. It is intended to be used with the proper judgment and experience of the individual craftsperson. The specific causes listed are common, but do not necessarily represent all problem areas. Installing tile over plywood presents special problems for the contractor since any failure of the wood floor system will appear as a tile-related failure. Awareness of the more common pitfalls and their results will help to prevent these mistakes.

TCNA Handbook methods F142 and F143.

1. The use of masonite, hardwood flooring, wood strip flooring, parquet flooring, composite panels, luan, fire-resistant or pres sure-treated plywood, and wafer board are specifically unsuitable substrates/surfaces for the installation of tile.

2. Install tile only over plywood underlayment of grade C–C plugged exterior or plywood APA-rated Sturd-I-Floor Design Systems per APA Form E30.

3. Structure is to be designed to support the intended use of the floor system and finish materials. Subfloor minimum 19/32 in. (15.1 mm) EGP and underlayment minimum 19/32 in. (15.1 mm) EGP. Supporting structure is to be strong enough to limit deflection to 1 in. in 30 ft. (25mm in 9m); maximum allowable is L/360 system. Large-format tiles, particularly in large areas, may require a more rigid system.

4. Leave a gap between the tongue and groove EGP sheets of approximately 1/8 in. (3 mm). Also leave a gap at the perimeter of the room, not to be filled, but free to move slightly.

5. Plywood must be securely fastened to floor joist with 4D ring shank nails spaced 6 in. (152 mm) on center at edges of plywood and 8 in. (203 mm) on center at center of plywood sheets. Underlayment plywood should not be glued to the plywood subfloor. Offset joints in the subfloor from joints in the underlayment. Adjacent sheets of plywood should be within 1/32 in. (0.8 mm) of level.

6. Use of improperly installed tongue and groove plywood for subfloors and/or underlayment to receive ceramic tile may contribute to cracked tile outlining the plywood sheets due to expansion and contraction.

7. Any shrinkage of plywood occurring AFTER the installation of tile may damage the tile.

8. Plywood is not a suitable subsurface for the installation of ceramic tile where plywood is exposed to excessive moisture or hu midity, such as steamrooms, gang showers, Roman tubs and pools, etc.

Install tile only over plywood underlayment of grade C–C plugged exterior or plywood APA-rated Sturd-I-Floor Design Systems per APA Form E30.

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NTCA Reference Manual | 2024 / 2025

Chapter 2 | Substrates