TecTips #695
Tile Installations Over Plywood Floors Supported by Engineered Joist Systems
PROBLEM: Building contractors are beginning to replace the typical 2" x 10" solid-sawn floor joist with engineered wood products such as I-joists and truss floor systems. These systems are designed to increase the on-center spacing between joists from 16" to 19.2" and, in some cases, to as much as 24" on-center. The current 1995 edition of the Tile Council of America Handbook stipulates that joist spacing should not exceed 16" on center and deflection not to exceed L/360 of a given span.
SOLUTION: SCHLUTER SYSTEMS recently had tests conducted by the Tile Council of America using ASTM Method C-627, "Standard Method for Evaluating Ceramic Floor Tile Installation Systems". Three different flooring systems incorporating the SCHLUTER DITRA Matting, 3/4" tongue and groove exterior grade plywood over 24" on-center 2" x 4" truss joists, and various underlayments and bonding mortars were tested. Each system achieved a light commercial rating (see Page 10 of 1995 TCA Handbook).
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Test Table - Table Installation Techniques for Truss Flooring-Type Subfloors |
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| Subfloor | Underlayment | Bond Coat | Tile | Result |
| 3/4" plywood over 24" joist o.c. | 3/8" plywood & DITRA matting | latex modified thin-set mortar | 12"x12"x5/16" porcelain paver | light commercial |
| 3/4" plywood over 24" joist o.c. | 1/4" C.B.U. & DITRA matting | latex modified thin-set mortar | 12"x12"x5/16" porcelain paver | light commercial |
| 3/4" plywood over 24" joist o.c. | DITRA matting & medium set mortar | polymer modified medium set mortar | 12"x12"x5/16" porcelain paver | light commercial |
SCHLUTER DITRA matting is a thin, 3/16" ribbed membrane that detaches the tile and mortar bond coat from the substrate. Through its design, tensions developed in the substrate are not transferred directly to the tile surface. SCHLUTER DITRA matting is user friendly and bonded to substrates with latex modified thin-set mortars.
PRECAUTIONS: Although these systems have passed independent testing, the following situations may lead to job problems if not properly reviewed before installation.
WAIVER OF GUARANTEE: Because site conditions, material and other circumstances vary, ConestogaTile cannot guarantee satisfactory results of the above recommendations. TecTips represents what ConestogaTile considers the best recommendations at the time of publication.
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