2024-25_NTCA_Reference_Manual_dig_dwnld

Interior Installation of Tile over Exterior Glue Plywood (EGP)

Problem

Cause

Cure

Expansion and contraction of plywood.

Plywood installed with grain parallel to joists. Moisture changes or high levels of moisture in wood.

Turn top sheet perpendicular to floor joists because greater shrinkage occurs perpendicular to grain. Offset joints of subfloor and underlayment.

Allow 1/4 in. (6 mm) perimeter joints that will not be filled with setting materials.

Tile not in true plane.

Excessive variation of thickness be tween plywood sheets.

Follow proper fastening procedure.

Sand sheets thoroughly to level them.

Floor joists out of plane.

Beat-in not adequate (see “Proper Bed ding Procedures” on page 190).

Use appropriate flowable/leveling un derlayment material.

Tile cracks at perimeter of plywood sheets.

Tongue and groove plywood used.

Don't use tongue and groove plywood as the underlayment.

Gap not left between sheets.

When tongue and groove plywood is al ready installed as an underlayment, saw cut through to the depth of the bottom of the tongue. When height will not allow for overlay of another sheet of plywood or backer board, the application of a crack sup pression elastomeric membrane system may prevent cracks transferring to the ceramic tile. sheets: 1/8 in. (3 mm) for organic adhesives ( TCNA Handbook method F142); 1/4 in. (6 mm) for epoxy mortar ( TCNA Handbook method F143). Follow bonding materials manufacturer's recommendations for treatment of joints between sheets. Space required between

Excessive deflection due to joists in ex cess of 16 in. (406 mm) on center.

Plywood not fastened sufficiently.

Certain surfacing materials such as marble may require different installation systems or subfloor preparation.

Floor buckles.

Gap not left between sheets.

Expansion and contraction of plywood due to moisture exposure.

Shrinkage or warpage of joists or sup porting structure.

Excessive deflection due to joist spac ing in excess of 16 in. (406 mm) on cen ter or joist span may be too great. Gird ers/floor joists not properly fastened. For latex mortar/grout installations fol low the bonding materials manufactur er’s recommendation for treatment of joints between sheets.

Protect and properly store plywood in a dry area.

Proper vapor barrier in place. Proper ven tilation.

Joists and supporting structure should be dry (and conditioned to use level) be fore application of tile.

NTCA Reference Manual | 2024 / 2025

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Chapter 2 | Substrates