2024-25_NTCA_Reference_Manual_dig_dwnld

Tenting

Problem

Cause

Cure

“Tenting,” commonly observed as unbonded tiles elevated above their originally bonded position, is caused by a variety of situations. This situation can occur in all tile installations, but is often encountered in installations of porcelain tile set with dry-set mortar in a thin-set manner with a direct bond directly to concrete without the use of a crack isolation membrane, and no consideration for movement and expansion joints per EJ171. The low bond strength of the mortar to the porcelain tile, relatively low flexibility of dry-set mortars, the thin cross section of the installation and specific substrate conditions, may combine to produce detrimental effects. Listed below are the more frequent causes of “tenting” along with their respective reasons and preventive measures.

Setting directly over movement joints.

Lateral movement causes a weakened or total loss of bond when the dry-set, latex Portland cement, or EGP mortars have been applied over and subsequently fill the existing movement joints. These joints now become restricted in their movement capabilities resulting in buck ling, cracking and/or other detrimental effects to the tile installation. Young (uncured) concrete will exhib it a reduction of surface area during its curing stage. The degree of shrinkage is mostly a function of the amount of water used during mixing – the more water, the more shrinkage. When tiles are applied to green concrete they immediately are subjected to counteracting stresses. These stresses result from the concrete reducing in surface area and the tile maintaining its original dimensional characteristics. Therefore, the weakest link of the system, the bonding mortar, fractures and the tiles separate from the concrete slab. Because the directional stresses are more concentrated towards the center of the slab, tiles in this area loosen first and further delamination continues at a reduced rate towards the perimeter. The tiles that are still bonded along the perimeter areas now pinch in ward, causing the center tiles to lift up ward.

Follow ANSI and TCNA Handbook recommendations for properly constructing and treating movement joints.

Shrinkage of concrete slab.

Allow the concrete slab to cure at least 28 days, but preferably longer. Lower temperatures and high humidity will extend the curing time. Check with membrane manufacturers for crack isolation membrane options and concrete moisture limitations.

Lack of movement joints will cause pressure to build up and result in tenting of tile.

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NTCA Reference Manual | 2024 / 2025

Chapter 8 | Thin-Bed Method Installations