2024-25_NTCA_Reference_Manual_dig_dwnld

Tenting

Problem

Cause

Cure

Not enough or no movement joints.

The purpose of expansion/control joints is to allow movement of concrete and/or masonry substrates, without cracking. By dividing a large concrete or masonry expanse into separate, smaller units the overall stresses are minimized by dis tributing them more evenly throughout the area. These smaller areas now work independently and move at a much low er rate. Each unit is also sealed along the entire perimeter with an elastomeric sealant, which acts as a shock absorb er, by stretching and compressing. Elevated structural concrete slabs are de signed to deflect over time and when live loads are applied. This downward move ment causes a change in the surface di mension of the slab. The dry-set, latex Portland cement or EGP mortar now frac tures due to the stresses caused by the deflecting structural concrete slab. Loose tiles then lift or “tent” above the deflected floor areas. Differential movement is the result of the varying degrees of movement be tween dissimilar construction products. This movement is caused by changes in moisture, temperature and in some cases settling. Since each dissimilar component will move at a different rate, stresses will develop that may be detri mental to the tile installation. Examples of different building products common ly used together in tile installations are plywood, steel and wood studs, cemen titious and gypsum backerboards.

Assure that movement joints are includ ed in drawings and positioned in accor dance with ANSI and TCNA Handbook recommendations. When in doubt, sug gest using more than required, which will result in shorter distances between the joints. Accommodate for movement in the perimeter of the installation by placing a soft joint in lieu of grout or tile.

Thin setting directly over structural slabs.

Follow ANSI A108 and TCNA Handbook method EJ171 for treatment and location of movement and expansion joints.

Climate controlled conditions: Allow components to stabilize, by run ning the climate control system and maintaining normal humidity and tem perature levels for several days. Then follow ANSI and TCNA Handbook an ti-fracture/crack suppression recom mendations for the movement joints and tile installation. Non climate controlled areas: Follow ANSI and TCNA Handbook move ment joint recommendations. Crack-iso lation systems should also be consid ered for this situation.

Differential movement and structural set tling.

Expansion and contraction due to climate conditions, without adequate movement accommodation, can result in tile tenting.

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Chapter 8 | Thin-Bed Method Installations