Floor tiles are meant to stay put. Andrew Golle offers an extensive guide to minimising movement.

Floor finishes such as tiling are viewed as static and solid components of a building.

However, functional failure of tiling can often be attributed to movement – differential and compounded.

Years ago, we devoted most of our time to rectifying de-bonded floor tiling using the Tile Reglue Injection Method.

We investigated and rectified bond failure in which tiles had dislodged from the substrate, sometimes even exploding and lodging in the ceiling above.

Less severe areas had peaked or tented in localised areas, or had just become drummy, because the bond to the substrate had sheared.

Although the problem was movement related, many in our industry attributed such failures to tile growth. This was often a minimal contributing factor – as with any porous product subject to moisture absorption and thermal movement.

The main source of movement was concrete shrinkage and substrate deflection, both of which apply a compounding stress on the tiling system. This reduces the available surface area that a hard floor covering is bonded to, thereby giving the tiles nowhere to go but up. Hence peaking and exploding tiles.

Differential movement, through cyclic expansion and contraction, subjects the tiling system to continuing elastic shear stress.

These movement-related failures are exacerbated by installation faults, which drastically reduce the expected performance and shear tolerances of the tiling system.

Incorrect or missing movement joints, incorrect screed placement, substandard adhesive contact coverage and poor substrate preparation all result in a compromised tiling system that is unable to withstand normal building movement.

REQUIREMENTS
AS 3958.1 – 2007 Guide to the Installation of Ceramic Tiling provides codes of practice for accommodating building movement and maintaining tiling system performance.

Scope – ‘The designer should make known to the tiling contractor any structural or environmental conditions, such as excessive deflection or the likely hood of excessive or delayed shrinkage onset of the substrate.’

5.4.2.1 – ‘The setting out should be related to siting of movement joints.’

5.4.5.2 – ‘Movement joints….should be sited at locations where stress might reasonably be expected …’

These requirements are often ignored due to a focus on aesthetics.

Tile cuts at perimeters and doorways take precedence over concentrated substrate shrinkage around structural elements such as beams, thickenings or set-downs in slabs.

When removing peaked or drummy tiles we often discover tell-tale shrinkage cracking either side of an internal beam or thickening. Perimeter tiles are often pinned down by timber skirting boards, restricting any lateral movement.

The above requirements mean that planning for building movement must begin at the design stage.

Designers should advise tiling installers of any substrate-related factors that may result in movement stress. Elements that may restrict lines of movement should be noted on plans, such as:

– slab deflection

– internal beams

– set-downs

– benches

– columns

– walls

If necessary, movement joint positions should be indicated on plans to assist tilers, who must use the information to select the best tiling system for accommodating expected movement.

The three methods of movement accommodation are set out below.

ISOLATION
This may involve installation of tiling to a floating mortar screed, over a slip sheet or a crack isolation mat.

This type of mat is a bonded sheet membrane directly adhered to structure, with a bonded tiling system or floating tiling system over it. The mat separates the tiling system from direct stresses associated with cracks.

Bonded tiling systems can be adhered to some mats of a compatible and flexible material, such as SBR (styrene butadiene rubber – modified latex).

Unbonded tiling systems can be placed over isolation membranes that are not compatible with adhesives, such as a torch-applied bitumen membrane with heavy-gauge corflute as a slip sheet, or PVC sheet membrane.

This system is used extensively in external applications where movement at construction joints exceeds the capabilities of a bonded flexible system. The floating screed above acts as a ballast to hold the membrane down, and also as a drainage cell for sub-tile water movement.

Topical movement joints should still be located directly over the offending construction joint, even though an isolation mat is incorporated.

TRANSFERENCE
This relies on the direct transfer of structural movement through finishes above.

Movement joints must be directly over construction joints or active cracks and projected through the tiling and screeds.

We have witnessed transference cracking projecting through tiles where the tiler has missed the set-out by as little as 20mm.

Large areas of mortar screeds should also be placed with formed cold joints over structural movement joints by inserting compressible foam between bays.

SUPPRESSION
This method is viable for minor active cracks, bonded flexible membrane systems or when siting intermediate movement joints at structural elements where localised shrinkage is expected.

Suppression acts like shock absorption. Bond-breaking tapes or sealants are placed over the crack or beam, providing a slip joint and preventing the cracking point from acting on the layer above.

These slip joints are mirrored through different strata, and the final tiling movement joint is sited somewhere in the slip zone.

An example would be where the tiler sites a movement joint relative to a 300mm internal concrete beam.

A 300mm tiling silicone joint can’t be installed to span the width of the beam in order to deal with cracks either side of the beam. Instead, the dominant crack is selected, and 50mm vinyl tape or neutral cure silicone is set at 50mm over the crack.

Note: some tilers may install a polyurethane sealant over the crack. However, this may not act as a bond-breaking slip joint, as most polyurethanes are paintable and the tiling system may bond.

The solution is 300mm to 400mm of a Class III water-based membrane (compatible with the tiling system) applied over the span of the beam and bond breaker onto sound bonded substrate.

This allows the 50mm zone to separate and ripple but not rip, where otherwise direct contact to the crack occurs. This two-stage application is repeated on any screeds above the substrate if not a direct thin bed adhesive application.

Once the appropriate movement accommodation system is selected, tilers must align the set-out to movement joints, then adjust planned cuts aesthetically.

They should consult the floor plans with footing details and confirm the position of intermediate beams or thickenings by visual inspection.

Cracks may not be visible during the early stages of shrinkage.

(Waiting for the cracks to appear strained my marriage in our last house. We delayed tiling for nine months to allow the initial shrinkage to dissipate.)

Concrete continues to shrink over many years, applying compounding stress to the tiling. Delaying hard finishes at the initial onslaught will reduce the level of compounding stress.

Starting a full tile at the dominant movement joint, or cutting a tile directly over the joint is necessary if transference is required. Most active cracks, or structural shrinkage cracks, will run close to parallel and perpendicular to the structure.

Suppression is a good technique to apply if the crack is slightly askew to square set-out. Branch cracks running off structural cracks can be accommodated with suppression, but not transferred through tiling as a cut. These branch cracks are normally inactive shrinkage or plastic cracks, appearing during slab curing.

Installation of movement joints is often badly done or compromised by other building elements.

Perimeter movement joints are required in all tiling installations over 10m2. They are often compromised when tiles are installed before timber skirting fitting-off. The skirting is then cramped down and shot onto walls, thereby pinning the perimeter tiling.

We have cut out the three grout joints surrounding a perimeter tile, resulting in the tile lipping upwards. Timber skirting should be packed up minimally to clear the tiling and the gap can be soft sealed at pre-paint.

Intermediate movement joints must be planned at set-out and established as the tiles are being laid. Adhesive must be cleaned out of the selected joint while wet – not scraped out the following day.

This is crucial to allow segregated bays to cure independently, where the adhesive system is at its weakest while still green.

Cement-based adhesive reaches optimum strength in about seven days. Failure to isolate the intended segregated bays during curing stages will compromise the final bond integrity through differential shearing movement.

All movement joint installations require three key criteria:
Isolation – between layers in the tiling system.

Joint sealants should not extend into the layer below, where independent movement between layers is required to accommodate differential movement.

Wall adhesion – ensures that the sealant bonds to side walls of tiles only, and not to the backing substrate.

A bond-breaking tape or contaminant is installed at backing substrate level. Adhesion of the joint sealant to the backing substrate negates isolation between strata layers and does not allow for separation between bays, as intended.

Correct width to depth ratio – ensures compressive movement of the sealant according to its movement capabilities.

AS 3958.1- 5.4.5.5 gives the required ratios relative to the class of sealant. An elasto/plastic sealant, for example, requires 1:1 to 2:1. Industry practice follows the 2:1 ratio when using medium-modulus polyurethane sealants, which are more elastic than they are compressive.

A 10mm wide joint would therefore require a sealant depth of 5mm. This can be achieved by installing a closed cell polyethylene bond-breaking foam rod of the required diameter to match the joint size.

The foam rod is installed with a depth gauge – such as a set square adjusted to the correct depth and run along the movement joint over the loosely placed rod.

A low-modulus plastic sealant, such as silicone, may have a ratio of 1:3. Floor tiling with a 3mm joint size may include silicone filling to full depth, as long as a bond breaker is installed to the backing substrate to maintain two-wall adhesion.

The correct size grout joint is also a factor that helps with movement accommodation. AS 3958.1- 5.4.6(c) requires a minimum floor joint size of 3mm for dust-pressed floor tiles.

It is common practice in Victoria and NSW to install large-format floor tiling with a 1.5mm grout joint, or even smaller. This is a breach of the Standard and compromises independent movement between tiles. In some cases, tiles are actually abutting each other at the base of the joint.

Finally, as mentioned, the Standard recommends that movement joints be installed wherever movement is likely, and not just at the specified spacings.

Structural components, localised shrinkage, loading, substrate deflection, pinning by skirting, or obstruction of runs by dividing walls and fixtures all need to be considered.

Taking these steps will increase your chances of installing a tiling system that will perform as expected. Skipping steps may compromise the system, resulting in having to call Tile Reglue Injection Method to rectify the drummy ones that have come loose from the substrate.