Why tiles lift or crack
There are many reasons why tiles lift or crack and the reason may be easily identified or hidden within the mortar, the foundation or even deeper below
By Gareth Greathead
One of the biggest flooring problems experienced in homes must be the lifting or cracking of tiles. By gathering an understanding of common problems, why they occurred and how they can be resolved, you can ensure that you or your tiler get the job done right.
Buckling or tenting
Tenting of tile flooring occurs when compressive forces induced into the tile system break the bond between the tile and substrate. The problem is regularly caused by expansive forces created due to moisture or thermal exposure. This, together with the inherent shrinkage of concrete substrates and lack of movement joints within the tiling system, exceeds the limits of the adhesive, causing problems.
Moisture and thermal exposure
Ceramic tiles expand with moisture and thermal exposure. Expansion and contraction after elevated thermal exposure is ongoing while expansion from the absorption of moisture is generally permanent. For this reason, water removal from the surface of the tile should be addressed immediately, either manually or through adjustment of the gradient to promote drainage of water.
Looking at the date of manufacture of the tiles you purchase may be helpful as tiles are most vulnerable to water ingress soon after manufacture. However, there are industry standards in place which prevent tiles making it onto the shelf earlier than they should. This provides good reason not to purchase tiles without SABS approval – the approval means they have been tested according to our environmental conditions.
Fractures and separations
Buckling may fracture the tile system; however, fractures in tile systems may also be the result of impact, rolling loads, differential foundation movement and substrate fractures.
Differential foundation movement
This results in fractures that often coincide with movement joints or other irregularities in the supporting substrate. This movement results in compressive forces being transferred upwards, affecting the surface above.
Tiles can be damaged by objects dropped from above. Chips and radial type fractures are common and are typically located in high traffic areas, such as kitchens or bathrooms.
Heavy items like office chairs or trolleys distribute load amongst isolated spots and can cause cracking.
When ceramic tiles are properly bonded to the substrate, the tiles and substrate will behave as a homogenous material. As such, fractures in the concrete substrate below are frequently reflected in the ceramic tile. Pool decks and other exterior applications are especially susceptible to tile fracturing caused by fractures in the supporting substrate.
A commonly observed fracture in the supporting substrate is the untreated shrinkage fracture. Shrinkage typically results in only cosmetic fractures. If thermal/moisture effects and foundation movement have been eliminated as potential causes of tile fractures, it is likely that the tile fracture has been caused by shrinkage of the concrete substrate.
This type of failure is commonly found at newly developed complexes where contractors are under pressure to commission the structure. The majority of movement takes place within the first two years of the concrete being poured, making the problem evident within that time frame.
Hollow-sounding tiles are difficult to ignore, but are not necessarily reason for concern. This condition may be encountered during normal use, such as walking with hard soled shoes or moving of furniture across the tiles. At other times it may result in less resilient tiles, leading to frequent impact/load breakages.
The presence of hollow-sounding tiles may be indicative of incorrect installation, exposure to moisture or thermal sources, material defects or the installation of a cleavage membrane (a membrane that separates the concrete from the mortar to limit transfer of unwanted forces). If hollow tiles are encountered in isolated patches inside your home or you are buying a new home, you will need to know how to test for hollow tiles.
The test can be done by dragging or tapping a blunt object over the tiles. A hollow tile will often make a dull sound, which lacks a more solid pinging tone. The test can be done using a few lengths of chain attached to a length of tube. Alternatively, a metal object like a ball bearing can be used to test smaller areas without hassle.
Hollow-sounding areas could signal several things: the thin-set adhesive could have debonded from the back of the tiles or from the substrate. If the substrate is a bonded mortar bed, it’s possible that the mortar bed has debonded from the concrete slab.
What signals debonding is a change in sound from one area of an installation to another, or from one portion of the tile to another. At least 80% of the entire floor should be directly attached to the substrate below; anything less indicates a potential problem that is likely to become worse as time goes on.
This may not be easy to ascertain without specialised equipment; checking if the hollow-sounding tiles are grouped together or are spread out randomly is more realistic. If they are spread out randomly, one has to question whether the solid sounding tiles will join those already debonded at a later stage.
If poor installation is to blame, it is likely that the bond may never have developed adequately, leaving the tiles un-bonded.
Poor bonding may be a result of improper substrate preparation, poor mortar mixes and obstructions including, but not limited to, paint, dirt, laitance, curing compound and paint overspray. ‘Dirty’ surfaces are likely to affect the strength of the bond between the mortar bed and the substrate.
This happens when an installer applies ‘spots’ of adhesive on each corner of a tile and one in the centre, and presses it into place to save on material costs. This leaves voids under the tile and ultimately throughout the entire installation. When this method has been used, you’ll hear the hollow sound where there is no adhesive (the voids) and you’ll hear solid sounds where the tile is spot-bonded. Again, a thinset coverage of 80% at the bonded side of the tile is industry standard and the only sure way to ensure tiles are bonded to the substrate effectively.
A vital step in correct installation is the setting of the tile into the mortar bed. Tiles should be installed while the mortar bed is workable and should be forcibly pushed into the mortar using a tile mallet designed for the purpose. This process is referred to as ‘beating in’ amongst professional tilers.
In some cases, tiling problems can be resolved more easily with the use of advanced materials and techniques developed over the years.
This is a workable paste used to bind building blocks such as stones, bricks and concrete masonry units together, to fill and seal the irregular gaps between them and sometimes add decorative colours or patterns to masonry walls.
This is the tile adhesive sold in shops and is used to attach tile or stone to surfaces such as cement or concrete. It is an adhesive mortar made of cement, fine sand and a water-retaining agent.
This is thinset cement, to which polymers have been added, and is commonly called latex-Portland cement mortar. Today, there are over 10 000 polymers available to cement chemists when formulating their products. Many of these polymers are, in fact, acrylics and not latex-based chemicals. The use of these polymers allows specific properties to be imparted to the cement; commonly freeze/thaw resistance, improved flexibility and improved adhesion. There are also polymers used to make the cement more water resistant, waterproof or elastic, so that it acts like an anti-fracture membrane (softens blows received from above).
This is derived from the traditional method of packing a mortar bed over a surface before installing the tile. The tile is adhered to the mortar bed either while the mortar bed is green (still curing) or after the mortar bed has cured. This is mainly used when floors need to be levelled out or gradients formed to aid drainage.
A mortar bed may be reinforced with wire and either set over a cleavage membrane (that allows the mortar bed to ‘float’ free of the substrate) or bonded to the substrate; hence, the use of the terms ‘floating mortar bed’ or ‘bonded mortar bed’.
In the case of the floating mortar bed, the tile layer is unaffected by minor cracking and movement in the substrate. This can be very important in applications over concrete where cracking in the concrete could result in cracking in the tile. It is also used on structures where vibration and deflection can be expected (as occurs in some exterior walls).
Tips to avoid lifting and cracking
• Movement joints within tiling systems can be created using a polyurethane sealant available in caulking cartridges.
• Buckling/tenting may be avoided with the application of movement joints by providing expansion along the perimeter before skirtings are added.
• Movement joints in-between the tiled area are essential outdoors, in sunrooms and beneath skylights, and help control cracks due to expansion, contraction and movement of the slab underneath the thinset.
• Movement joints should be made to coincide with those already present on the substrate below.
Helpful tiling aids
As with most things in life, having the knowledge is nothing without the right tools. With tiling becoming such a popular DIY task, suppliers have developed a range of tools and aids that cater for home installations. Consider these before attempting to resolve any tiling issue you may be experiencing.
A thinset coverage of at least 80% of the tile ensures that the tiles bond effectively
Tiles should be forcibly pushed into the mortar using a tile mallet