Spot bonding of tile / stone – Spot fixing

Standard AS3958-1 2007: 54.6.2c states "spot fixing where the tiles are fixed with four or five dabs of adhesive is not recommended and should be avoided at all tiles".

This technique should ensure that the final bed of adhesive does not exceed 3mm and voids behind the tiles are kept to a minimum.  

To internal applications where there are dry conditions, notched trowel techniques should be employed using the following size trowel in relation to the tile format.

1. 200x200mm - 8mm notched trowel

2. 250x250mm - 10mm notched trowel

3. 300x300 - 12mm notched trowel

4. 400x400mm and above  -12mm notched trowel and buttering of the tile

It is not acceptable to spot bond tiles on floors.  For residential dry area applications, tiles are suppose to have at least 80% thin-set contact with full support at all edges and corners.  Tiles in wet areas should have at least 95% thin-set coverage.  Areas of the tile that are unsupported are susceptible to damage if subjected to live loads.

The tile should not crack or flex, or become de-bonded.  That suggests that the substrate is not stable and may have excessive deflection.   To determine if the problem is an isolated condition or systemic, you have to remove and inspect tiles in different areas.   If both the problem tiles and tiles without any sign of a problem are found to be deficient then you would likely need to replace all of the tiles.

I assume that when you say scratch coat you are referring to the mortar bed under the tile to which the tile is bonded.  If after the tile is removed, if the mortar bed is structurally sound, it can be reused if properly prepared.  If the problem resides in the substrate because of excessive deflection, then you have to repair the substrate.

Moisture damage can be a big problem with tiles that have been applied through the dot and dab method. Even waterproof adhesive cannot compensate for gaps where the area between the back of the tile and underlying surface is not filled. Moisture can cause mould and dampness which over time will not only cause damage to the underlying surface but may also result in the tiles becoming loose. Moisture can also penetrate through to the surface of the tiles, causing permanent staining.

Dot and dab tiling does not provide a secure fix on floors. Areas beneath the tile which are not covered with a layer of adhesive will be weaker and therefore more likely to crack under pressure.

We have seen many more failure of stones installed at sill, jambs and as coping at both internal and external areas. These has become common even when we use high-end tile & stone adhesives for the installation. The main reason for this failure is the irregular dimension of stones. Then what is the right size of the stone when we planned to install them at sill, jambs and as coping stones?

India standard – IS 1127 - Recommendations for dimensions and workmanship of natural building stones for masonry work, clearly explains about the size of the sill and coping stones dimensions.

So, when we limit the size of the stone as per the above dimensions, we have seen many success in stone installation.