Using the correct notch trowel on installations is vital to ensure 100% coverage of mortar under the tiles.
100% coverage of mortar under the tiles means that there are no void spots (air pockets). This is essential to ensure maximum strength of the installation. All stone is immensely strong in compression, but if there are air pockets within the mortar, the stresses can be tensile in nature (bending forces). This bending force can lead to cracks forming in the tiles.
With slate, although it is calibrated to be relatively even in thickness, there are still differences from one tile to the next. For this reason, always use a cement-based mortar that is recommended for installing slate (medium bed) and use an appropriate notch trowel (1/2″ by 1/2″ square notch and larger). This will ensure that there is enough depth of mortar to allow the tiles to be adjusted, so that the top surface is level with adjacent tiles.
To be absolutely sure that the trowel selected is appropriate, always refer to manufacturer’s instructions on the appropriate cement-based medium bed mortar, for notch size and shape recommendations.