The bond coat must be applied to produce a minimum coverage for the item to be bonded. It is recommended that tile, when set and beat in, has coverage of mortar on the back of the tile greater that 80% in dry interior areas and 95% in wet or exterior areas. It is also recommended that the bond coat be at least 3/32″ thick between the tile and setting bed. That is verified by systematically pulling up several tiles in a field of tile while the installation is in progress.
This is the reason you will see various sizes of notched trowels recommended for certain sizes of tile. If the trowel size is not sufficient to provide the coverage, a larger trowel size needs to be selected. If 100% coverage is desired, tiles need to be back-buttered. Back buttering is a good practice especially if the tile has a lug back or is an out of plane shape.
Back buttering is not difficult. The process is designed to even out the back of the tile prior to setting the tile and beating it in. For some tiles, this may mean only a thin “skim coat” of the bonding mortar is applied using a straight non-notched finish trowel. Other out of plane tiles may require a thicker coat. The idea, again, is to even out the back of the tile prior to setting.
Is it possible to apply “thin set” mortars too thick? Yes it is. Portland cement “thin set” mortars cure by hydration. While they cure a certain amount of shrinkage will occur. Their bonding strength, if applied too thick, can actually fracture tile. This is the reason you will see “medium set” mortars available. Medium set mortar is manufactured for specific installation requirements. It is especially useful when setting tile on irregular setting beds and tile that varies in thickness.
The procedure for applying the setting bed is simple. Without covering or disturbing the layout lines or ledger, a sufficient amount of bonding mortar is applied to the substrate using the flat side of the notched trowel first. Scoop out enough mortar to skim coat the whole surface to receive tile. Use the flat side of the trowel to push the mortar firmly into the substrate. Then apply enough mortar to comb out the desired area using the notched side at a 45° angle to a uniform depth. Be careful to come as close to the layout lines as possible without obscuring the lines. This can be accomplished with a little practice.
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