Installing stone or ceramic tiles on vertical or overhead surfaces always carries the risk of those tiles falling. The obvious choice is to make the mortar more sticky, to give it greater bonding strength. Depending on the amount of strength you desire for your tiling project, increase the stickiness of factor appropriately.
Things You'll Need
Plastic spray bottle
Premixed tile mortar with latex
Powdered latex bonding agent
Moisten the application area using a garden hose outside or plastic spray bottle indoors. Also moisten the backs of the tiles or stones by squirting them with a spray bottle as you butter their backs with mortar before you set them into the bed of mortar. The water will increase the curing, or drying, time of the mortar, which eventually creates a stronger bond between the tiles and the installation surface.
Use a good quality tile mortar with latex to set your tiles or stones. The latex additive in this type of mortar helps it stick better, especially on vertical surfaces. Apply a thick layer and set stones close enough together that they will not require additional mortar between them.
Mix in powdered latex bonding agent, an additive commonly used to turn any type of mortar into a stickier, flexible finished product. The latex additive functions exactly like the premixed latex mortar, but you can vary its stickiness factor by changing the amount of latex you add to the mortar.