Why is surface drainage needed at all under single grain mortars, often also known as drainage mortars?
Extensive research and testing at the Materials Testing facility in Darmstadt came to the conclusion that single grain mortars, when lying in a pool of water, transported the water upwards and thereby created damp stains on the natural stone coverings, as these coverings are absorbent. The single grain mortars must therefore be considered capillary-active and using single grain mortar alone is not enough to prevent damp stains. Single grain mortars allow water to move very quickly from top to bottom, but they do require additional drainage laid on the substructure in the fall or at the seal level respectively. This additional drainage will raise the single grain mortar, breaking the capillary action and thereby preventing the single grain mortar transporting water upwards. Apart from damp stains, efflorescence may appear in the joint areas, as during the capillary transport of water lime leaches out of the single grain mortar. If single grain mortar is applied without drainage, that is
