The procedure for laying natural stone is similar to that for laying ceramic tiles, but it's more exacting. Natural stones stones don't have regular edges, and they are quite a bit heavier than ceramic or porcelain tiles, so placing them is a painstaking job that even professionals find difficult. Whether you're laying the stones on a concrete subfloor or on layer of fiber cement board, you want a mortar that's strong but remains flexible enough to resist cracking.
Mortar Isn't Concrete
As it is for concrete, Portland cement is the main ingredient in the mortar that tile setters use to lay ceramic or stone tiles. Unlike concrete, though, mortar contains no rocks or stones -- just sand. Moreover, the cement usually contains a variety of plasticizing ingredients, such as lime and latex additives, to keep it flexible. When it comes to mortar, stronger isn't necessarily better; if an unusual load is applied, it's better for the mortar to fail than the tiles, because if the tiles remain intact, they can always be reset.
Type M, S, N and O Mortar
The American Society for Testing and Materials -- ASTM -- recognizes four types of masonry mortar -- types M, S, N and O. There is one more type it no longer includes in its listings, although that type -- type K -- is sometimes the best choice for historical preservation projects because of its workability and low compressive strength.
- Type M mortar can have a compressive strength of 2,500 pounds per square inch and is generally reserved for use on large-scale commercial projects.
- Type S mortar has a compressive strength of 1,800 psi and is used in below-grade applications and for exterior projects, such as patios.
- Type N mortar has a compressive strength of 750 psi and is the all-purpose grade for most interior applications.
- Type O mortar has a low compressive strength of only 350 psi and is used primarily with materials with similarly low compressive strength, such as brownstone and sandstone.
When choosing mortar for setting natural stone tiles, the best all-around choice is type N unless the tiles will be exposed to unusual moisture conditions, in which case type S would be a better choice.
Thinset vs. Mortar
When you're looking for tile adhesive at a building supply outlet, you'll usually find several types of thinset, which is a type of mortar that is made with especially fine aggregate materials. It's often made with mastic or epoxy instead of Portland cement. Thinset is designed to provide good bonding strength when applied in a layer that is 3/16 inch thick or less and loses strength when applied in thicker layers.
When laying natural stone, the choice of adhesives is usually between thinset and type N mortar. Any thinset formulation recommended for ceramic or porcelain tiles will work with smooth and regularly shaped stones, but if your stones are highly featured and irregular, and you need to lay a thick bed of adhesive to hold them, choose type N mortar. Upgrade to type S mortar if you're laying the stones outside or in the basement.