For most doing-it-themselves, when a project requires mortar, they follow the directions and buy what is recommended in the instructions. They do not understand why they bought the mortar mix they did; they just did as instructed. For some, though, knowing why is as important as knowing how to use it; for them, here is a brief rundown on the standard types of mortar and its purpose.
Type M mortar is the strongest mortar and is generally only used on stone. It lasts forever. When you find a stone from a stone wall that has decades old mortar caked on it, you are probably looking at Type M mortar.
This type of mortar is medium strength and usually is used on below grade, exterior walls and surfaces. Below grade in this case means below the ground or surface. Mortar with an S rating can be used on masonry foundations, manhole tunnels, retaining walls and sewers. This type of mortar is also used for patios and other deck like platforms, where high foot traffic is common. Additionally, it holds up well when exposed to external corrosives like salt, sand, stones or dirt.
Mortar mix graded “N” is medium strength and considered the true “general purpose” mortar. It is used for above grade exterior walls and interior projects. Type M mortar is used on semi-soft masonry and has a flexibility to it that makes it resilient when pressure is put on it.
This makes it a better choice than harder mortars for some projects where shifting can occur as it will not crack as easily. That includes load bearing surfaces and soft-stone masonry. Another use for N rated mortar is stucco.
O mortar is the lowest strength of traditional mortared materials. It is used almost exclusively on non-load bearing interior walls. Mortar stucco mix is one good example of this type of mix if it is being used on a non-load bearing surface. Mortar with a rating of O is also commonly used as a patch for mortar that has crumbled because it is so easy to work with.
As the name implies, this type of mortar is used when installing glass blocks. Glass block mortar is like O, but is even more flexible, which helps keep glass blocks from breaking or cracking under stress.
K is almost exclusively used on historic preservation or restoration projects. It is extremely soft and its ingredients benign, which helps prevent damage from occurring to the articles or surfaces where it is being applied.
Most mortar mix labels come with highly visible rating letters and the instructions indicate what it should be used for. As always, if you have questions, consult a mortar or masonry expert!