United States
Hong KongMortar types
Mortar types M, S, N and O run from highest to lowest compressive strength. Mortar should generally be weaker than the units so that cracking occurs in the joint rather than through the block.
Four letters in a fixed order, and a selection principle that surprises people.
The order
M, S, N, O - from highest compressive strength to lowest.
The letters come from alternate letters of MASON WORK, which is worth knowing because the sequence is otherwise arbitrary and easily reversed.
Where each is used
| Type | Typical use |
|---|---|
| M | Below grade, foundations, retaining walls, high compressive load |
| S | Structural, where lateral strength matters |
| N | General purpose above grade |
| O | Interior non-loadbearing, and repointing historic work |
Mortar should generally be weaker than the masonry units. Movement then cracks the joint, which can be repointed, rather than the block, which cannot. Using Type M everywhere is a mistake, not a margin of safety.
Why that matters for repointing
Repointing soft historic brick with hard modern mortar traps moisture and forces movement into the brick face, which spalls.
It is the reason Type O exists, and it is a favorite question because the intuition runs the other way.
Grout is not mortar
Mortar bonds units together. Grout is fluid and fills cores and cavities, often around reinforcement.
Different material, different job, and questions offer one for the other.
Cold weather
Forty degrees Fahrenheit, the same threshold as concrete. Below it, cold weather masonry provisions apply.
Common questions
What are the mortar types?
M, S, N and O, from highest to lowest compressive strength.
Should mortar be stronger than the units?
Generally not. Weaker mortar cracks in the joint, which can be repointed, rather than through the block.
Why does repointing use soft mortar?
Hard mortar on soft historic brick traps moisture and forces movement into the brick face, which spalls.
What is the difference between mortar and grout?
Mortar bonds units together. Grout is fluid and fills cores and cavities, often around reinforcement.
What is the cold weather threshold?
40 degrees Fahrenheit, the same as for concrete.