Recommended Guide for Choosing the Right Mortar Mix Type

Choose Between N,M,S and O Type of Mortar

Everything is built one brick at a time / Getty Images

How to Choose the Right Mortar Mix

There are many types of mortar mix available in the market and sometimes can get a little bit confusing to determine which one is the right one for the application. I would suggest starting by analyzing two very important characteristics of mortars because they will hold and provide the strength on masonry units. Mortar mixes are classified based on compressive strength and their bonding properties and flexibility.

Each mortar mix has a specific Portland cement, hydrated lime and sand proportions referred to volumes used during mixing.

Choosing a mortar mix is not an exercise based only on the resistance and strength of each mortar, it must depend on its use, adhesion, and sealing requirements. Let's discuss the differences between N, O, M and S types. Many builder confuse the term mortar and grout, but you need to be clear that mortar is the element that keeps all bricks joint together providing structural capacity to the wall.

Applications and Uses of Mortar Type N

A mortar mix type N is usually recommended on exterior and above-grade walls that are exposed to severe weather and high heat. Type N mortar mix has a medium compressive strength and it is composed of 1 part Portland cement, 1 part lime and 6 parts sand. A type N mortar is described as a general purpose mortar mix, used in above grade, exterior, and interior load-bearing installations.

It is also the preferred mortar mix for soft stone masonry. This is the mortar most often used by homeowners and the best one for general application. Type N mortar typically achieves 28-day strength in the range of 750 psi.

Mortar Mix Type O Uses and Applications

Mortar mix type O is referred as that mix with a low compressive strength, about 350 psi, used on interior or non-load–bearing walls.

In the appendix for ASTM C 270, Type O mortar is listed as an alternative to Type N for interior areas but its exterior use shall be limited due to its low structural capacity and not recommended in areas subjected to high winds. Type O Mortar mix is ideal when repointing due to its consistency and can be applied easily. It is used in above grade, non-load bearing situations ideally in interior environments. 

Should I Use Mortar Mix Type S?

Offering a high compressive strength, over 1800 psi, and with high tensile bond strength, mortar mix type S is the ideal product to be used in masonry at or below grade. It performs extremely well to fight soil pressure, wind or under seismic conditions. This mortar mix can be used for below grade areas, for example, masonry foundations, manholes, retaining walls, sewers, and brick patios and brick pavements. Type S mortars are required to have a minimum of 1800 psi and their mixes usually give you strengths between 2300 to 3000 psi.

When to Use Type M Mortar?

Type M mortar mix has the highest amount of Portland cement and it is recommended for heavy loads, masonry below grade, foundations, retaining walls and driveways. Mortar mix type M will provide with at least 2,500 pounds per square inch compressive strength.

A type M mortar with its high strength yet poor adhesion and sealing can be a bad choice for one area of the job and just what is needed in another. Type M is preferred with stone because the strength of the mortar simulated that of the stone being used.

Mortar Mix: ASTM Requirements

Masonry mortars are specified by ASTM C 270, Specification for Mortar for Unit Masonry. Mortars are evaluated by ASTM C 780, Preconstruction and Construction Evaluation for Mortars for Unit Masonry. Following ASTM standard the average required compressive strength of each mortar types is:

  • Type M 2,500 psi
  • Type S 1,800 psi
  • Type N 750 psi
  • Type O 350 psi

There is also a type K-mortar but that is rarely used. That type of mortar is used for historic or ancient buildings that require a special mix. At very low compressive strengths, 75 psi only, type K mortar is normally one that you will rarely see and it is very difficult to manage due to its composition.