Type N Mortar Mix
Type N mortar is usually recommended on exterior and above-grade walls (including stone veneer) 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. It is considered to be a general-purpose mix, useful for above grade, exterior, and interior load-bearing installations. It is also the preferred mortar mix for soft stone masonry (such as limestone). Type N is the mortar most often used by homeowners and is the best choice for general application. It typically achieves 28-day strength in the range of 750 pounds per square inch (psi). Thinset mortar is a moisture-resistant adhesive made of Portland cement, water, and fine sand, which is used with projects involving cement board, shower tile, backsplashes, and shower pans.
Type O Mortar Mix
Type O mortar mix has relatively low compressive strength, at only about 350 psi. It is used primarily for interior, above-grade, non-load–bearing walls. Type O can be used as an alternative to Type N for some interior applications, but its exterior use is limited due to its low structural capacity. It is not recommended in areas subjected to high winds. However, type O mortar mix is ideal for repointing and similar repair work on existing structures, due to its consistency and ease of application.
Type S Mortar Mix
Offering a high compressive strength of over 1,800 psi and high-tensile bond strength, type S mortar is suitable for many projects at or below grade. It performs extremely well to withstand soil pressure and wind and seismic loads. Type S is the common choice for many below-grade applications, such as masonry foundations, manholes, retaining walls, and sewers, as well as at-grade projects like brick patios and walkways. Although type S mortar must have a minimum compressive strength of 1,800 psi, it is often mixed for strengths between 2,300 and 3,000 psi.
Type M Mortar Mix
Type M mortar mix has the highest amount of Portland cement and is recommended for heavy loads and below-grade applications, including foundations, retaining walls, and driveways. While type M mortar provides at least 2,500 psi of compressive strength, it offers relatively poor adhesion and sealing properties, making it unsuitable for many exposed applications. Type M is preferred for use with natural stone because it offers similar strength to that of stone.
Type K Mortar Mix
Type K mortar is rarely used for new construction but may be specified for restoration or other specialty applications. It offers a very low compressive strength of only about 75 psi. Because of its softness, type K is primarily used for restoring the masonry on historic or ancient buildings that require a special mix that is not significantly stronger than the existing masonry.
