Mortar is usually classified according to its strength and weatherability: The table at right describes the correct proportions of ingredients for each.
► Type M has the highest compressive strength, at least 2,500 pounds per square inch (psi). This durable mix is recommended for load-bearing walls, masonry below grade, and masonry that is not reinforced with steel.
► Type S has a relatively high compressive strength (1,800 psi) and the best tensile strength of any mortar listed here; so it best resists wind and soil movement.
► Type N offers medium compressive strength (800 psi) and is suitable for all above-grade uses, including those subject to heavy weathering, such as chimney mortar.
► Type O has a low compressive strength (325 psi) and is limited to non-load-bearing, interior uses. However, it is sometimes specified for repointing chimneys with soft, old brick that would be destroyed by stronger mortar (see "The mortar mix," on p. 190, for more information).
► Type K is an extremely low strength (100 psi) mortar and is not recommended.
Of the mortar types listed here, type N is the most versatile. A simplified version of its proportions is 1 part portland cement, 1 part lime, and 6 parts sand (or 1 part masonry cement and 3 parts sand). Before portland cement became widely used in the nineteenth century, mortar was usually a mixture of lime and sand (animal hair was often added to reduce cracking). If brickwork 100 years old or older needs repointing, use type O so it won’t destroy the brick (roughly, 1 part portland cement, 2 parts lime, and 4 parts fine sand).
Dry mixing. When mixing mortar, mix the ingredients dry first to ensure a uniform mixture. That done, create a pocket in the middle, and add water gradually. As you add water, be fastidious about turning out the material in the corner of the mixing pan, so that there will be no dry spots. Mortar should be moist, yet stiff. A batch that’s too wet will produce a weak bond. Once the mix is nearly right, its texture will change radically if you add even a small amount of water.
