Which mortar is commonly used for outdoor structures due to its water resistance?

Type M mortar is commonly used for outdoor structures because it has a high compressive strength and excellent water resistance. This makes it particularly suitable for applications exposed to harsh weather conditions, such as retaining walls and foundations. The formulation of Type M mortar typically includes a higher ratio of cement to lime, leading to durability and resistance to moisture, which is essential for outdoor environments where water ingress can be an issue.

The other types of mortar, such as Type N and Type S, while they do have their advantages, do not offer the same level of compressive strength or water resistance as Type M. Type N is often used for general masonry work and is suitable for above-grade applications where moderate strength is sufficient. Type S is used for structural support and is better than Type N in terms of strength but is still not as water-resistant as Type M. Type O, on the other hand, has the lowest strength and is mainly suitable for interior non-load bearing applications. Therefore, Type M’s properties make it the ideal choice for outdoor structures.