Which type of mortar is recommended for masonry installed below grade?

Type M mortar is the recommended choice for masonry installed below grade due to its high compressive strength and low water absorption characteristics. This type of mortar is designed to withstand the pressure from soil and moisture that is typically encountered in below-grade applications. Its strong bonding properties make it suitable for structural applications, including foundations and retaining walls, where durability and resistance to water infiltration are crucial.

In addition, Type M mortar's formulation, which includes higher cement content compared to other types, provides excellent resistance to moisture and helps prevent deterioration from the elements. This is particularly important for below-grade masonry where constant soil moisture and potential hydrostatic pressure can lead to structural failures or water damage if an inadequate mortar type is used.

Type N, while versatile for general above-grade use, does not offer the same strength and is less effective against water intrusion. Type O is primarily used for interior non-load-bearing walls, and Type S, although stronger than Type N, may not provide the significant compressive strength needed for below-grade applications. Hence, Type M is the clear choice for these conditions.