For new construction, ASTM C270 “Standard Specification for Mortar for Unit Masonry”, defines three mortar Types: N, S, and M, listed from lower to higher compressive strengths. Within the masonry industry, a widely accepted rule of thumb is to use Type N mortar for new construction unless there is a structural or durability reason to use a higher strength mortar. For veneer, Type N mortar will often be the best choice. Higher compressive strength mortar may be required for structural masonry, below-grade masonry, or masonry in high seismic regions.
In its non-mandatory appendices, ASTM C270 offers additional guidance on selecting the appropriate mortar Type for every project. Appendix X.1 relates the type of structural member and its location (above or below grade) to the preferred mortar Type.
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