I'm laying a CMU stem wall below grade and then brick veneer on the house above.

Below Grade (CMU Stem Wall): Use Type M.

• Reason: Highest compressive strength (min. 2,500 psi) and best durability for soil pressure, moisture, and freeze-thaw cycles.
• Code: IRC R607.1 / TMS 602 Table 3 — Type M or S required for foundation walls below grade. Type M is preferred for unreinforced CMU stem walls handling high lateral loads.

Above Grade (Brick Veneer): Use Type N.

• Reason: Medium strength (min. 750 psi), high workability, and lower cement content. It accommodates differential movement between the brick veneer and wood framing without cracking the brick units.
• Code: IRC R703.8.1 / TMS 602 — Type N or S permitted for veneer. Type N is industry standard for clay brick veneer over wood backup.

Do not use Type S for the veneer unless the brick is extremely hard/dense (e.g., some pavers or high-PSI face brick) and the engineer specifies it. Type S’s higher stiffness often cracks softer clay brick during thermal expansion.

Mixing Tip: For Type M below grade, use a 1:¼:3½ (Portland:Lime:Sand) or 1:3½ Masonry Cement:Sand. For Type N above, use 1:1:6 or 1:6 Masonry Cement:Sand. Keep sand damp, not wet. Tool joints concave for weather resistance.

For your CMU stem wall below grade and brick veneer above, use the following mortar types based on ASTM C270 and IRC/ACI standards:

Below-grade CMU stem wall (in contact with soil): Use Type M mortar.

• Why: Type M has the highest compressive strength (2,500 psi) and excellent resistance to moisture and freeze-thaw cycles. It’s required for below-grade masonry (IRC R404.1.1, ACI 530/ASCE 5/TMS 402) because it withstands lateral soil pressure and groundwater exposure better than weaker types.
• Note: Ensure proper drainage and waterproofing (e.g., dampproofing per IRC R406) behind the wall — mortar alone isn’t a water barrier.

Above-grade brick veneer (non-structural, exterior): Use Type N mortar.

• Why: Type N (750 psi) is the standard for exterior brick veneer (IRC R703.4, ASTM C270). It offers good workability, flexibility to accommodate minor movement, and adequate bond strength without being too rigid — which helps prevent cracking in veneer due to thermal expansion or foundation settlement.
• Avoid Type S or M here: Overly strong mortar can crack the brick or prevent necessary movement, leading to water intrusion or spalling.

Summary:

• Below-grade CMU: Type M
• Above-grade brick veneer: Type N

Do not substitute — using the wrong type risks durability, code compliance, and long-term performance. Always mix per manufacturer specs and tool joints properly for water resistance. If the stem wall is structural (retaining soil), confirm design with an engineer — but for standard residential stem walls, Type M is correct.