Standard Concrete Masonry Unit (CMU) Sizing & Mortar Rules
Concrete masonry units are referred to by their nominal dimensions, which include a standard 3/8-inch mortar joint. An 8×8×16 block actually measures 7 5/8" wide by 7 5/8" high by 15 5/8" long.
| Nominal Size (W×H×L) | Face Area | Blocks / 100 Sq Ft | Core Volume / Block | Typical Application |
|---|---|---|---|---|
| 6×8×16 Inch | 0.889 sq ft | 112.5 blocks | 0.16 cu ft | Light interior partitions, storage sheds |
| 8×8×16 Inch (Standard) | 0.889 sq ft | 112.5 blocks | 0.22 cu ft | Residential retaining walls, foundation crawlspaces |
| 12×8×16 Inch | 0.889 sq ft | 112.5 blocks | 0.33 cu ft | Heavy earth retaining, deep basement perimeter walls |
Mortar Selection: Type S vs Type N vs Type M
Mortar binds blocks together while accommodating slight thermal movement. Selecting the wrong mortar formulation can cause joint cracking or water intrusion:
Type S Mortar (Recommended)
1,800 PSI Compressive StrengthThe standard industry recommendation for exterior retaining walls, below-grade foundation blocks, and walls subjected to high lateral soil loads or seismic activity. Provides excellent flexural bond strength.
Type N Mortar
750 PSI Compressive StrengthMedium-strength general purpose mortar used for above-grade exterior veneer and non-loadbearing interior partitions. Too flexible for heavy retaining walls holding back saturated soil.
Type M Mortar
2,500 PSI Compressive StrengthHighest compressive strength available, designed for heavy concentrated vertical structural loads, below-grade foundations with severe groundwater contact, and manholes.
Crucial Retaining Wall Construction Rules
1. Poured Concrete Strip Footing Rule
Never place concrete blocks directly on gravel or soil. Pour a solid concrete footing twice as wide as the block thickness (a 16-inch wide footing for standard 8" blocks) and at least 8 inches thick, reinforced with two continuous lines of #4 (1/2") steel rebar. Embed rebar dowels that project upward into the block cells.
2. Hydrostatic Pressure Relief (Drainage is 90% of Success)
Retaining walls don't fail from the weight of dry dirt—they fail from trapped hydrostatic water pressure. Backfill the immediate 12 inches behind the wall with #57 crushed clean stone, install a 4-inch perforated PVC drain pipe sloping to daylight at the footing base, and install 1.5" PVC weep holes every 6 to 8 feet along the bottom course.
3. The 4-Foot Height Permit Barrier
Under International Residential Code (IRC) Section AH106, retaining walls exceeding 4 feet in total height measured from the bottom of the footing to the top of the wall require an engineered design signed by a registered structural engineer, including soil geogrid reinforcement tie-backs.