Cement Block (CMU)

The first decision matters more than any other: which block category fits the job. Get that right and everything else — strength, install method, how much labor it takes — falls into place. Get it wrong and you’re either handling blocks too heavy for the application or building a wall that can’t carry the load.

For broader project planning, explore WBC’s building materials resources.

Start with the basics. Concrete blocks — formally called concrete masonry units (CMU) — are precast rectangular building units made from Portland cement, water, and aggregates, cured under controlled conditions to achieve a specified compressive strength before delivery to the job site. Unlike cast-in-place concrete, which is poured wet into a form and cures on site, CMU blocks arrive ready to lay, requiring only mortar between units and, for structural applications, grout and reinforcement in the hollow cores.

Here’s the part that trips people up at the counter. Blocks are sold by nominal thickness — 4-inch, 6-inch, 8-inch, and 12-inch, the most popular referred to by their thickness. The standard nominal block runs 8x8x16 inches. But their actual dimensions are 3/8 inch less than the nominal dimensions, to allow for 3/8-inch mortar joints between blocks in any orientation. So an “8-inch” block doesn’t actually measure 8 inches in any direction. Plan your courses around the nominal size — mortar joint included — not the actual block.

Most blocks are hollow. The hollow area is typically 40–50% of the gross cross-sectional area. Most concrete blocks have two cores, but three- and four-core units are also produced. Those cores aren’t just for weight savings — they’re the conduit for rebar and grout when the wall needs to carry real load.

One ordering rule: buy extra. Always order 5–10% extra for cuts and breakage. Running short mid-course in the middle of a Montana build season costs you more in downtime than the extra blocks ever will.

Block Types and What Each One Does

Eight categories cover most of what gets built. Weight, strength, and best use separate them.

Block Type	Weight (8” unit)	Compressive Strength	Best Use
Standard hollow CMU	38–43 lbs	1,900–3,000 PSI	Foundations, load-bearing walls
Lightweight CMU	22–28 lbs	1,900–2,500 PSI	Interior partitions, upper floors
Solid CMU	55–70 lbs	2,500–4,000 PSI	Below-grade walls, fire separation, columns
Split-face CMU	40–48 lbs	1,900–2,500 PSI	Visible exterior walls, decorative landscape
Ground-face / glazed CMU	40–50 lbs	2,000–3,000 PSI	Architectural walls needing smooth finish
Segmental retaining wall block	30–80 lbs	3,000–5,000 PSI	Landscape retaining walls, terracing
Interlocking (dry-stack) block	25–50 lbs	2,500–4,000 PSI	Garden walls, raised beds, no mortar
AAC (autoclaved aerated concrete)	15–25 lbs	500–900 PSI	Non-structural interior partitions, insulation

Standard hollow CMU is the workhorse. It dominates commercial building, industrial structures, garages, foundation walls, and any application where structural performance, fire resistance, and dimensional consistency matter.

Lightweight versus normal weight comes down to handling, not strength. C90 units come in two weight classifications: normal weight at 125–135 PCF density and lightweight at less than 105 PCF. Normal weight units use gravel or crushed stone aggregate; lightweight units use expanded shale, clay, or slate aggregate. Both meet the same compressive strength minimums — the lightweight classification refers to density, not strength. Out West, there’s a regional twist: in the western United States, where they are easily obtainable, porous lava rock gravels are used for weight reduction. That’s where pumice block comes from.

Solid block earns its keep below grade. Solid CMU has no cores or only small voids constituting less than 25% of the gross area. Solid units are heavier and more expensive but provide greater cross-sectional area for compressive loads without grouting, are preferred for below-grade walls where water infiltration through unfilled cores is a concern, and achieve higher fire ratings per unit of thickness. That water-infiltration point matters for Montana basements and crawl spaces — fewer voids means fewer paths for moisture.

AAC sits at the opposite end. Autoclaved aerated concrete blocks run half the weight of standard CMU, offer excellent insulation, and cut with a handsaw — faster installation for non-structural interior walls. But look at that PSI: 500–900. That’s partition-wall territory, not structural.

The governing standards are worth knowing. Almost all structural CMU in the US is manufactured to ASTM C90, the standard specification for loadbearing concrete masonry units. C90 establishes minimum requirements for compressive strength, dimensional tolerances, water absorption, and material composition. The minimum net area compressive strength for C90 units is 1,900 PSI. Lightweight units fall under a separate spec — ASTM C331 requirements for lightweight units.

The block lineup carried under the Western Building Center umbrella reflects these categories: solid block in 1-5/8x8x16 and 4x16x16, lightweight 8x8x16, an 8x8x16 bond beam, pumice half block, partition block, plus segmental and landscape products like planter wall, flagstone wall, and pier blocks for deck framing.

Matching Block to the Job

Self-select before you reach the counter. Most projects sort cleanly.

Foundations and load-bearing walls. Use 8” or 12” standard CMU to ASTM C90, cores filled with grout and vertical rebar. One common foundation type for American suburban houses is the crawl space foundation, which consists of a concrete block wall on the perimeter on which dimensional lumber floor joists are supported.

Below-grade walls. Lean toward solid units. Fewer voids for water, higher fire rating per inch of thickness — both matter where the wall sees moisture and freeze-thaw.

Retaining walls under 4 feet. Segmental blocks earn the nod here. Segmental retaining wall blocks come with pre-engineered geometry that eliminates custom rebar design for walls under 4 ft. These are blocks designed to be set back each course and used with a sand base, without mortar or reinforcing — a gravity wall.

Interior partitions. Lightweight CMU or AAC. Lighter to lift, faster to set, and the insulation is a bonus on interior walls that don’t carry load.

Decorative or visible walls. Split-face or ground-face CMU — the integral texture eliminates the need for exterior finishing and delivers an architectural finish at a masonry price. The split-face look comes from the manufacturing: special textures are produced by splitting a ribbed or solid two-block unit, and such factory-produced units are called “split-rib” or “split-face” blocks.

Installation and Reinforcement Basics

A block wall is only as good as the footing under it and the rebar inside it.

Start level. Build on a level concrete footing. Then check constantly — every course gets verified for level and plumb, because a course out of true at the bottom compounds the whole way up.

Mortar selection isn’t one-size-fits-all. Choose type M, S, N, or O based on the load the wall carries and the climate it lives in — a real consideration where freeze-thaw cycles work the joints all winter. Butter the block ends to seal the joint against water.

Reinforcement is where a stack of block becomes a structural wall. Vertical rebar is placed in selected cores at spacing specified by the structural engineer or building code — typically 24–48 inches on center for residential walls, 16–32 inches for commercial — and the cores containing rebar are filled with grout, a high-slump fluid concrete mix that flows into the cores and consolidates around the rebar without vibration. The result is a composite system in which the CMU carries compressive load and the grouted rebar carries tensile and shear forces.

Grout type depends on block size. Fine grout — no coarse aggregate — is used for 4-inch and 6-inch CMU, while coarse grout with aggregate up to 3/8 inch handles 8-inch and larger cores where there’s room for it to flow.

That grouting also drives fire rating. An 8-inch hollow CMU wall reaches a 2-hour fire rating; fully grouted, that same wall climbs to 4 hours. For seismic country — and parts of western Montana sit in active zones — full grouting matters structurally too. High-seismic zones typically allow only fully grouted walls in their building codes. Reinforced this way, concrete block walls are better able to resist lateral forces such as wind load and seismic forces.

For deck and post applications, pier blocks do a different job — the 12x12x8 pier and the bracket-style pier blocks support framing rather than building a continuous wall.

Frequently Asked Questions

Why is an “8-inch” block not actually 8 inches? Blocks are sized by nominal dimension, which includes the mortar joint. The actual dimensions are 3/8 inch less than the nominal dimensions, to allow for 3/8-inch mortar joints between blocks in any orientation. Lay out your courses using the nominal size so the joints work out right.

What’s the difference between hollow and solid block? Hollow block has cores running through it — typically 40–50% of the gross cross-sectional area — that take rebar and grout. Solid CMU has no cores or only small voids constituting less than 25% of the gross area. Hollow is the standard for grout-reinforced structural walls; solid is preferred below grade and for fire separation.

Do I need rebar in a short garden or retaining wall? Often not, if you use the right block. Segmental retaining wall blocks have pre-engineered geometry that eliminates custom rebar design for walls under 4 ft. For low garden walls, 4” or 6” standard CMU or decorative cap blocks work, with no permit required under 3 ft in most jurisdictions. Confirm local requirements before you build.

Which block holds up best below grade in a cold climate? Solid units. They’re preferred for below-grade walls where water infiltration through unfilled cores is a concern, and achieve higher fire ratings per unit of thickness. Concrete blocks are suitable for use in walls below ground, due to their durability and resistance to frost, where they may be exposed to moisture and varying temperatures. Pair that with the right mortar type for freeze-thaw and tight joints.

How much block should I order? Add a cushion. Order 5–10% extra for cuts and breakage. Corners, openings, and the occasional cracked unit eat into your count faster than you’d expect.

What makes AAC different from regular block? Weight and what it’s for. AAC is made by adding aluminum powder to the concrete mix, which creates gas bubbles during curing, resulting in a block up to five times lighter than traditional concrete blocks. At 500–900 PSI it’s a non-structural choice — partition walls and insulation, not foundations.