My name’s Tom Halloran, and I’ve spent close to twenty years laying stone and block around Austin, Texas. I build mostly in limestone down here, but concrete block is the workhorse behind more homes than people realize, from foundation stem walls to the back-up wythe hiding behind a pretty stone veneer. I got a call last month from a homeowner up in Hamden, Connecticut who wanted to understand what her 1960s block foundation actually was before a contractor started “fixing” it. Fair question. So here’s the plain-English version of concrete block, or CMU, and why it behaves the way it does.
What CMU actually is
CMU stands for concrete masonry unit. It’s the gray hollow block you’ve seen on every job site, made from Portland cement, aggregate, and water, cast into a mold and cured. The standard nominal size is 8 by 8 by 16 inches, though the real block runs 3/8 inch smaller in each direction to leave room for the mortar joint. The hollow cells, called cores, are what make block efficient. They cut weight, and they give you a place to run vertical rebar and grout where the wall needs strength.
Not all block is the same. You’ll see lightweight, medium, and normal-weight units depending on the aggregate. Heavier block resists water and sound better, lightweight is easier to lay. For a foundation or a retaining application you want normal-weight units and, often, grout-filled reinforced cells — that’s true whether the house is a 1960s split-level in Hamden or a mid-century ranch in the Austin suburbs.
Where the strength actually comes from
A plain block wall is strong in compression, meaning it handles weight pressing straight down very well. Where it’s weak is tension and bending, which is what happens when soil pushes against a basement wall or settlement twists it. That’s why a real structural block wall gets steel. I drop vertical rebar into the cores at a set spacing and fill those cores with grout. Horizontal joint reinforcement, that little ladder-shaped wire, goes in the mortar bed every couple of courses to tie the wall together and control cracking.
In a New England climate, that reinforcement matters even more because freeze-thaw cycles and frost heave load foundation walls hard every winter. Down here our enemy is expansive clay pushing sideways, but the engineering answer is the same: steel and grout where the bending happens, not just more block.
| CMU detail | Typical spec |
|---|---|
| Nominal block size | 8 in. x 8 in. x 16 in. |
| Actual block size | 3/8 in. smaller each dimension (for mortar joint) |
| Vertical rebar spacing | Every 16, 24, or 48 in., depending on engineering |
| Mortar for foundations/load-bearing | Type S |
| Mortar for non-structural, above-grade | Type N |
| Installed reinforced wall | $10-$20 per sq. ft. |
| Single 8-in. block unit | $2-$4 each (before mortar, grout, labor) |
The mortar joint is where I check first
Block goes up in a running bond, each unit offset half a block from the one below, which spreads load and looks right. The mortar isn’t glue; it’s a bedding material that seats the units, seals the wall, and transfers load. Most foundation and load-bearing work uses Type S mortar for its higher strength, while softer Type N shows up in above-grade non-structural work. A sloppy joint, either too thick or unfilled, is a weak spot and a water path — and that’s true whether I’m looking at a block foundation or repointing an old brick chimney. The tools change, but tuckpointing and joint repair on any masonry wall lives and dies by the same rule: a good joint keeps water out, a bad one invites it in.
Water is the real enemy
Concrete block is porous. Left unprotected below grade it wicks moisture, and in a freezing climate that trapped water expands and spalls the face right off the block. Any below-grade block wall should have a damp-proof or waterproof coating, drainage at the footing, and grade sloped away from the house. If you’re dealing with a wet block basement, our guide on waterproofing a basement masonry wall walks through the options from interior sealers to full exterior excavation. For insulating a block wall, the U.S. Department of Energy’s guidance on insulation is a solid, unbiased starting point.
What a block wall actually costs, and where not to cut corners
Ballpark, an installed reinforced CMU wall runs $10 to $20 per square foot depending on height, rebar spacing, and access. A standard 8-inch block itself is roughly $2 to $4 each before mortar, grout, and labor. Reinforcement, grout-filled cells, and waterproofing add cost, but they’re not the place to cut corners on anything structural. A cheap unreinforced wall that bows in ten years is the most expensive block you’ll ever buy — the sand and lime in the mortar are cheap, so skimping on them to save a few dollars a bag never made sense to me.
Questions I get asked on site
Is concrete block as strong as poured concrete? For most residential walls, a properly reinforced and grouted block wall performs comparably to poured concrete. The strength comes from the steel and grout inside, not the block alone, so the reinforcement detailing is what matters most.
Why is my block foundation cracking or flaking? Usually water. Below-grade block that absorbs moisture will spall in a freeze-thaw climate — the kind of thing I see a lot in older New England foundations around places like Hamden — and horizontal cracks often signal soil pressure. Get the cause diagnosed before you patch, because sealing over a structural problem just hides it.
How much does it cost to repair a bowing or cracked block foundation? Spot repair and reinforcement can often be done within that same $10 to $20 per square foot range for the affected section, but a full re-do of a badly bowed wall — with excavation, waterproofing, and new reinforced block — runs well past that. I always walk a homeowner through spot repair versus a full re-do before we talk numbers, because the diagnosis decides which one you actually need.
Do all the block cores need to be filled with grout? No. Only the cells with rebar, plus bond beams and any cells an engineer specifies, are typically grouted. Filling every core adds weight and cost without much benefit on a standard wall.
What’s the right repair material for a historic or heritage masonry home? For homes built well before CMU became common, roughly the 1900s through the 1930s, you’re usually dealing with solid brick or stone rather than block, and that changes the mortar mix entirely. Historic masonry needs a softer, more breathable mortar than modern Type S — matching hardness matters as much as matching color when you’re repointing or restoring brick on a house that old.
