How to lay and grout a CMU block wall: a field execution guide

What this guide covers, and what the overview owns

This guide is about the field execution of a concrete masonry unit (CMU) block wall: how a mason lays it, reinforces it, and grouts it so the finished wall is what the engineer drew. A block wall is a system of four parts that have to work together. The units. The mortar that bonds them in the joints. The grout that fills the reinforced cells. The steel the grout encases. Get the four working as one and the wall stands. Treat them as separate trades and the wall cracks, leaks, or fails the section it was supposed to carry.

There is a companion masonry construction overview on this site, and the split between the two is deliberate. The overview owns the material-by-material comparison of brick, block, and stone; the choice between a single-wythe, cavity, or veneer wall; the water-management story of flashing, weeps, and drainage; the difference between a control joint and an expansion joint; cold-weather and hot-weather masonry; efflorescence; and the generic special-inspection narrative. Read it for any of those. This guide does not repeat them.

What this guide does cover is the part a mason lives in. The 8 in module and the dimension math. Coursing, story poles, and bond pattern. The footing and dowels that start the wall. The hard line between mortar and grout. Grouting and consolidating the cells. Placing the steel. The laying sequence and tooling. Bracing the wall before it can stand on its own. Estimating the block and the grout. And the fire, sound, and structural ratings the assembly delivers.

What is a CMU?

A CMU is a precast concrete block, most commonly the unit nominally 8 in high, 8 in deep, and 16 in long, with hollow cores called cells separated by the cross walls called webs and bounded by the outer face shells. People call it a CMU, a concrete block, a cinder block, or just block. The cells are what make a block wall different from any other wall: they are the channels that take the rebar and the grout that turn hollow masonry into a structural section.

Units split a few ways that change the order and the wall. Hollow versus solid: a hollow unit has open cells for grout and rebar and is most of what gets laid, while a solid unit is mostly massed concrete for courses that need the bearing or the fire mass. Lightweight versus normalweight: the lightweight block uses lighter aggregate, lays easier on the mason, and shifts the fire and sound ratings for the same wall thickness. The unit standard, ASTM C90 for loadbearing concrete masonry units, sets the dimensions, the minimum face-shell and web thickness, and the strength.

The width of the block, the 6 in, 8 in, 10 in, or 12 in face dimension, is the number the engineer cares about most, because it sets the wall thickness, the size of the cell available for grout and bar, and a large part of the wall's strength and fire rating.

Nominal versus actual: the 3/8 in joint math

The nominal 8 by 8 by 16 is not the size you measure with a tape. The actual block is 7 5/8 in by 7 5/8 in by 15 5/8 in, made 3/8 in short in every direction on purpose, so that the unit plus one 3/8 in mortar joint adds back up to the clean 8 in and 16 in module. Lay the actual 15 5/8 in block with a 3/8 in head joint and you get 16 in face to face. Stack the actual 7 5/8 in unit on a 3/8 in bed joint and you get 8 in of course height. The module only works if the joint is held at 3/8 in.

This is the math that runs the whole wall, so carry it. One course is 8 in. Three courses is 24 in, or 2 ft. Two units laid end to end is 32 in. A wall called out to a multiple of 8 in tall and 16 in long uses whole and half block and almost no cutting. The undersized unit and the 3/8 in joint are a system, and a mason who lets the joint creep to 1/2 in to chase a high spot has thrown the coursing off and will pay for it at the top of the wall.

The blunt version: the 3/8 in joint is a dimension, not a feeling. Tool it fat to make a course come out and the next opening lands off the block. Hold it and the wall lays itself out.

The block shapes you actually stack

A block wall is not one shape repeated. The field unit is the stretcher, the standard block with flanged ends that meet in the head joints. But a real wall needs a handful of special shapes, and running short of one is what stops a crew mid-course. Order the shapes off the elevation at the same time you order the field block, because the corners, bond beams, and lintels are exactly the units a wall sits and waits on when they are not on the first delivery.

The common ones are below. Count each off the drawings, not from a guess, and add the half blocks the bond pattern needs at every opening and corner.

Coursing, story poles, and the dry layout

Coursing is the vertical and horizontal math of the wall: the unit plus its joint, repeated, landing on the openings, the bond beams, the bearing points, and the top of the wall without a sliver course or a fat joint. A mason works coursing off a story pole, a marked stick that lays out every course height up the wall so each lead gets built to the same marks. Mark the pole once to the actual coursing, 8 in a course for standard block, and every corner comes up level with every other corner.

The horizontal layout matters as much as the vertical, and it gets decided before the first block. Lay the first course out dry, no mortar, across the footing to confirm the openings and corners fall on the module. This is where you find that the opening called 3 ft 10 in instead of 4 ft 0 in puts a cut block on each jamb for the full height of the opening, on every course, with a head joint that never works out. That cut costs labor and looks rough for the life of the building.

The dry run is fifteen minutes that saves a day of cutting. Snap the wall lines, set the dry block to the module, mark the corners and the jambs, and confirm the math closes before any mortar goes down. The wall that was laid out dry comes up clean. The wall that was started wet and figured out as it went is the one full of cuts.

Running bond, and why stack bond needs more steel

Running bond, where each course is offset half a block from the course below, is the standard pattern for structural masonry, and the offset is what ties the wall together along its length. The code treats a wall as running bond when the units overlap by at least one quarter of the unit length. Less overlap than that, block stacked so the head joints line up in a continuous vertical line, is stack bond.

Stack bond looks clean and modern, and it is weaker for a clear reason. The stacked head joints make a built-in vertical plane of weakness, and the units do not interlock to carry load and resist cracking across the joints. Testing has put the out-of-plane flexural capacity of an otherwise-equal stack-bond wall on the order of 8 to 11 percent below the running-bond version, and for unreinforced masonry not laid in running bond the code takes the flexural tension across that continuous joint as essentially zero.

You can build stack bond and make it work, but you pay for the pattern with steel. TMS 402 requires a minimum horizontal reinforcement for masonry not laid in running bond, commonly cited near a ratio of 0.00028 times the gross vertical cross-section, to carry the continuity the bond pattern gave up. Choose stack bond for the look and the engineer designs the reinforcing to replace what running bond would have done for free. Lay stack bond on a wall drawn for running bond and you have quietly removed strength nobody added back.

The footing and the dowels that start the wall

A block wall starts on a concrete footing, and a reinforced wall starts on dowels cast into that footing to match the vertical bars above. The footing carries the wall load to the soil and gives the first course a level, true bearing surface. Get the footing out of level and every course above inherits the error, because the first course takes the correction and the coursing fights it the rest of the way up. Footing design and sizing is its own topic and the project drawings and the engineer set it.

The dowels are the connection between the footing and the wall, and they are set when the footing is poured, long before the block goes up. They have to come up in the right cells, at the spacing of the vertical bars, with enough length above the footing to lap the bars above. The classic field failure is dowels that miss the cells, set off the wall layout or knocked out of plumb during the footing pour, so the vertical bar cannot drop down the cell to lap them. Then someone bends the dowel sideways to chase the cell, and a sharp cold bend in a dowel is not the splice the engineer drew.

Set the dowels to the wall layout, not the footing form, and brace them so the concrete pour does not move them. Confirm they land in cells while the footing is still plastic and a tap still fixes it. A dowel found wrong after the footing has set is a repair detail, not a hammer tap.

What is the difference between mortar and grout in a block wall?

Mortar and grout are two different materials that do two different jobs, and confusing them is the single most common conceptual error on a block wall. Mortar bonds the units together in the thin 3/8 in joints. Grout fills the cells, solid, to encase the reinforcing steel and add mass and strength. They are not interchangeable, and no building code lets you fill cells with mortar in place of grout. The mortar type, M, S, N, or O against ASTM C270, is picked by the project specification for the exposure and the loading, and the overview guide carries the type-by-type table; this guide cares that you keep mortar and grout straight.

The difference you can see is water. Mortar is stiff, troweled, and stays where you put it on the unit, with a low enough water content to hold a furrow and carry a block without squeezing flat. Grout is fluid, with a high slump, made to be poured or pumped so it flows around the rebar and into every corner of the cell. ASTM C476 governs grout and a typical grout slump runs 8 to 11 in, which would be soup as mortar and is exactly right as grout.

Try to fill a cell with stiff mortar and it bridges and leaves voids around the bar, and the reinforced section the engineer designed is not the wall you built. Try to lay block with grout-wet mortar and the units float and the joints fail. Keep the two materials, and the two jobs, separate in your head and on the ground.

Grouting and consolidating the cells

Grout is a fluid mix of cement, sand, sometimes pea gravel, and enough water to flow, placed into the cells to encase the steel and turn hollow masonry into a solid reinforced section. It is closer to a high-slump concrete than to mortar. ASTM C476 sets the requirements, and grout commonly carries a slump of 8 to 11 in and a strength on the order of 2000 psi or the value the spec names, whichever governs. The high slump is intentional. Grout has to travel down a narrow cell, past the bar and the webs, and fill every corner, and a stiff grout will not.

Grouting comes in two methods. Low-lift grouting builds the wall in shorter increments, commonly to about 4 ft or 5 ft, grouts that lift, then builds and grouts the next. High-lift grouting lays the full wall, or a tall section to story height, then grouts it from the bottom in pours and lifts. High-lift is faster on a tall wall but demands cleanouts and careful consolidation, and the maximum pour heights and lift limits come from TMS 602, not from how fast the crew wants to move.

Consolidation is the step that makes or breaks the grout, and it is the one rookies skip because the cell looks full from the top. Freshly placed grout has to be worked to drive out the voids: puddled with a rod for short lifts, commonly up to about 12 in, and mechanically vibrated for taller lifts, with reconsolidation after the grout has given up some of its water and settled. Skip it and a void hides around the bar where you cannot see it, and the bar the wall depends on is sitting in air instead of grout.

Cleanouts are the other piece of high-lift work. When the grout pour gets tall, commonly cited over about 5 ft 4 in per TMS 602, openings are cut at the base of the grouted cells so mortar droppings can be cleaned out and an inspector can confirm the grout reaches the bottom. The crew that grouts a tall wall with no cleanouts is betting the cell is clean and the grout got down, with no way to look and no way to fix it after.

Do you fill every cell in a CMU wall?

No. Most CMU walls are partially grouted, meaning only the cells that hold a vertical bar, plus the bond-beam courses, get filled, and the rest of the cells stay hollow. The drawings show which cells: the reinforced cells at the bar spacing, the cells at jambs and corners, the cells under bearing points, and the bond-beam courses. A fully grouted wall, every cell solid, is built where the design calls for the mass or the strength or the fire rating, common on foundation walls, retaining walls, shear walls, and high-seismic work.

Reading the grouting off the drawings is what keeps the wall right and the estimate honest. Grout every cell when the design only wanted the reinforced ones and you have poured money and weight the wall did not need. Grout only some of the reinforced cells, or miss a bond-beam course, and you have built a weaker wall than the one on paper. The grouting pattern is a structural decision the engineer made, so build the cells the drawings show solid, and confirm any change with the engineer rather than the truck driver waiting to pour.

Placing the vertical and horizontal steel

You reinforce a block wall with vertical bars set in grouted cells, horizontal steel laid in bond beams and bed joints, or both, in the size and spacing the structural drawings call for. This guide is about getting that steel placed and held; the bar sizes, cover, and lap theory are the same discipline carried in the rebar placement and cover guide. The field job is to put the right bar in the right cell, lapped, centered, and held there until the grout sets it.

Vertical reinforcing is the workhorse. A bar runs up a cell, lapped to the dowel from the footing the length the drawings call out, and that cell is grouted solid so bar and masonry act together. Spacing might be every cell, every other cell, or wider, and at jambs and corners the bars tighten up. The bar has to stay where the drawing places it through the pour, not get shoved against a face shell by the grout, which is why positioners and ties matter and why a loose bar floating in a cell is a defect even before the grout hides it.

Horizontal reinforcing comes two ways and they are not interchangeable. Joint reinforcement, the ladder-type or truss-type welded wire laid in the bed joints commonly every other course or as the spec sets, ties the wall across head joints and helps control cracking. Bond-beam steel, deformed bars laid in the channel of bond-beam units and grouted, gives the wall a continuous horizontal structural member. Build the bond-beam course by knocking out or using the cast-down web, laying the bar continuous through the course with its lap, and grouting it solid. All of this gets inspected before the grout buries it, because once grouted you cannot see the steel or fix it.

How do you lay a block wall?

You lay a block wall by building the corners or leads first, running a line between them, and filling the courses to the line, level and plumb, on the specified bond and mortar. The first course is the one that matters most. It goes down in a full mortar bed on a clean footing, leveled and aligned dead-on, because every course above it takes its line from it. A first course set a quarter inch out of line is a wall a quarter inch out of line all the way up.

The leads, the stepped-up corners built first, set the height and the plumb. A taut mason's line stretched lead to lead gives every block in the course its top and its face. The mason butters the head joints, sets the block to the line with a slight tap, and checks level and plumb as the course runs. The joint stays at 3/8 in by the module, the line, and the eye, course after course, so the wall holds its dimension.

Bedding is where face-shell and full bedding part ways. Bed the block on full mortar on the face shells for an ordinary course, and on the webs too where the cell below will be grouted, so the grout does not leak into the cell beneath it. Strike the excess and tool the joint while the mortar is thumbprint hard. The rhythm is the craft. A mason who keeps the line, the level, and the joint consistent builds a wall that is plumb and tight, and one who chases the line builds a wall that waves and shows it in every raking light for thirty years.

Tooling the joints for a weather-tight wall

The mortar joint is held at 3/8 in and tooled to a profile that sheds water, most often the concave joint for any wall exposed to weather. The joint is not just the gap between blocks. It is the bond, and the way it is finished decides whether water runs off the face or soaks into it. Tooling does two things at once: it compresses the mortar against the units, tightening the bond and closing the surface, and it shapes the joint.

The concave and the V joints are the tooled, weather-resistant profiles. They press the mortar in and shed water, and they are what most exterior block calls for. Raked, struck, and flush joints are flatter or recessed, hold water or shade lines, and get chosen for appearance more than weather. On an exterior structural wall, the concave joint is the default for a reason.

Timing is the whole trick. Tool the joint when the mortar is thumbprint hard, firm enough to hold the shape and still dark with moisture, not so wet it smears and not so dry it crumbles and will not compress. Tool it too early and the joint is weak and stays wet-looking. Tool it too late and you burnish a hard, light-colored line that never bonded. The window is short, and a good mason times the tooling to the set, wall by wall, weather by weather.

How do you brace a block wall during construction?

A freshly laid block wall has almost no strength until the mortar cures and the cells are grouted, and an ungrouted wall in the wind is one of the real hazards on a masonry job. The mortar carries the units, and that is it. There is no grout and no continuous steel yet to take a lateral load, so a tall, freestanding green wall can come down in a gust that a finished wall would shrug off. Block walls have killed people this way, which is why bracing during construction is taken seriously and governed by safety regulation, not left to the crew's read of the weather.

The practice is to brace the wall against wind from the moment it is tall enough to need it until it is grouted, cured, and connected to the permanent structure that braces it for good. The bracing is engineered: temporary braces, their spacing, and the wind speed they are designed for come from a bracing plan, and the masonry industry publishes a standard practice for bracing masonry walls during construction that sets restricted and evacuation zones tied to the wind speed. When the wind hits the limit, the crew clears the zone behind the wall rather than working under it.

This is firmly engineer and safety territory, not a rule of thumb. Confirm the bracing design and the wind thresholds against the project's bracing plan, the masonry bracing standard, and the applicable OSHA requirements before the wall goes up, and have the plan in hand before the wall is tall enough to fall. The blunt version: an ungrouted block wall is temporary work until it is braced or grouted, and treating it as finished because it looks finished is how the wall ends up in the dirt or on someone.

How many blocks do I need per square foot?

For standard 8 by 16 face block, figure about 1.125 blocks per square foot of wall, before openings and waste, which is the number every estimator carries. So a 10 ft by 100 ft wall, 1000 sf, runs about 1125 blocks gross, less the openings, plus a waste allowance for breakage and cuts. The 1.125 comes straight out of the module: a 16 in by 8 in face is 128 square inches, and 144 square inches in a square foot divided by 128 is 1.125.

Mortar and grout come off the same takeoff but behave differently. Mortar runs roughly on the order of a few bags of masonry cement per 100 block, which varies with the joint thickness, the unit, and whether you mix masonry cement or portland and lime, so use the supplier's yield for the actual mortar and joint. Grout is by volume: a fully grouted 8 in wall takes a bit over 2 cubic yards per 100 sf of wall, and a partially grouted wall scales down by how many cells get filled, counted off the bar spacing on the drawings.

The number that wrecks an estimate is the grout on a partially grouted wall. Price every cell grouted and you bid high; price none and you miss the reinforced cells entirely. Count the grouted cells from the reinforcing layout, not from the gross wall area, and the takeoff lands where it should. These are field rules of thumb for a first pass; the real quantities come off the drawings and the supplier yields.

Fire, sound, and structural ratings of CMU assemblies

Concrete masonry is used heavily for fire walls, sound walls, and shear walls because the mass of the wall does all three jobs well, and every one of those ratings belongs to the assembly, not to the block alone. The fire-resistance rating of a CMU wall is set by its equivalent thickness, the amount of solid material in the wall accounting for the hollow cells, and by the aggregate in the units. Grout the cells solid and the equivalent thickness goes up, so a grouted wall reaches a higher fire rating at the same nominal thickness than a hollow one. A relatively modest grouted CMU wall can reach multi-hour ratings, which is why block is the default for fire separations in commercial and industrial buildings.

Sound is the same mass story. The sound transmission class of a CMU wall climbs with the weight per area, so denser units and grouted cells block more sound, and a masonry demising wall can reach an STC well up into the 50s and beyond when grouted. That is why block is the common pick where a code-required sound rating separates occupancies. Use the tested or calculated rating for the specific assembly rather than assuming a thickness buys a number.

The structural rating is where the steel and grout earn their place. In seismic regions and high-wind zones, block walls are built as fully grouted, reinforced masonry, because the bars and grout are what let the wall take the reversing in-plane and out-of-plane loads an earthquake or a hurricane throws at it. Unreinforced masonry has almost no capacity against those forces. The bar sizing, the spacing, the grouting extent, and the diaphragm connections all step up sharply with the seismic design category and the wind exposure, and they are governed by TMS 402 and the adopted building code. This is engineer-of-record territory: build exactly what the structural drawings show, because a reinforced wall built with missed grout or short laps is an unreinforced wall wearing the drawings of a reinforced one.

The grout pour is the hold point

On a block wall the grout pour is the point of no return, and the field QC has to treat it that way. The visible checks happen as the wall goes up: plumb, level, alignment, joints consistent at 3/8 in, faces in plane. A wall that waves or leans was not checked against the line and the level as it was built, and there is no fixing it after. But the checks that matter most are the hidden ones, and the grout buries them for good.

Before the grout goes in, the reinforcing gets verified: bar size, spacing, position in the cell, cover, and lap to the dowels. The cells get confirmed clean of mortar droppings, through the cleanouts on a high-lift wall. During the pour, the grout gets confirmed reaching the bottom and being consolidated and reconsolidated. After, grout and mortar samples may be taken for strength. The overview guide carries the generic special-inspection narrative; the point here is narrower and blunter. Once the cells are full, the inspection is over whether anyone looked or not, so the steel and the cells are inspected before the truck pours, not after. Structural masonry usually carries special inspection at a level the design and the adopted code set, and the grout pour is the hold point that inspection exists for.

What to document

A block wall hides almost everything that matters inside grouted cells and behind finished joints, so the record is what proves the wall that got covered was the wall that was designed. The grout pour seals the steel away. The document is the only thing left to check it against when a question comes up two years later.

Capture the unit type and strength, the mortar type and grout class against the spec, the reinforcing as placed before grouting with sizes, spacing, and laps, the cells grouted and the lift method, the cleanout and consolidation on high-lift work, and who inspected each hold point. Photograph the steel and the clean cells before the grout, tagged to the wall and the elevation, so the record is built as the wall is built rather than reconstructed from memory. A field tool like FieldOS makes that stick by tying the photo and the sign-off to the exact wall before the pour covers it.

Common mistakes

• Confusing mortar and grout, or trying to fill cells with stiff mortar instead of fluid grout.
• Leaving reinforced cells with voids around the bar because the grout was never consolidated.
• Letting the 3/8 in joint creep to chase a course, throwing the coursing off at the top of the wall.
• Starting the wall wet with no dry layout, so every opening lands off the module and full of cut block.
• Vertical bars not lapped to the dowels, shoved against a face shell, or short on lap length.
• Grouting every cell on a partially grouted design, or missing reinforced cells and bond-beam courses.
• Skipping cleanouts on a high-lift pour, so a dirty cell and a short pour cannot be seen or fixed.
• Leaving a tall green wall unbraced in the wind before it is grouted, cured, and tied to the structure.
• Laying stack bond where the design assumed running bond, without the extra horizontal steel it requires.

Field checklist

Standards and references

The construction of structural masonry in the United States runs on TMS 602, the Specification for Masonry Structures, paired with its design code TMS 402, both published by The Masonry Society and adopted through the building code. For the work in this guide, TMS 602 is the document that matters most. It sets the grout placement, the maximum pour and lift heights, the cleanout requirements, the consolidation, and the inspection. TMS 402 is the design side, the strengths, the reinforcement rules, and the bond-pattern and seismic provisions the drawings are built from. The adopted edition and local amendments control.

The materials carry their own ASTM specifications. ASTM C90 is the standard for loadbearing concrete masonry units. ASTM C270 is the specification for mortar, where the M, S, N, and O types live. ASTM C476 is the specification for grout for masonry, the cell fill, where the high slump comes from. For bracing the wall during construction, the masonry industry publishes a standard practice for bracing masonry walls during construction, and the applicable OSHA requirements apply on top of it.

Two cautions. The grout slump and lift heights, the cleanout trigger, the block-per-square-foot and grout yields, and the seismic detailing read as fixed numbers but are set by the specification, the adopted code edition, the supplier yields, and the engineer, so confirm them against the project documents rather than memory. And the special-inspection level for structural masonry is set by the code and the design, so verify what the job requires with the building official before treating any of it as optional.

Units, terms, and conversions

Concrete masonry has its own vocabulary, and the same wall reads differently across a drawing set, a unit spec, and a supplier sheet, so the execution terms are worth pinning down. The block is sized nominal, including the 3/8 in joint, while the unit is actually 3/8 in smaller in each direction. Width is named by the nominal face dimension: 6 in, 8 in, 10 in, 12 in. Mortar is specified by ASTM C270, grout by ASTM C476, the units by ASTM C90.

The terms below are the ones that decide whether the wall is built the way it was drawn.

CMU

Concrete masonry unit, the precast concrete block; nominal 8 by 8 by 16, actual 7 5/8 by 7 5/8 by 15 5/8

Cell and web

Cell is the hollow core that takes grout and rebar; web is the cross wall between cells

Face-shell bedding

Mortar on the outer face shells only; full bedding adds mortar on the webs where the cell is grouted

Story pole

A marked stick laid out to the coursing so every lead is built to the same course heights

Low-lift vs high-lift grouting

Grouting the wall in short increments as it rises, versus laying it tall and grouting from the bottom in lifts with cleanouts

Cleanout

An opening cut at the base of a grouted cell to clear droppings and inspect the grout on a high-lift pour

Bond beam

A horizontal grouted, reinforced course built in channel units, continuous through the wall

Running vs stack bond

Half-block offset that interlocks the wall, versus aligned head joints that need added horizontal steel

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