Introduction
Concrete masonry units (CMUs) include plain hollow and solid blocks, lightweight units, architectural finishes, lintel and bond-beam shapes, and adhered veneer units. The right choice starts with the job: a small nonstructural garden feature, partition, exposed decorative wall, or veneer can be a reasonable DIY project; a load-bearing, retaining, foundation, tall, wind-exposed, reinforced, or heavily opened wall is not a choose-a-block-and-build project. Related guidance: Revitalize Your Foundation: A Step-by-Step Guide to Repointing Concrete Blocks from Within.
Before buying, confirm the local permit and code requirements, wall purpose, footing or supporting surface, drainage, exposure, openings, and manufacturer data. Appearance and a block’s hollow cores do not establish structural capacity, weather resistance, fire rating, or insulation value. For loadbearing units, ASTM C90 covers hollow and solid CMU but recognizes that properties such as finish, density, strength, fire, thermal, and acoustic performance may need separate specification.
Key takeaways
- Choose CMU by wall function, code, specified performance, exposure, and manufacturer documentation—not by appearance alone.
- Standard hollow CMU is common for small walls and partitions; solid, lightweight, architectural, lintel, and veneer units have more specific uses.
- Hollow cores do not make a wall well insulated. Insulation and moisture control depend on the complete wall assembly.
- Use the specified mortar and follow its product instructions; do not alter packaged mortar with unmeasured sand or water.
- Lay only on a verified, suitable base; keep work level, plumb, aligned, and protected from rain, freezing, and rapid drying.
- Get professional design before building structural, retaining, foundation, reinforced, grouted, tall, or high-exposure masonry.
Table of Contents
- Introduction
- Key takeaways
- Overview of Concrete Masonry Blocks (CMUs) and When to Use Them
- Choosing the Right Block for Your DIY Project
- Mortar and Masonry Mixes — Composition and Best Practices
- Tools, Materials Checklist, and Safety Gear for DIY Masonry
- Step-by-Step DIY Guide: Laying and Repointing Concrete Blocks
- Common Mistakes and How to Avoid Them
- Aftercare, Maintenance, and When to Call a Pro
- Conclusion
- FAQ
Overview of Concrete Masonry Blocks (CMUs) and When to Use Them
CMU is a manufactured concrete unit laid in mortar. Standard units are commonly sold in nominal face sizes near 8 by 16 inches, although widths, shapes, finishes, and specialty units vary. Nominal dimensions include the intended mortar joint; an “8-inch” unit is typically about 7-5/8 inches in actual width. A wythe is one continuous thickness of masonry. Cores are the voids in hollow units, and face shells are the outer webs that receive mortar in ordinary face-shell bedding. See the CMHA guide to CMU shapes, sizes, and specifications when comparing locally available units.
| Block or unit category | Typical DIY-relevant use | Weight and finish | Important selection check | Typical role |
|---|---|---|---|---|
| Standard hollow CMU | Small garden walls, utility walls, partitions, or designed masonry walls | Plain; weight varies by density and width | Verify unit specification, strength, absorption, exposure, and code requirements | Structural or nonstructural, depending on design |
| Solid CMU | Caps, localized bearing, bond courses, and small landscape features | More material and usually heavier than a comparable hollow unit | Being solid does not by itself make it suitable for a structural wall | Structural or decorative, depending on design |
| Lightweight CMU | Partitions and walls where easier handling matters | Reduced unit weight; usually plain | Confirm strength, absorption, fire, sound, and thermal properties | May be loadbearing only when specified |
| Split-face or architectural CMU | Exposed decorative walls, façades, and landscape features | Textured, ground-face, polished, or colored finish; product-dependent weight | Check finish, absorption, freeze-thaw, water-repellent, and installation requirements | Usually architectural; structural use requires a specified product and design |
| Glazed architectural block | Interior partitions and decorative exposed surfaces | Factory-applied smooth, glossy face | Verify approved exterior exposure, UV/color retention, impact, fire, freeze-thaw, and moisture ratings | Generally decorative unless product documentation states otherwise |
| Lintel or bond-beam unit | Openings or reinforced courses where shown on plans | Special shape that accommodates reinforcement and grout | Use only where the design calls for it; support over openings is not optional | Structural specialty unit |
| Concrete brick or adhered masonry unit | Veneer and decorative cladding | Brick- or stone-like appearance; product-dependent | Adhered units need a suitable structural backing and attachment system | Nonstructural decorative finish |
Block Materials and Surface Finishes
Aggregates and pigments affect appearance, while the product specification governs performance. Split-face CMU is not automatically unsuitable in wet areas; its durability depends on the particular unit, mortar-joint workmanship, drainage, movement accommodation, flashing or weeps where the wall assembly needs them, and exposure. A textured face is not a water-management system.
Do not assume hollow CMU supplies adequate insulation. Cores, density, thermal mass, insulated cores, cavity insulation, and continuous insulation all affect the finished assembly. Choose a finish only after confirming that coatings, sealers, water repellents, and cleaners are compatible with the unit and the wall design.
Choosing the Right Block for Your DIY Project
First classify the project: decorative freestanding feature, nonstructural partition, veneer, or structural masonry. Then confirm the base, wall height, soil and drainage conditions, property-line limits, openings, attachments, climate, and local code. A block wall that supports a roof, floor, beam, gate, slope, or retained soil needs project-specific design; so do walls in seismic, hurricane, flood, high-wind, or fire-rated conditions.
Check the manufacturer’s data for specified compressive strength, density classification, absorption, approved exposure, finish, and any integral water-repellent system. Reinforcement, grout, bond beams, lintels, flashing, weeps, control joints, and sealant are not universal add-ons. They are installed only where required by the approved design or the wall assembly. Control-joint layout is especially dependent on wall length, openings, restraint, geometry, reinforcement, and code; do not apply one fixed spacing rule to every wall.
Matching Block to Climate and Exposure
For freeze-thaw, coastal, wet, or highly exposed work, select units and finishes approved for that exposure and follow the required detailing. Water resistance comes from the complete assembly: sound joints, compatible materials, drainage where needed, flashing and weeps in drained veneer or cavity systems, movement joints, and a correctly prepared base. Do not try to make a wall “more waterproof” by arbitrarily reducing mortar water.
For exterior walls, protect tops, unfinished openings, and drainage paths from water entry. Use sealant at designed movement joints, penetrations, openings, and dissimilar-material interfaces—not as a substitute for correct wall design.
Estimating Units and Handling
Calculate gross wall area, subtract openings, then divide the net area by the exposed face area of the selected unit. For nominal 8-by-16-inch-face CMU, that is about 112 units per 100 square feet before waste; add a waste allowance and count corner, end, cap, lintel, bond-beam, and closure units separately. More openings, returns, cuts, and nonmodular dimensions mean more waste. Mortar yield varies with unit configuration, joint size, bedding method, product, and site loss, so use the bag yield or supplier estimate for the actual units.
Confirm unit weight before delivery and plan a safe lift, two-person handling, or mechanical assistance for heavy units. Keep stacks stable and do not overload a scaffold or platform. OSHA identifies falls, falling objects, instability, electrocution, and overloading as scaffold hazards; use a platform only when it is correctly selected, erected, accessed, and loaded under applicable requirements in the OSHA scaffold safety guidance.
Mortar and Masonry Mixes — Composition and Best Practices
Mortar selection is a specification issue, not a universal recipe. Types N, S, M, and O have different intended applications, and a project may use a proportion or property specification. Select the specified mortar for the wall, exposure, units, and design. For ordinary new CMU work, a generic primer or bonding agent between clean units and mortar is not normally part of the process; such products can apply to repairs, adhered veneer, surface-bonded systems, or specialty substrates.
How to Mix Mortar Correctly (Ratios & Consistency)
Use a preblended mortar exactly as its label and data sheet direct. For example, QUIKRETE Mortar Mix instructions identify a preblended product designed to meet ASTM C270 Type N and direct the user to add water; site-batched mortar follows its own specified proportions. Do not treat a generic ratio as interchangeable with every product.
Measure the water, mix thoroughly, and use mortar while it remains workable under the product instructions. The mortar should spread and support a unit without running out of the joint. Adding unmeasured sand or water changes proportions and can affect workability, bond, water retention, air content, and strength. Do not keep retempering or reformulating a batch to rescue it; discard material that has exceeded the permitted working time.
Additives, Bonding Agents, and Color Mortars
Use additives, pigments, integral water repellents, and bonding products only when they are compatible with the mortar, unit, exposure, and specified system. Test a small, inconspicuous sample of color mortar or cleaning method when appearance matters. This is different from repointing: repair work begins by removing unsound joint material, cleaning the joint, and following the repair product’s preparation instructions.
Tools, Materials Checklist, and Safety Gear for DIY Masonry
For a small, nonstructural CMU project, gather tools and materials before mixing mortar. Read the unit, mortar, cutting-tool, flashing, grout, reinforcement, and sealant instructions that apply to your job. Related guidance: Mixing Small Batches of Mortar with Consistent Water Content.
Essential Hand Tools and Power Tools
- Layout tools: tape measure, square, chalk line, string line, line blocks, straightedge, and level.
- Masonry tools: trowel, mortarboard or tub, jointer, mason’s hammer, cold chisel, brush, and buckets.
- Cutting tools: a masonry saw or angle grinder with the correct diamond blade; use the manufacturer’s guards and dust-control method.
- Mixing equipment: clean mixing tub, hoe, or appropriately sized mortar mixer.
- Materials: specified CMU and specialty shapes, specified mortar, clean water, and only the flashing, weeps, reinforcement, grout, and sealant shown or required for the wall.
Safety and Site Prep
- Wear safety glasses, gloves, work boots, and hearing protection when cutting or mixing.
- Control cutting dust with the tool’s approved wet-cutting or vacuum method and use suitable respiratory protection when dust cannot be adequately controlled.
- Keep bagged materials dry and off the ground. Store units on stable ground and keep walkways clear.
- Do not work from an unstable ladder or overloaded platform. Protect people below from falling tools and units.
- Check temperature, wind, and precipitation before starting. Do not lay on snow- or ice-covered supporting surfaces.
Step-by-Step DIY Guide: Laying and Repointing Concrete Blocks
This procedure is for a small, nonstructural new CMU wall on a verified base. It is not a design for a foundation, retaining, load-bearing, reinforced, grouted, or tall freestanding wall. Repointing is a separate repair task: it does not involve laying new units, and it requires removal of failed joint material and compatible repair methods.
Step-by-Step Process
- Confirm the scope. Verify permits, property limits, base condition, drainage, wall dimensions, unit layout, and the specified materials. Stop for professional design if the wall carries load, retains soil, has significant openings, or needs reinforcement, grout, a lintel, or a bond beam.
- Prepare and lay out. Clean the supporting surface and verify it is sound, level, and appropriate for the wall. Mark corners, openings, and elevations. Dry-lay or calculate the first course so cuts and specialty units are known before mixing.
- Set corners and leads. Establish a taut string line. Spread mortar on the bed and required head joints, set corner units first, and check level, alignment, and plumb. Build short leads only as your skill and layout allow.
- Lay each course. Butter and bed units as required by the selected system, seat them without knocking the line out of position, maintain the specified joint thickness, and check level, plumb, and bond pattern frequently. Keep cores, reinforcement, and grout locations exactly as designed.
- Finish and protect. Tool joints when the mortar has stiffened enough to take the finish without smearing. Brush off loose droppings before they harden. Protect fresh work from rain washout, freezing, excessive wind, rapid drying, and impact according to the mortar product and conditions.
Preparing Foundations, Layout, and First Course
A straight, durable wall begins with a suitable base, not with shims under a weak or out-of-level footing. Do not build over an unverified slab, unstable soil, snow, or ice. Establish the wall line and corner elevations before mixing; a mistake in the first course repeats through every course above it.
Laying Course-by-Course and Finishing Joints
Use the line as a guide, not an excuse to force mis-sized units into place. Check each course for level, plumb, straightness, and consistent joints. A unit that will not seat correctly should be lifted, corrected, and reset rather than hammered into alignment. Install control joints, flashing, weeps, reinforcement, grout, lintels, and sealant only where the design requires them.
Normal masonry conditions are often around 40–90°F, but product instructions control. Below 40°F, special cold-weather procedures may be needed; fresh masonry must be protected from rain, snow, and freezing. In heavy rain, stop before plastic mortar washes from the joints. Secure protective covers so they shed weather without rubbing, staining, or trapping damaging moisture against the wall. Related guidance: Transform Your Space: Coloring Concrete Mortar Joints with Masonry Dyes.
Common Mistakes and How to Avoid Them
Most failed DIY blockwork traces back to an unsuitable base, poor layout, wrong materials, inconsistent joints, or weather exposure while mortar is fresh. Correct a problem as you build; do not expect coating, caulk, or repointing to turn an improperly built wall into a sound one.
Mistakes During Mixing and Application
- Using a universal recipe: Choose the specified mortar and follow its instructions instead of applying one ratio to every project.
- Altering packaged mortar: Do not add unmeasured sand or water to make a mix “stronger,” “more waterproof,” or easier to spread.
- Inconsistent bedding or joints: Spread mortar evenly, fully seat units, and tool joints consistently. Correct voids and misalignment immediately.
- Trying to lay new CMU like a repair: New work normally does not need a generic primer or bonding agent; repointing has separate surface-preparation requirements.
Structural and Weatherproofing Errors
- Choosing by appearance alone: Verify the unit’s specification and approved exposure, then design the wall around its intended use.
- Assuming hollow cores provide insulation: Plan insulation as part of the full wall assembly.
- Adding reinforcement, weeps, or flashing indiscriminately: These components are design-dependent. Omission can be serious where they are required, but unnecessary or misplaced components can also create problems.
- Ignoring openings and movement: Openings may require lintels, and control joints must be located for the actual wall design rather than copied from a generic rule.
- Working through bad weather: Rain can wash out plastic mortar, and freezing can damage fresh work. Pause and protect the work when conditions require it.
Aftercare, Maintenance, and When to Call a Pro
Let mortar cure and protect it for the period and conditions stated by the mortar manufacturer. Remove fresh droppings carefully before they harden; do not spread cement paste across an architectural face. Before applying a sealer, water repellent, coating, or cleaning chemical, verify compatibility with the specific CMU, mortar, and wall assembly.
Maintenance Schedule and Visual Checkpoints
- After severe weather and seasonally: inspect for new cracks, spalling, displacement, leaning, open joints, efflorescence, and water entry.
- Keep drainage working: clear designed weeps and drainage paths, and keep soil, mulch, or paving from bridging details intended to drain.
- Maintain interfaces: inspect sealant at designed movement joints, penetrations, and dissimilar materials; replace it only with a compatible product when it fails.
- Investigate recurring moisture: repeated efflorescence, staining, or joint deterioration signals a water-management problem, not simply a need for more sealer.
When to Call a Pro
Stop and consult a qualified designer, engineer, architect, or experienced masonry contractor before proceeding with load-bearing walls, retaining walls, foundations or basement walls, tall freestanding or wind-exposed walls, walls supporting roofs, floors, beams, gates, or heavy attachments, significant openings, reinforced or grouted masonry, bond beams, lintels, complex connections, or seismic, hurricane, flood, fire-rated, and property-line conditions. Also call a professional for leaning, settlement, widening cracks, major spalling, or persistent water entry.
Costs vary too much by location, date, product, delivery, access, permits, engineering, and wall complexity to make generic price figures useful. Request local material quotes and, for any project beyond a small nonstructural wall, compare them with an itemized professional estimate.
Conclusion
For a small DIY wall or decorative feature, select CMU by the job it must do, not by texture, weight, or price alone. Verify the unit and mortar requirements, start on a suitable base, lay a careful first course, inspect every course for line, level, plumb, and sound joints, and protect fresh mortar from damaging weather.
When the wall must carry load, retain soil, resist major wind or seismic forces, span openings, or use reinforcement and grout, the critical decision is to obtain a project-specific design before buying materials.
FAQ
What are common concrete masonry block types?
Common categories include standard hollow CMU, solid CMU, lightweight CMU, split-face and other architectural CMU, glazed units, lintel and bond-beam shapes, and adhered veneer units. Their appearance does not determine whether they are suitable for structural use.
How do I choose the right block for a DIY project?
Start with the wall’s purpose, local code, base, height, exposure, openings, drainage, and manufacturer documentation. For a small nonstructural feature, choose units approved for the intended exposure and finish. Obtain professional design for structural, retaining, foundation, tall, reinforced, grouted, or heavily opened walls.
What mistakes should I avoid when laying blocks?
Avoid building on an unsuitable base, using the wrong or modified mortar, skipping layout, letting joints vary, assuming every wall needs the same reinforcement or drainage details, and working through rain or freezing conditions without the required protection.
What basic tools do I need for block work?
You need layout tools, a level, straightedge, trowel, mortar tub or board, jointer, mason’s hammer or chisel, brush, buckets, and suitable cutting equipment. Use eye, hand, hearing, foot, dust, and respiratory protection appropriate to the cutting and mixing methods.