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.
