How to Build a Concrete Block Wall: Step-by-Step Guide

June 1, 2026

Before You Start

Before laying a single block, estimate your full material list. Use the Concrete Block Calculator to get block count, mortar bags, and courses for your wall dimensions. Then gather tools and materials:

Tools needed:

Mason’s line and line pins
Level (4-foot minimum)
Rubber mallet
Mason’s trowel (brick trowel)
Jointing tool (for finishing mortar joints)
Tape measure
Angle grinder or masonry saw (for cut blocks)
Chalk line

Materials:

CMU blocks (see CMU Sizes and Coverage Reference for size selection)
80 lb pre-mixed mortar bags, or masonry cement + sand
Water
Rebar and wire ties (for reinforced walls)
Bond beam blocks (for top course and over openings)

Step 1 — Prepare the Footing

A concrete block wall must rest on a concrete footing — not on soil, gravel, or slab alone.

Footing requirements:

Width: minimum 2× the wall thickness (8-inch wall → 16-inch footing minimum)
Depth: below the frost line for your climate (check local building code)
Thickness: minimum 8 inches
Footing must be level across its full length — use a long level and check both directions

If adding vertical rebar (structural or retaining walls), set rebar stubs in the wet footing concrete at the correct spacing before it cures. Rebar should extend at least 18 inches above the footing to lap with the wall rebar.

Let the footing cure fully before starting — minimum 3 days, 7 days preferred. Never lay blocks on green concrete; the footing will flex and crack your first mortar joints.

Step 2 — Dry-Lay the First Course (No Mortar)

Before mortaring, dry-lay the first full course without mortar. This lets you:

Verify block count and spacing
Plan where cut blocks fall (aim for cuts at openings, not mid-wall)
Locate corner positions exactly
Mark the footing with a chalk line for reference

Adjust block positions so cut pieces are at least half a block length. Avoid cuts smaller than 1/3 block — they are structurally weak and visually poor.

Step 3 — Mix Mortar

Pre-mixed 80 lb bags: add water per bag instructions — typically 5–6 pints per bag. Mortar should hold its shape when squeezed in your hand but not drip. Consistency is similar to peanut butter.

Type S mortar (higher strength, for structural and below-grade): 1 part Portland cement : 0.5 part lime : 4.5 parts sand. Use Type S for all exterior, loadbearing, or below-grade work.

Type N mortar: 1 part masonry cement : 3 parts sand. Suitable for above-grade non-loadbearing walls.

Mix only what you can use in 30–45 minutes. Mortar that stiffens cannot be re-tempered with water — discard it.

Step 4 — Lay the First Course

The first course is the most important — every subsequent course depends on it being level and plumb.

Snap chalk lines on the footing marking the inside and outside faces of the wall.
Set corner blocks first. Butter the footing with a full bed joint of mortar (3/4 inch thick). Set one corner block at each end. Check level in both directions, plumb on both faces. Tap gently with the rubber mallet to adjust.
Run mason’s line between corner blocks, aligned with the top outside edge of the blocks.
Lay blocks between corners, working from both corners toward the middle. Butter the bottom (bed joint) and one end (head joint) of each block before setting.
Check level constantly. Every third or fourth block, check level, plumb, and alignment to the line.
Closure block: Fill the last gap in the middle. Butter both ends of the closure block and both ends of the adjacent blocks. Slide it in straight — do not tilt.

Step 5 — Build Corners (Leads)

In professional masonry, corners are built first as “leads” — stepped pyramids of blocks — and then the wall is filled between them. This gives accurate alignment for every course.

Building a lead:

Lay first-course corner block (already done in Step 4)
Lay second-course corner block perpendicular — it overlaps the first at 90°
Continue: each course adds one block, stepping back by half a block
A 6-course lead steps back 3 blocks from corner to wall end

Check each course of the lead:

Level across the top
Plumb on all faces
Course height (use a story pole or measure: each course = 8.375 inches with 3/8” joint)

With leads at both ends, stretch a mason’s line at each course height and fill the wall to the line.

Step 6 — Running Bond Pattern

Standard CMU walls use running bond — each block centered over the joint of the course below. This distributes load across multiple blocks and is required for most structural applications.

Alternate courses start with a half block at the end to maintain the offset
Every course at corners rotates 90° so the head joints interlock
Stack bond (joints aligned vertically) has much lower lateral strength and typically requires engineer approval and additional reinforcement

Step 7 — Door and Window Openings

Plan openings before starting. Openings require:

Rough opening: Frame the opening with wood or steel temporary forms. The rough opening width = door/window width + 1 inch for framing on each side.

Lintel: Above every opening, a lintel (or bond beam block) spans the gap and carries the load of blocks above. Minimum lintel depth: 8 inches for spans up to 4 feet. Deeper or longer spans require engineering.

Build to each side of the opening simultaneously, keeping courses level. Install the lintel form or precast lintel after the full opening height is reached.

Step 8 — Bond Beam at Top of Wall

The top course should always be a bond beam — grouted solid with horizontal rebar.

Switch to U-shaped bond beam blocks at the final course
Install horizontal rebar (typically 2 × #4 continuous)
Pour and consolidate grout — rod or vibrate to eliminate voids
Set anchor bolts while grout is wet if attaching a top plate

For full bond beam specifications, see the CMU Reinforcement Reference.

Step 9 — Tool the Mortar Joints

As you work, mortar joints will reach a “thumbprint” hardness — firm enough to hold a thumbprint but not fully hard. This is the time to tool them.

Concave tooling (standard for CMU): Run a round jointing tool along all head and bed joints. This compresses the mortar, seals the joint face, and improves water resistance.

Tool all joints before the mortar hardens fully. Hardened mortar cannot be tooled.

Step 10 — Curing and Cleanup

Keep the wall damp for at least 3 days in hot or dry weather — cover with burlap if needed
Do not load the wall structurally for at least 7 days
Remove any excess mortar from block faces with a wet brush while still workable, or a dilute acid wash after curing
Inspect joints for gaps or voids — fill with fresh mortar within the first day

For accurate pre-project material estimates, use the Concrete Block Calculator before starting. For cost expectations, see the Concrete Block Wall Cost Guide.