How to Lay Cinder Blocks: Mortar, Joints, and Setting Technique
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Full bio →Tools You Need Before Starting
Gather these tools before mixing mortar — once mortar is mixed, the clock is running:
| Tool | Purpose |
|---|---|
| Brick trowel (10–12 inch) | Spreading mortar |
| Mason’s line and line blocks | Keeping courses straight |
| 4-foot level | Checking level and plumb |
| Rubber mallet | Tapping blocks into position |
| Jointing tool (convex or V-joint) | Tooling mortar joints |
| Story pole (or marked rod) | Checking course height |
| Mixing trough or wheelbarrow | Mixing mortar |
| Margin trowel | Cleaning up excess mortar |
| Spray bottle of water | Wetting dry blocks and tools |
Use the Cinder Block Calculator before starting to confirm your block count — running out of blocks mid-course is a common and avoidable mistake.
Step 1: Mix the Mortar
Pre-mixed mortar (80 lb bags labeled “mortar mix” or “masonry mortar”) is the simplest choice. Add water according to bag instructions — typically 3.5–4 quarts per 80 lb bag.
Correct mortar consistency: Hold a trowel of mortar sideways for 3–4 seconds. Mortar should hold its shape without sliding off. If it slumps, it’s too wet — add more dry mix. If it crumbles at the edges, it’s too dry — add water in small increments.
Working time: Mixed mortar is workable for approximately 90 minutes at 70°F. Mix only what you can use in that time. Discard mortar that has begun to set — do not add water to “retemper” stiffened mortar; it will produce a weak joint.
ASTM C270 specifies mortar types for masonry:
| Mortar Type | Compressive Strength | Best For |
|---|---|---|
| Type S | 1,800 psi | Below-grade, retaining walls, exterior |
| Type N | 750 psi | Above-grade exterior, interior |
| Type M | 2,500 psi | Foundations, severe weather exposure |
Most residential block projects use Type S or Type N pre-mixed mortar from a home improvement store — the bag will be labeled accordingly.
Step 2: Lay the Bed Joint (Bottom Mortar Layer)
The bed joint is the horizontal layer of mortar that the block rests on.
- Wet the footing or previous course with a spray bottle before applying mortar. Dry concrete absorbs water from the mortar too quickly, weakening the bond.
- Spread mortar in a continuous strip approximately 1 inch thick and as wide as the block (7⅝ inches for standard 8×8×16). Spread 3–4 blocks’ worth at a time — don’t spread more than you can set before mortar begins to skin over (~10 minutes in warm weather).
- Furrow the center of the bed joint by dragging the trowel tip down the middle. This creates a slight valley that compresses to fill any voids when the block is set.
The finished joint thickness should be 3/8 inch — this is the standard that makes block coverage calculations accurate.
Step 3: Set the Block
- Butter the head joint first — the vertical joint on the end of the block you’re about to place. Hold the block, apply mortar to one end face (about 3/4 inch thick), then set the block.
- Set the block firmly onto the bed joint, pushing it toward the previously laid block so the head joint squeezes to 3/8 inch.
- Tap level — use a rubber mallet to tap the block down to the correct height. Check level in two directions (lengthwise and across the block width).
- Check plumb — the block face should be in line with the course below. Check plumb with your level on the face of the block.
- Check alignment — the front face of the block should align with the mason’s line stretched taut from one end of the course to the other.
Common beginner mistake: Setting the block and then tapping it while checking level, but not checking plumb or alignment. All three must be correct — don’t move on until the block is right.
Step 4: Scrape Excess Mortar
As you set each block, mortar squeezes out from the joints. Scrape it off immediately with the edge of your trowel, before it stiffens. Excess mortar that stiffens on the block face is much harder to remove.
Return scraped mortar to your mixing trough if it’s still workable. Discard it if it’s beginning to set.
Step 5: Running Bond (Staggered Joint Pattern)
Every block course should be offset by 8 inches (half a block) from the course below. This running bond pattern creates a wall that transfers loads across joints rather than concentrating stress at vertical lines.
Aligning vertical joints on top of each other (stacking bond) produces a wall with 40–60% less structural strength. Never use stacking bond for structural or load-bearing applications.
Starting the stagger:
- Course 1: Start with a full block at the corner
- Course 2: Start with a half block (8×8×8 manufactured half block, or a full block cut with an angle grinder or masonry saw)
- Course 3: Back to full block at the corner
- Repeat alternating
Step 6: Tool the Mortar Joints
Tooling compresses the mortar surface, improving water resistance and appearance. Tool when mortar is “thumbprint hard” — it holds a clear thumbprint but does not stick to your thumb.
Timing is critical: Too early and the joint deforms. Too late and the mortar is too hard to tool cleanly.
Joint tool types:
- Concave (rounded) joint: Most common. Weather-resistant, easy to tool. Deflects water.
- V-joint: Attractive for decorative walls. Good weather resistance.
- Flush joint: Troweled flat. Weakest weather resistance; use only for walls that will be parged or covered.
Tool the horizontal (bed) joints first, then the vertical (head) joints.
Common Block Laying Mistakes
| Mistake | Effect | Fix |
|---|---|---|
| Mortar too wet | Weak joint, block sinks | Add dry mix in small increments |
| Not wetting previous course | Poor bond, joint failure | Spray block surface before laying mortar |
| Skipping head joints | Weak wall, water infiltration | Butter both end faces before setting |
| Tooling too early | Smeared, distorted joint | Wait until thumbprint hard |
| Not checking plumb | Leaning wall, difficult upper courses | Check plumb every 3–4 blocks |
| Joints over 1/2 inch thick | Reduced wall strength | Keep joints at 3/8 inch |
| No mason’s line | Wavy courses | Stretch line for every course |
For the block quantities and mortar bag count needed for your wall, use the Cinder Block Calculator. For technical specifications on coverage and mortar quantities, see 8×8×16 Cinder Block Coverage and Mortar Reference. For a full retaining wall build guide, see How to Build a Cinder Block Retaining Wall.