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newSchoolWorks.com › VoTechWorks › Building & Construction › Masonry › Concrete Masonry Unit (CMU) Block Construction › Lesson 4
BLD-MASO04-03 · Lesson 4 · 14 clock hours · practice

Laying the First Course: Dry Bond, Layout, and Corner Setup

Objective. Students will dry-bond a running-bond CMU wall segment on a prepared footing form, mark head-joint locations, and lay a fully mortared first course that is level across its length within 1/8 inch and square at the corners within 1/8 inch over a 10-foot run.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why the First Course Is the Foundation of Everything

In CMU construction, every course you lay above the first one inherits whatever errors are in it. A first course that is out of level, out of square, or has inconsistent head joints will compound upward, creating bowed walls, raked corners, and joints that fight you all the way to the top plate. Getting the first course right is not optional — it is the entire game.

Step 1: Dry Bond Layout

Before you touch mortar, you dry-bond the wall. Dry bonding means you set blocks on the footing without mortar, spacing them to work out the running-bond pattern so that head joints fall in consistent locations and you avoid awkward cuts.

Running bond staggers vertical joints by half a block length from one course to the next. For standard 16-inch CMU stretchers, this means the head joint of the second course will land at the 8-inch mark of the block below it. To start a running-bond wall at a corner, you alternate between a full stretcher and a half block (8 inches long) so each successive course offsets properly.

During dry bond, lay stretchers end to end across the footing, leaving roughly 3/8-inch gaps where head joints will fall. Adjust spacing so the gaps are as equal as possible from one end to the other — this prevents you from arriving at the far corner with a tiny sliver of block. Once the spacing looks right, use a pencil to mark each head-joint location directly on the footing form. These pencil lines become your guide when you lay in mortar. Photograph the layout for your lab log before you disturb anything.

Step 2: Reading the Story Pole

Your instructor has prepared a story pole for this module. A story pole is a straight board marked with the height of each course, including the mortar joint, so you can hold it against the wall at any point and instantly verify that your coursing height is correct. Standard CMU coursing for 8-inch block with a 3/8-inch mortar bed runs three courses to 24 inches. Your story pole reflects the design height for this lab segment. Check your corner blocks against the story pole before laying any additional courses.

Step 3: Setting the Corner Block

The corner block is the control point for everything that follows. Apply a full mortar bed to the footing at the corner location — approximately 1 inch thick, furrowed down the center to allow the block to seat without trapping air. Set the corner block and check it on all three axes:

Use a rubber mallet to tap the block to final position. Do not use a metal hammer directly on CMU — you will crack the face shell. Once you are satisfied, check the height against the story pole. The top of the block must match the first course mark. Get instructor sign-off before proceeding.

Step 4: Laying the First Course Between Corners

With both corner blocks set and approved, stretch a mason's line between them using line blocks hooked to the outside face of each corner block. Set the line at the top-face height of the corner blocks. The line gives you a continuous reference for face alignment and height as you fill in the course.

Apply mortar to the footing in manageable sections — do not spread so far ahead that the mortar skins over before you set the block. Butter the head joints on each block before setting it: apply mortar to one end of the block and press it firmly against the previously set block so the head joint fully closes. A proper head joint is 3/8 inch thick (tolerance: ± 1/8 inch, meaning no less than 1/4 inch and no more than 1/2 inch).

Check level at every third block using your 4-foot level. If a block is high, tap it down. If it is low, remove it, add mortar, and reset — never shim with fragments. Pull the block no closer than 1/16 inch from the mason's line but never let it touch the line; touching the line bows it and ruins the alignment for every block behind you.

Tolerances and the Completion Check

When the first course is complete, your instructor will measure:

If any measurement falls outside tolerance, you reset the deficient section before advancing. This is industry standard, not a penalty — on a job site, an out-of-tolerance first course gets torn down and relaid.

Safety

CMU blocks are heavy (standard 8×8×16 stretchers weigh roughly 35–40 pounds each). Use proper lifting mechanics: back straight, lift with the legs, keep the load close to your body. Wear safety glasses at all times — mortar splatter and block chips are common. Leather gloves protect against abrasion and mortar burns; wet mortar is caustic and will damage skin on prolonged contact. Steel-toed boots are required in the lab. Keep the work area clear of block scraps and mortar droppings on the footing surface — a slip on a wet mortar spill can cause a serious fall. Never stand on a mortar board.

Key terms9

Dry bondThe practice of setting CMU units on a footing without mortar to work out the running-bond spacing and locate head joints before any mortar is applied.
Running bondA masonry pattern in which vertical head joints are staggered by half a unit length between successive courses, distributing loads and adding wall strength.
Story poleA straight board marked with the cumulative height of each course (block plus mortar joint) used as a quick reference to verify correct coursing height at any point on the wall.
Head jointThe vertical mortar joint between the ends of adjacent masonry units in the same course; standard thickness is 3/8 inch.
Mortar bed (bed joint)The horizontal layer of mortar on which a masonry unit is set; standard thickness is 3/8 inch in most CMU work.
Mason's lineA taut string stretched between corner blocks or line pins that gives masons a continuous reference for face alignment and course height as they lay units.
FurrowingDrawing the tip of a trowel down the center of a mortar bed to create a shallow groove that allows air to escape and helps the block seat evenly without a trapped air pocket beneath it.
3-4-5 diagonal checkA method of verifying a right angle at a corner by measuring 3 feet along one leg, 4 feet along the other leg, and confirming that the diagonal between those two points measures exactly 5 feet.
Coursing heightThe combined vertical dimension of one masonry unit plus one mortar bed joint; for standard 8-inch CMU the nominal coursing height is 8 inches per course.

Worked examples4

1. You are dry-bonding a 10-foot (120-inch) wall using standard 16-inch CMU stretchers. How many full stretchers fit, and what is the theoretical head-joint spacing if you target 3/8-inch joints?
  1. Step 1 — Count blocks: With 3/8-inch head joints between blocks, each block occupies 16 + 0.375 = 16.375 inches nominally. Divide 120 ÷ 16.375 ≈ 7.33 blocks. That does not come out even, which is why dry bonding exists — you adjust.
  2. Step 2 — Try 7 full stretchers: 7 blocks × 16 inches = 112 inches of block. Remaining space = 120 − 112 = 8 inches. Spread over 6 interior head joints (between 7 blocks) plus the two end conditions. If both ends land at a corner or control joint, you divide 8 inches among 6 joints: 8 ÷ 6 ≈ 1.33 inches per joint — far too wide.
  3. Step 3 — Adjust: Mix in half blocks (8 inches each). In running bond you already need half blocks at alternating corners. For this 10-foot segment, try 6 stretchers + 1 half block: (6 × 16) + 8 = 104 inches of block. Remaining for joints = 120 − 104 = 16 inches. Spread over 6 head joints between 7 units: 16 ÷ 6 ≈ 2.67 inches — still too wide.
  4. Step 4 — Correct approach: Work from nominal coursing. Standard nominal CMU is 16 inches including one joint, meaning the system is designed so that 10 feet = 7.5 nominal 16-inch modules (7 stretchers + 1 half block) with 3/8-inch joints already built into the module dimension. Dry bonding lets you see the actual gaps and mark them; adjust spacing so all head joints are as close to 3/8 inch as possible, splitting any excess evenly.
Answer: A 10-foot wall works out to 7 full stretchers and 1 half block (or equivalent combination) in running bond. The dry-bond exercise lets you confirm that head joints are approximately 3/8 inch before you commit to mortar.
2. After laying three blocks, you check level with your 4-foot level and see that the bubble is touching the outer graduation line on the right side. The level reads approximately 1/8-inch high on the right end over a 4-foot span. Is that within tolerance, and what do you do?
  1. Step 1 — State the tolerance: The spec requires the completed first course to be level within 1/8 inch across its full length. The reading here is 1/8 inch over 4 feet.
  2. Step 2 — Evaluate: 1/8 inch over 4 feet is at the outer edge of the allowable tolerance. Over a 10-foot run, if this slope continued uniformly, the error would accumulate to approximately (10 ÷ 4) × 0.125 = 0.3125 inch — which exceeds the 1/8-inch end-to-end tolerance.
  3. Step 3 — Decision: Do not accept the deviation and move on. While the mortar is still workable, tap the high (right) end of the last block down with the rubber mallet and re-check level. Continue checking at every third block to prevent the error from compounding.
Answer: 1/8 inch over 4 feet is at tolerance for a short span but would exceed tolerance if projected across 10 feet. Correct the block now by tapping down and re-checking before proceeding.
3. You want to verify that your corner is square using the 3-4-5 method. You measure 3 feet along one wall and 4 feet along the other wall. Your diagonal measurement between those two points is 5 feet 1-1/4 inches. How far out of square is the corner, and does it pass the 1/8-inch tolerance?
  1. Step 1 — State the target: A perfect right angle produces a diagonal of exactly 5 feet (60 inches) when the legs are 3 feet and 4 feet.
  2. Step 2 — Convert measured diagonal: 5 feet 1-1/4 inches = 60 + 1.25 = 61.25 inches.
  3. Step 3 — Calculate error: 61.25 − 60 = 1.25 inches out of square.
  4. Step 4 — Compare to tolerance: The allowable tolerance is 1/8 inch (0.125 inch). The actual error of 1.25 inches is ten times the allowable tolerance.
Answer: The corner is 1.25 inches out of square, which far exceeds the 1/8-inch tolerance. The corner block must be reset before the course continues.
4. Your mortar bed is supposed to be 3/8 inch thick. You spread a bed, set a block, and measure the joint at three points: 1/2 inch, 3/8 inch, and 1/4 inch. Which readings are within tolerance, and what does a tapered bed tell you?
  1. Step 1 — State the tolerance: Head and bed joints are 3/8 inch ± 1/8 inch, meaning acceptable range is 1/4 inch to 1/2 inch.
  2. Step 2 — Evaluate each reading: 1/2 inch = at the upper limit — acceptable. 3/8 inch = exactly on target — acceptable. 1/4 inch = at the lower limit — acceptable.
  3. Step 3 — Evaluate the taper: All three readings are individually within tolerance, but the bed varies from 1/4 inch to 1/2 inch across the block length (a 1/4-inch taper). A tapered bed means the block is not sitting level — one end is lower than the other.
  4. Step 4 — Action: Re-check the block with your level. Tap the high end (1/2-inch joint side) down until the block is level and the joint thickness is uniform across the full length.
Answer: All three readings individually pass the ± 1/8-inch tolerance, but the 1/4-inch taper indicates the block is not level. Tap it to a uniform joint thickness and re-check level before continuing.

The math in this lesson2

Learners fit 16 in CMU stretchers in a 120 in wall, check a 3-4-5 corner, and compare bed-joint readings to 3/8 in.
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Brush up in the math halls at newSchoolWorks.com:
In class: Dry-bond layout exercise: each student dry-bonds a 10-foot wall segment using stretcher and half blocks to establish running bond without mortar, adjusts spacing to achieve equal head joints, and marks each head-joint location on the footing form with a pencil line — then photographs the layout for the lab log. · Corner lead-up: instructor demonstrates setting a corner block plumb and level, applying mortar to footing, setting the unit, checking all three axes (plumb, level, range), and tapping to final position; students each set one corner block, and instructor checks with a level and tape before they proceed. · First-course lay: students lay the full first course between two corners, pulling a mason's line set to the face of the corner blocks, checking level at every third block, and adjusting mortar bed thickness to maintain the coursing height established by the corner; course is checked end-to-end by instructor before students move to the next lesson.
Materials: Prepared footing forms or slab surface, CMU stretcher and half blocks, story pole (instructor-prepared for this module), mason's line and line blocks, 4-foot level, 2-foot level, steel square, tape measure, pencil, chalk line, mortar boards, trowels, rubber mallet, NCCER Masonry Level 1 unit masonry module
Check: Instructor measures completed first course: level tolerance ≤ 1/8 inch across full length, corner square within 1/8 inch (3-4-5 diagonal check), all head joints 3/8 inch ± 1/8 inch; any measurement outside tolerance requires student to reset the deficient section before advancing.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Masonry — Unit Masonry Fundamentals
1. What is the primary purpose of dry bonding a CMU wall before applying mortar?
NCCER Masonry — Unit Masonry Fundamentals
2. What is the standard nominal thickness of a mortar bed joint in CMU construction?
NCCER Masonry — Unit Masonry Fundamentals
3. When setting a corner block, which three axes must you check before the mortar sets?
NCCER Masonry — Unit Masonry Fundamentals
4. A mason's line is set to the face of the corner blocks. During laying, a block is pressed so that it touches the line. What is the most likely result?
NCCER Masonry — Unit Masonry Fundamentals
5. You use the 3-4-5 method to check a corner. You measure 3 feet along one leg, 4 feet along the other, and the diagonal is exactly 5 feet. What does this confirm?
OSHA 10 — Construction Safety and Health
6. Which personal protective equipment (PPE) is required at all times in the CMU lab to protect against mortar splatter and block chips?
NCCER Masonry — Unit Masonry Fundamentals
7. A student checks level at the midpoint of the first course and finds the bubble is 3/16 inch off level over a 4-foot span. What is the correct action?
NCCER Masonry — Unit Masonry Fundamentals
8. Why does a story pole improve accuracy when laying the first course of CMU?
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