Corner Lead Fails Plumb, Level, and Joint Tolerances
During a performance verification, a masonry trainee builds a 6-course, 8-inch CMU corner lead with 4-foot legs. The evaluator records: first course level within 3/16 inch over 4 feet; corner plumb 5/16 inch out of plumb over the 4-foot height; bed joints vary from 1/4 to 1/2 inch; head joints vary from 1/4 to 5/8 inch; the sixth course top is 50 1/4 inches above the base; mortar is struck flush on the exterior face and not tooled; several blocks are laid with the hollow cores not aligned. The trainee used a 4-foot level and a line, checked the first course, and then laid the remaining courses without rechecking plumb. The evaluation criteria require nominal 3/8-inch joints, plumb within 1/4 inch in 10 feet, and level within 1/4 inch in 10 feet. The trainee argues that 5/16 inch out of plumb is acceptable because it is less than 3/8 inch.
1. Determine whether the lead passes or fails each stated criterion, showing the calculations.
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The lead fails several criteria. For plumb, the stated tolerance is 1/4 inch in 10 feet. Over 4 feet, the proportional allowance is about 0.10 inch, not 0.25 inch, so 5/16 inch out of plumb fails. The trainee's 3/8-inch comparison is not the stated tolerance. For level, 3/16 inch over 4 feet also exceeds the proportional allowance of about 0.10 inch. For height, six courses at 8 inches nominal equal 48 inches; the measured 50 1/4 inches is 2 1/4 inches high, which is far outside normal coursing tolerance. Bed joints of 1/4 to 1/2 inch are at the edges of a common 3/8-inch plus-or-minus-1/8-inch tolerance, but the 5/8-inch head joint exceeds the upper bound. Flush, untooled mortar and misaligned cores are workmanship failures.
2. What is the root cause of the out-of-tolerance height and plumb, and what should the evaluator coach?
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The root cause is that the trainee leveled the first course, then laid the remaining courses without carrying a consistent level line or checking plumb each course. Thick joints were used to adjust height, which raised the lead to 50 1/4 inches and exaggerated plumb error. Misaligned cores show that the trainee was not maintaining a consistent bond and laying pattern. The evaluator should coach the trainee to establish a corner pole or story pole, mark each course height, check plumb at every course, keep bed joints consistent at about 3/8 inch, and tool the joints after thumb-set. The trainee should also learn to check level and plumb before the mortar sets, when correction is still possible. Rechecking only at the base is not enough.
3. What should happen to the lead: accept, rework, or remove?
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The lead should not be accepted as a reference lead. It fails the stated plumb, level, height, head-joint, and workmanship criteria, and if it were used as a corner lead, the errors would be repeated across the wall. Because this is a training evaluation, the evaluator should fail the exercise, explain the specific measurements, and require the trainee to remove and rebuild the lead under observation. The trainee should demonstrate the corrected sequence: set the first course accurately, establish a height and plumb reference, lay each course to the line, check plumb after each course, and tool the joints. A re-evaluation should use the same criteria, with the trainee explaining the tolerance calculations before starting.