Diagonals 20 mm Out on a Front Elevation About to Be Bricked
You are an apprentice bricklayer on a two-storey detached house in a housing scheme. The blockwork to both gable ends and the rear wall is up to first-floor level; the front elevation is now being set out for the facing brick. On the setting-out grid the building is 12.4 m × 8.6 m, so both diagonals should measure the same: 15.09 m. Your labourer reads 15.09 m and 15.11 m using a 30 m steel tape that he has hooked over a wall tie halfway across the building. The string lines look square to the eye and each quoin is within 5 mm of its setting-out pin. At the front-right corner a dumper clipped the blockwork the previous afternoon. The foreman wants the first course laid before the concrete lorry arrives at 11:00 and says that 20 mm on a diagonal that long is nothing to worry about.
1. Before any facing brick is laid, what checks do you carry out - and what do you do about the 20 mm discrepancy?
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I would not lay a brick until the measurement itself is trustworthy and the geometry is resolved.
First, re-measure properly. A tape hooked over a wall tie is bridged, not straight, so the reading is meaningless; I re-run both diagonals corner to corner, tape clear of obstructions, tape held level at the same course, no sag, both reads taken to the same points on the quoins. I repeat each diagonal twice.
Second, verify squareness independently rather than relying on one test - a 3-4-5 check on the longest available multiple at a corner, and a check that the front face is parallel to the rear wall at both ends.
Third, treat 20 mm as a real error. A discrepancy of that order is outside normal setting-out tolerance on a building of this size and will not close cleanly; it usually means one corner has moved, and the dumper strike is the obvious candidate.
So the action is: report to the site engineer, get the damaged corner re-checked and the pin/line reset, and only start laying once the diagonals agree within a few millimetres. Starting the front elevation before the setting-out is signed off locks the error into both lift faces.
2. The foreman insists you start anyway. Explain in technical terms what happens over the following months if the front elevation is built out of square to the existing returns.
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If the front face is built off a skewed line, the error does not stay at the corner - it propagates through every course.
At the quoins the brickwork has to accept the return that is already built. If the angle is not 90°, the courses will either taper away from the return (opening a wedge-shaped joint that has to be pointed with mortar thicker than the joint design allows) or converge (forcing the brick to be cut back, producing a ragged, non-vertical quoin). Once the quoin is out of plumb, the lift above is out of plumb too.
On the front elevation itself, window and door openings set out from the corners will not sit square to the reveals, so frames will not fit without packing, and the brick reveals will not be parallel to each other. Cills and thresholds will not line up with the blockwork behind.
The real cost shows at roof level. The wall plate and trusses need a square bearing; a skewed elevation means the truss spacing has to be fudged, and the eaves line runs out of parallel with the gable. That may show as an uneven soffit or as verge cuts.
Finally, the wall ties and cavity will be skewed relative to the inner leaf, and the cavity width will vary along the front, compromising the insulation and the DPC lap. A 20 mm error noted now is a few hours' work; discovered after the lift, it is demolition.