Diagonals That Do Not Match on a House Set-Out
You are setting out a single-storey house measuring 10.2 m by 6.8 m on a cleared plot in Anuradhapura. The ground is level but the surface is loose gravel. You drive four corner pegs using a steel tape and the 3:4:5 method, then run string lines. When you measure the diagonals you get 12.31 m and 12.27 m. The owner's boundary survey peg sits 450 mm outside the line of the front wall, and the local authority requires the building to keep a 600 mm side setback. Your supervisor is away and the excavator arrives in two hours. Two of the helpers have never set out a building before. The owner says not to worry, the wall will be straight enough.
1. The diagonals measure 12.31 m and 12.27 m. What does this tell you, what could be causing it, and what would you do before the excavator starts?
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What it means
- A rectangle is square only when both diagonals are equal. A 40 mm difference means the corners are not true right angles.
- As a working rule the difference should be no more than about 10 mm at this size. 40 mm is roughly four times too much and will show up later as rooms that are not rectangular, wall and roof layout problems, and constant rework.
Likely causes
- Tape not held level or not pulled taut (sag over a 12 m pull on loose gravel).
- A corner peg knocked, leaning, or measured to a different face each time.
- The 3:4:5 check done at only one corner, or done carelessly.
- String line sagging or pinned off the centre of the peg instead of the mark.
What to do
- Do not let the excavator start; the trench lines are not yet trustworthy.
- Check the tape against a known length, then re-measure both diagonals from the same fixed points, holding the tape horizontal and tight.
- Re-do 3:4:5 at every corner.
- Correct by moving the corner nearest the short diagonal by about half the error (about 20 mm), then re-measure until the diagonals agree within tolerance.
- Only then dust the trench lines in lime and hand the setting out to the machine operator.
2. You find the front wall face will sit 450 mm from the boundary peg, but a 600 mm setback is required. The owner wants the digging to start. What is your judgement and your next step?
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- This is not a building-skill problem, it is a legal and boundary problem, and you must not dig.
- First confirm what the peg actually represents by asking for the survey plan or the surveyor's pegs. If the peg marks the boundary, the wall is 150 mm inside the required setback.
- Options, in order of preference:
- Shift the whole building back (check that the rear and other side setbacks still comply before doing this).
- Obtain written approval or a variation from the local authority before any work.
- What you must not do: move the boundary peg, shave the setback, or dig on a verbal promise that it will be sorted out later.
- Tell the owner and the supervisor the same day, in writing or in the site diary with a sketch and the date, and record that excavation was held up for this reason. If the excavator is already booked, that cost is far smaller than demolishing a wrong wall.
3. How would you use the two inexperienced helpers during the set-out, and what would you record before handing over for excavation?
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Using the helpers
- Give each one a single, clear job: one holds the zero end of the tape on the marked point and calls the reading back, the other holds the far end or places pegs and string lines.
- You personally read and confirm the tape and the diagonals; two people never confirm their own work.
- Explain the 3:4:5 idea in plain language and show it once on the ground, then have them repeat it while you watch.
- Keep them clear of the excavator track area and check their pegs before the lines are trusted.
Records before handover
- A simple sketch plan with all overall dimensions, the two diagonals, and the offset from the boundary peg to each wall face.
- The datum or level reference used, and where it is physically marked.
- The date, who set out, who checked, and any instructions or warnings received (such as the setback issue).
- Get the supervisor or owner to sign or acknowledge, so that responsibility for the dimensions is clear.