Dimensioning, Scales and Annotation of Technical Drawings
This domain tests whether a drawing can be built from the sheet without a ruler. Dimensioning follows EN ISO 129, scales follow EN ISO 5455, and the examiner looks for three things: completeness, no duplication, and correct units.
Dimensioning rules
- Extension lines start 2–3 mm clear of the object and run past the dimension line; dimension lines sit outside the building outline, the first about 10 mm away and subsequent chains 7–10 mm apart.
- Terminators are arrows or 45° oblique strokes; small dimensions take the arrows outside the extension lines.
- Figures sit above the dimension line, readable from the bottom or the right-hand edge of the sheet, and are never crossed by a line.
- All dimensions are in millimetres with no unit symbol written: 3600, not 3.60 m or 3600mm. Levels are the exception - in metres with sign and three decimals: ±0.00, −0.15, +3.00.
- Dimension to structural faces, not to plaster lines, and give an overall dimension outside the chain.
- Do not double-dimension the same distance in a closed chain; that creates conflicting information.
Scales and annotation
- Preferred scales: 1:1, 1:2, 1:5, 1:10, 1:20, 1:50, 1:100, 1:200, 1:500. Plans at 1:50 or 1:100, details at 1:20 or 1:5, site plans at 1:200 or 1:500.
- Openings are referenced by a mark (Π1, Θ2) with size 900×2100 and given in a window/door schedule (πίνακας).
- Every sheet carries a title block, north point, scale bar and, where relevant, a revision cloud.
Worked example
A room is dimensioned at 1:50. The clear structural width is 3600 mm, the partition is 100 mm, and the total depth is 4200 mm.
- On paper: 3600 ÷ 50 = 72 mm; 4200 ÷ 50 = 84 mm; the partition = 2 mm. Two metres of scale bar = 40 mm.
- Chain check: 3600 + 100 + 4200 must equal the overall figure placed outside the chain. If it does not, the error is in the arithmetic, not the drawing.
- Changing to 1:100 halves every paper dimension; going from 1:100 up to 1:50 doubles them (×2, not ×0.5).
Traps
- Adding a unit symbol to millimetre dimensions, or mixing metres and millimetres in one chain.
- Dimensioning from plaster or finish lines that the builder cannot set out.
- Using a ruler on the printed sheet instead of reading the written figures.
- Extension lines touching the object, or dimension lines crossing each other.
- Missing overall dimensions, so no single figure can be checked.
- A scale bar absent from a reduced or photocopied sheet.