Technical Drawing
Technical drawing tests whether you can read a set of building drawings and get usable information off them - scale, projection, line types, levels and dimensions - without scaling something that is already written down.
Scales and projection
Location and site plans at 1:500 or 1:1250; floor plans at 1:100; sections and elevations at 1:50; details at 1:20, 1:10 or 1:5. Ghanaian practice follows first-angle projection, where the view seen from above is drawn below the elevation; in third angle the plan sits above. Check which convention the sheet uses before interpreting it.
Line conventions and symbols
- Thick continuous: outlines of the work.
- Thin continuous: dimension lines, extension lines, hatching.
- Dashed: hidden edges.
- Long dash-dot (chain): centre lines, grid lines and section lines.
- Hatching: concrete as dots and small triangles; blockwork as diagonal lines; earth as sloping lines; timber as grain lines. Grid lines are labelled A, B, C one way and 1, 2, 3 the other. Levels appear as a triangle with RL +12.500. Doors and windows are described in schedules, not by scaling the plan.
Reading a drawing
Start at the title block: project, drawing title, number, scale, revision. Then north point, grid, overall dimensions, wall thicknesses, openings, levels and notes. Cross-check plan against section - a wall shown 225 mm on plan must show 225 mm on section.
Worked example
A wall measures 84 mm on a print stated to be 1:50. Real length = 84 × 50 = 4200 mm = 4.20 m. The same 4.2 m wall on a 1:100 sheet would measure 42 mm. Before trusting either answer, check the printed scale bar: if 1 m is shown as 20 mm, the drawing really is 1:50. Photocopying shrinks paper, so the stated scale can be wrong - measure the scale bar, and where a dimension is written on the drawing, use the written figure and never scale it.
Traps that cost marks
Scaling dimensions that are already annotated; mixing millimetres and metres mid-calculation; reading a first-angle view as third angle; ignoring a revision cloud and building to a superseded detail; missing the level symbol and building to the wrong floor level; forgetting that sections and plans must agree.