Twin toilet block: scale choice, setting-out dimensions and missing DPC and lintels
An NGO is building a two-roomed toilet block at a school in Zomba. Overall size 4.8 m × 2.4 m; external walls 230 mm thick, internal partition 115 mm; two 800 mm doors and two 500 mm × 500 mm windows with sills at 1500 mm; wall height 2.4 m to ceiling; roof of corrugated iron sheets at 25° pitch; a damp-proof course is required 150 mm above finished ground level; lintels over every opening with 150 mm bearing each side. The drafter submitted a 1:100 plan with all dimensions taken to wall centre lines, doors and windows drawn without lintels, and poche lines drawn so closely that the 115 mm partition reads as solid black. The section is a bare outline: no DPC, no lintel, no roof pitch. The client has asked for a plan and a section that a local mason can set out from without a telephone call.
1. Recommend the drawing scales and line weights for the plan and section, and justify the choice against what was submitted.
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Scale: 1:50 for plan and section; 1:20 for the eaves, DPC and lintel detail.
- At 1:50 the 230 mm external wall becomes 4.6 mm on paper and the 115 mm partition becomes 2.3 mm - both can be outlined, poche-filled and read at arm's length on site. At 1:100 the partition is 1.15 mm, which is why it has gone solid black; the drafter has tried to poche a line that has almost no width.
- 4.8 m at 1:50 is 96 mm, comfortably inside an A3 or A2 sheet with room for three dimension strings and notes. 1:100 saves space the drawing does not need.
- Details at 1:50 are still too small to show a 150 mm DPC as more than 3 mm, so the eaves and the DPC/lintel junction belong at 1:20 or 1:10.
Line weights (thick-to-thin hierarchy):
- 0.7 mm: outlines of walls cut by the horizontal section plane (plan) and by the vertical cutting plane (section).
- 0.25–0.35 mm: poche or widely spaced 45° hatching inside cut walls - light enough that openings and fittings still read, and at 2–3 mm spacing so the partition does not blacken.
- 0.35 mm: fittings, door leaves, window frames, sanitary ware, floor and roof outlines seen beyond the cut.
- 0.25 mm: dimension lines, extension lines, leader lines, text and centre lines (chain).
Erase every construction and setting-out line before issue. A mason reads weight as meaning - the cut wall must be the heaviest thing on the sheet.
2. Explain how the plan should be dimensioned so a mason can set out the block, and why dimensioning to centre lines is wrong here.
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Fault: centre-line dimensions give the mason positions he cannot measure from. Strings, profiles and blockwork are set from a face or a corner, and an opening is built to the clear distance between reveals, because that is the size the door or window frame will be ordered to. A centre-line dimension also forces the builder to halve two wall thicknesses on site, which is where errors creep in.
Correct practice:
- Dimension to structural openings - the clear masonry opening between reveals (e.g. 800 mm for a door, 500 mm for a window), not to the frame or the centre line.
- Establish one datum, normally an external corner of the building, and work as far as possible from it.
- Use running (cumulative) dimensions along one line for the positions of openings, then a separate overall dimension outside it. Running dimensions from a single datum do not accumulate rounding errors the way a chain of small dimensions does.
- Three strings of dimensions, outside the plan, nearest to farthest: openings and piers; room sizes and wall thicknesses; overall length and width. Never repeat the same dimension in two strings.
- Give the overall diagonal or a note on squareness if the building is to be set out with a 3-4-5 triangle; and state that all dimensions are in millimetres.
- Show door swings, window positions, the partition, and a north point; add the DPC level and the finished floor level as levels, not dimensions.
3. The section must show construction the mason can build. List what must be added and how each item is drawn and annotated.
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Cut walls: outlined thick, poche-filled lightly, with the foundation concrete, footing and floor slab shown below the DPC. Ground line drawn as a thin continuous line with short hatching below.
DPC: a distinct band at 150 mm above finished ground level, drawn as a thick line with a break or as a narrow filled strip right through the wall thickness, continuous around the building and not interrupted at the internal partition junction unless the specification says so. Label 'DPC' with a leader. Its height above ground is dimensioned, because that 150 mm is what keeps rising damp out of the wall.
Lintels: over each door and window opening, shown as a separate rectangle in the section with 150 mm bearing each side clearly dimensioned and labelled 'lintel'. The section must be cut through an opening to show this, or an additional section through a window is needed - a solid-wall section hides the very thing the mason needs.
Roof: show the wall plate, rafters and corrugated sheets, with the 25° pitch marked as an angle from the horizontal on the section (or as a rise-to-run ratio) and the eaves overhang dimensioned. Show the ceiling line at 2.4 m.
Vertical dimension chain: finished floor level to ceiling (2.4 m), ceiling to eaves, eaves to ridge, plus sill heights (1500 mm) and door head heights. Mark levels with the inverted-triangle symbol and RLs rather than stacking more dimensions.
Labels: every material should be named once by leader - concrete, blockwork, DPC, lintel, wall plate, sheeting - so the drawing doubles as a specification.