The plan is a cut, and the cut explains the line weights
Everything the imaginary knife passed through is drawn heavy and usually filled with a tone, called poche. Walls, columns and the jambs of openings are cut, so they are the darkest things on the sheet. Everything below the cut that you can see from above is drawn at a medium weight: floor finishes, casework, fixtures, stair treads. Everything above the cut is drawn as a dashed hidden line, because you cannot see it but you need to know it is there. A beam over, an upper cabinet, a soffit and a change in ceiling all appear that way.
Line weight is therefore not a style choice, it is a channel of information running in parallel with the geometry. A reader scanning a busy plan gets the structure of the building from the dark lines alone, before reading a single word. That is why a plan printed too light, or scaled down until the weights merge, becomes so much harder to read than it looks: half the message has been thrown away.
The cut height itself is a convention rather than a rule. It is chosen so that the slice passes through doors and windows in a typical wall, which is why doors show as openings with swings and windows show as glazed gaps. Where an element does not suit that height, the drafter simply draws it as if the cut suited it, because the plan is a communication device rather than a photograph.
- Heavy and filled: cut by the section, normally walls and columns.
- Medium: visible below the cut, such as fixtures, casework and floor patterns.
- Light: annotation, dimensions, grid lines, leaders and tags.
- Dashed: above the cut or hidden behind something, such as a beam or an overhead cabinet.
Read the edges first
Before the building, read the sheet. The title block down the right edge gives the sheet number, the project, the issue date and status, and the revision table. The drawing title sits under the plan with its own number and a bar scale. The north arrow sits near the plan because it belongs to the drawing rather than to the paper, and plans are routinely rotated to fit the sheet, so the arrow is the only thing that tells you which wall is the north wall.
The general notes deserve a full read once. They are short and they change the meaning of everything else on the sheet. One note will say what face dimensions are taken to, which shifts every number by half a wall if you assume wrongly. Another will say what the typical partition is, so that the plan does not have to tag every wall. Another will tell you not to scale the drawing, which is not a formality: it is a statement that the numbers, not the paper, are the record of intent.
Walls, and how thickness reads
Because the cut passes through them, walls are drawn as two parallel lines with the space between them filled. The width of that band is the real thickness of the wall at the scale of the drawing, so an exterior wall simply looks fatter than an interior partition and you can sort a plan into structure and infill at a glance. On a small building plan at one eighth of an inch to the foot, a twelve inch exterior wall is an eighth of an inch on the paper and a six inch partition is a sixteenth, which is enough of a difference to see and not enough to measure comfortably. Read the difference, and get the number from the dimension string.
Openings are gaps in the band, which is exactly what they are in reality. That is worth stating because beginners often look for a symbol where there is only an absence. A door opening is a gap with a leaf and an arc drawn in it; a window opening is a gap with the glazing line drawn across it; a cased opening with no door at all is simply a gap, and it is easy to walk past on a first read.
Doors, swings and what the arc is telling you
A door in plan is drawn open at ninety degrees, with an arc tracing the path of the leading edge. That single symbol answers three questions simultaneously. The end the leaf is attached to is the hinge side. The side of the wall the arc sits on is the direction of swing. The size of the arc is the floor area the door needs to open into, which is why furniture, fixtures and other doors are never drawn inside it on a well coordinated plan.
Variations follow the same logic. A pair of doors gets two leaves and two arcs meeting in the middle. A sliding door is drawn as a leaf running alongside the wall rather than across the opening, with no arc, because it does not sweep. An elevator door is two short leaves in the shaft wall with no arc at all. A revolving door, a folding partition and an overhead coiling door each have their own mark, and the symbols legend on the general sheets draws every one the set uses.
The swing is also a code and accessibility question, not just a geometry question. Which way a door swings out of a room, whether it swings into the required width of a corridor, and how much clear floor space sits beside the latch are all things somebody has checked, and they are all readable from the plan. If a door looks like it swings into a stair landing or across an exit path, that is worth a question.
Tags, and the discipline of not writing on the drawing
A plan does not describe things, it names them. Each family of thing has a tag with its own shape, so the shape can be recognised at speed on a crowded sheet. A room tag is a name over a boxed number. A door tag is usually a hexagon or a rectangle holding a door number. A window tag is a diamond or an oval holding a type mark such as W3. A keynote is a hexagon holding a short key that resolves in a legend on the same sheet. An equipment tag names an item that appears in an equipment schedule.
The rule to internalise is that a tag is a key into a table, not a description. A door tag does not tell you the door is ninety minute rated; it tells you which row of the door schedule to read, and that row tells you the rating. Following the tag is one extra step and it is the step where the real answer lives. Where a tag has no table behind it, the set will use a leader with a note on the end instead, and that note is the answer in full.
Door numbers commonly carry the room number they serve, so 105A is the first door into room 105. It is a convention rather than a law, but where it holds it saves a lookup every time, and where it does not hold the door schedule has a column naming the room each door serves.
An order to read a plan in
Consistency beats speed while you are learning, and the order below is one that experienced readers converge on because each step gives context for the next. Skipping the first three steps is what produces the confident, wrong answer.
- Read the title block, the drawing title, north, the scale and the revision table.
- Read the general notes, especially the one about what face dimensions are taken to.
- Find the grid, and learn which letters run which way, so you can name any location.
- Trace the outline of the building and the main circulation: entrances, corridors, stairs, lifts.
- Read the room tags and build a mental map of what is where before touching a single dimension.
- Read the dimension strings from the outside in: overall, then grid, then locating.
- Follow the callouts off the sheet last, once you know what you are asking about.