Plan symbols

The lesson

A floor plan is a horizontal cut. Imagine slicing the building through at about four feet above the floor, lifting off everything above the cut, and looking straight down at what is left. Everything the knife passed through is drawn heavy and filled in. Everything below the cut that you can see is drawn lighter. Everything above the cut that matters, a beam, a ceiling change, an overhead cabinet, is drawn as a hidden line. Once you hold that picture in your head, most plan symbols stop needing to be memorised.

Walls are the clearest case. Because the cut passes through them, a wall is two parallel lines with the space between filled or shaded, and the width of that band is the real thickness of the wall. That is how you tell an exterior wall from an interior partition at a glance, without reading a single word: the exterior wall is simply fatter on the paper. A door or a window opening is drawn as a gap in that band, which is literally what it is, because the knife passed through an opening rather than through a wall.

A door in plan is a leaf and an arc. The leaf is drawn open at ninety degrees and the arc traces where it sweeps. That tells you three things at once: which side of the opening the hinges are on, which way the door swings, and how much floor the swing needs. A pair of doors gets two leaves and two arcs. A sliding door gets a leaf drawn beside the opening rather than across it, and an elevator door gets two short leaves in the shaft wall with no arc at all, because it does not swing.

Windows are simpler and easier to miss. A window is drawn as the wall thickness carried across the opening with a thin line through it representing the glazing. Compared with a door there is no arc and no leaf, so at a small scale a window can read as a thin patch in a wall. Look for the change in the poche and for the tag that goes with it, because a window is almost always tagged and the tag is what tells you the size and type.

Tags are the plan's way of not writing paragraphs on itself. Each family has its own shape, and the shape is the fastest thing to recognise on a busy drawing. Read the shape first, then the contents, then go to the schedule the shape sends you to.

• A room tag is a name over a boxed number: the number is the address, the name is for humans.
• A door tag is a hexagon or a rectangle holding the door number, and it leads to the door schedule.
• A window tag is a diamond or an oval holding a type mark such as W3, and it leads to the window schedule.
• A keynote is a hexagon holding a key that resolves in the keynote legend on the same sheet.
• A leader is an arrow with a note on the end, used when there is no schedule to send you to.

Room numbers usually carry the door numbers with them: door 105A is the first door into room 105, and 105B is the second. That is not a rule everywhere, but where it holds it is enormously useful, because a door tag tells you which room you are entering without any lookup at all. When a set does not follow it, the door schedule will say which room each door serves, and that column is the one to read.

Hatch patterns carry material. A pattern of diagonal marks and dots is concrete, a running bond pattern is masonry, a soft looping band is insulation, and a light uniform tone is a screened background belonging to somebody else's drawing. On a plan, hatch appears mostly at slabs, pads and areas of special construction; on a wall section it does most of the talking, because that is where the layers of an assembly are drawn.

Fixtures and casework are drawn to their real size in plan, which is why a toilet room reads correctly at a glance: the water closet and the lavatory occupy the floor they actually occupy. That is also why clearances can be checked on a plan at all. If a fixture looks tight in a room on the paper, it is tight in the room, and that is worth a note rather than a shrug.