Navigate the set

The lesson

A set of drawings is not a book with a beginning and an end. It is an index and a web of cross references, and reading it well means never hunting. If you find yourself flipping sheets looking for something, the set has already told you where it is and you have not read the instruction. Two systems do that work: the sheet number itself, which is an address, and the callout, which is a pointer from one drawing to another.

The sheet number splits into a discipline designator and a sheet type. The letter in front says whose drawing it is: G for general, C for civil, A for architectural, S for structural, M for mechanical, P for plumbing, E for electrical. That letter alone answers most questions about where to look. If you want to know how big a footing is, you are on an S sheet before you have thought about it, because footings are the structural engineer's work and the architect does not size them.

The first digit after the letter says what kind of drawing it is, and the convention runs roughly in order of how close you are standing to the building. Zero is general information for that discipline, one is plans, two is elevations, three is sections, four is large scale views, five is details, six is schedules and diagrams. So A-201 is an architectural elevation, S-101 is a structural plan, and A-601 is an architectural schedule sheet, and you knew all of that without opening any of them.

• The letter is the discipline: whose drawing it is, and therefore whose scope the answer lives in.
• The first digit is the sheet type: plans, elevations, sections, large scale views, details, schedules.
• The remaining digits are a sequence within that type, and carry no meaning of their own.
• A dash separates the designator from the digits, and the whole thing is read aloud letter first: A one oh one.

The sheet index on the cover is the authority. It lists every sheet in the set with its title, and it is what you check when a sheet you expected is missing, because a set that never had an A-121 is a different problem from a set that lost it. The index is also how you spot a set that has grown: a sheet added late gets a number that fits the pattern, and the index is the only place that records it.

Callouts are the other half. A callout is a symbol on one drawing that names a drawing on another sheet, and the notation is the same everywhere: the drawing number sits over the sheet number, written 3/A-401 when it is spoken or typed. The top half tells you which drawing to look for once you arrive; the bottom half tells you where to go. Section markers, detail bubbles and interior elevation markers all use it, and all three point somewhere real in a set that has been checked.

The shape of the symbol tells you what kind of drawing waits at the other end. A section marker is a circle with a tail and an arrow, and it usually has a cut line running across the plan; the arrow says which way you are looking. A detail bubble often has a dashed circle around the piece of plan being enlarged. An interior elevation marker is a circle with four arms, one per wall of the room, each arm carrying its own drawing number, so one symbol sends you to four drawings.

Match lines are a third kind of pointer, used when a drawing is too big for one sheet. A heavy dashed line runs across the plan with a label saying which drawing continues past it. Everything on your side of the match line belongs to this drawing and everything past it belongs to the one named, and the two overlap slightly so nothing falls in the gap. Treat a match line as a wall you cannot walk through until you have the other sheet in front of you.