Grid lines and columns

The lesson

A column grid is an address system for a building. It is a set of centre lines running in two directions, lettered one way and numbered the other, with a bubble at each end carrying the letter or number. Where two lines cross you have a named point that means the same thing on every sheet in the set, drawn by every discipline, from the foundation plan to the ceiling plan. That shared frame is what lets four consultants draw the same building without ever standing in the same room.

The convention is letters in one direction and numbers in the other, so that no intersection is ambiguous. Grid C-2 can only be one place. If both directions used numbers you would have to say which was which every time, and on a phone call at seven in the morning that is exactly the kind of thing that goes wrong. Letters normally run one way across the plan and numbers the other, in order, and the bubbles sit outside the building where they will not collide with anything.

Grid lines are drawn as centre lines, which is a specific line type: a long dash, a short dash, a long dash, repeating. That is deliberate. A centre line is not part of the building, it is a reference, and drawing it differently from a wall stops anybody from building it. The grid is also usually the thing dimensions are measured from, because a centre line is unambiguous in a way a face is not: it does not move when a wall gets thicker.

Columns sit at grid intersections, or at least they start there. On a plan a steel column is drawn as the shape of the section, which is why a wide flange column looks like a capital I with thick ends: you are looking down at the cut through the flanges and the web. A concrete column is a filled rectangle or circle. The size of the shape on the paper is the real size of the column, so at an eighth of an inch to the foot a twelve inch column is an eighth of an inch on the sheet, which is small enough that you must zoom to see the shape at all.

• A bubble carries the grid designation and sits outside the plan, at one or both ends of the line.
• The line itself is a centre line, drawn with the long dash short dash pattern so it is never mistaken for a wall.
• The spacing between adjacent lines is a bay, and the grid dimension string gives it.
• A column drawn away from its intersection is offset on purpose, and a dimension will say by how much.
• A grid added later gets a decimal or a letter suffix, so the existing numbering does not have to shift.

That last point matters more than it looks. Renumbering a grid after a set has been issued would invalidate every callout, note and shop drawing that referenced it, so instead a line added between 2 and 3 becomes 2.1, and a line added past E becomes E.1. If you find such a designation, it is a fossil of a change, and the revision table will usually tell you which revision put it there.

The grid is also how you carry a location between levels. A stair, an elevator shaft and a plumbing riser all have to line up floor to floor, and the way that gets checked is by grid: if the shaft is between B and C on Level 1 it had better be between B and C on Level 2. Reading two plans side by side is much easier once you stop looking at the outline of the building and start looking at where things sit relative to the grid.