Dimension strings

The lesson

Dimensions on a plan are not scattered notes. They come in strings, and a string is a chain of numbers laid end to end along one side of the building, each one measured between two things the builder can find. Strings sit in parallel rows, and each row answers a different question at a different level of detail. Read the rows from the outside in and the plan explains itself.

The outermost string is the overall dimension: one number, face to face or grid to grid, for the whole building in that direction. Inside it sits the grid string, one number per bay. Inside that sits the locating string, which places every partition, opening and offset. Each row is a finer sieve than the row outside it, and the numbers in a row add up to the number in the row outside. That is the check, and it is the reason strings are drawn in rows at all.

A dimension has anatomy, and reading the anatomy tells you what is being measured. Two thin extension lines run out from the points being dimensioned, with a small gap where they leave the object so they are not mistaken for part of the building. A dimension line runs between them, terminated at each end by a slash tick or an arrowhead. The number sits on that line, usually in a break in it. If a number has no extension lines reaching real geometry, be careful: it may be a note rather than a dimension.

The hard question is always what face the dimension is taken to, because the same wall can be dimensioned three ways. To the centre line, to the face of stud, or to the face of the finished surface, and those differ by the thickness of a stud and a layer of gypsum. The set states the rule once, in the general notes, rather than repeating it at every string. On a plan dimensioned to the face of stud, the layout crew snaps their lines on studs and the drywall goes on afterward, which is exactly why that convention is common.

• Overall: the whole building in one direction, on the outermost row.
• Grid: bay by bay, centre line to centre line of column grid.
• Locating: to each partition, opening, jamb or offset, on the innermost row.
• Clear, marked CLR: hold this distance no matter what happens to the wall thicknesses around it.
• Equal, marked EQ: divide the run into equal parts rather than measuring a stated number.

CLR and EQ are instructions rather than measurements, and they are the two you must never treat as ordinary numbers. A clear dimension across a corridor says the finished opening must be that width; if a wall gets thicker in a submittal, the wall moves and the clear stays. An equal dimension says the drafter cared about the spacing being even rather than about the exact figure, and the builder divides the actual run. Both are ways of saying which number is allowed to move when reality does not cooperate.

A well dimensioned plan is also deliberately incomplete. A drafter leaves one segment of a string undimensioned when the rest of the string plus the overall already fixes it, because putting the number in would let two numbers disagree. If you find a run with a missing number, that is usually the reason and you can solve for it. If you find a run where every number is given and they do not sum, the drawing has told on itself.