Drafting conventions: lines and dimensions

Based on National CAD Standard 3.1

Lines

Lines on a drawing have meaning, and their relative "weight" can confuse or clarify the information to be conveyed. Guidelines for line weights follow:

Line type	Width (mm)	Typical uses
Fine	0.18	Material rendering, patterns
Thin	0.25	Small text (3/32" - 3/8"), dimension lines, leaders, break lines, dotted lines, grid lines, etc.
Medium	0.35	Medium text (5/32" - 3/8"), object lines, property lines, door/window elevations, accent lines in schedule grid
Wide	0.50	Larger text (7/32" = 3/8"), titles, edges of elevations, profiling, drawing block borders
Extra wide	0.70	Largest text (1/2" - 1"), match lines, large titles, title block or sheet borders,schedule outlines
XX wide	1.00	Major title underlining
XXX wide	1.40	Border sheet outlines
XXXX wide	2.00	Border sheet outlines (children under 17 not permitted to use this thickness without parental consent)

Some definitions of important line types:

Leaders are the lines connecting text or symbols (notations, dimensions) to a particular point or item in the drawing.
- They terminate with an arrowhead, not necessarily touching the object being pointed to.
- They should be drawn the same way within a set of drawings: either straight or curved.
- Typically, if drawn straight, they are somehow angled so as not to confuse themselves with the things being represented (which presumably are not similarly angled...).
- Avoid crossing other lines, especially dimension lines or other leaders.
- Start the leader at the upper left of the notation (or, according to the Standard, the upper right, although for multi-line notations, I would start at the upper left or lower right).
break lines are used to reduce the size of a drawing by removing redundant parts of it, or terminating a section or detail, keeping everything else in scale.

Note that the break line should not remove something important from the drawing.
Centerlines are represented by a thin line interrupted by a dot (or short dash). They are drawn at the center of objects such as columns, or walls, or anything else where the fact that the object has a center is important (usually for dimensioning).
Dimension lines are one of the key elements in working drawings, as they indicate the sizes of things. Because they are so important, and so common, it is crucial to take the time to get them right:
- first, so-called "extension lines" are drawn at the start and stop points of the thing being dimensioned.
- then a thin dimension line connects the extension lines, extending a bit beyond (i.e, crossing it) so that it doesn't read as an object itself.
- finally, terminator marks, angled consistently in the same direction (but differently for horizontal and vertical dimensions) are drawn.
- note that the dimensions themselves are drawn above horizontal dimension lines, and to the left of vertical dimension lines (written vertically from bottom to top).
limit of construction lines essentially show the work area, but should be further defined to avoid ambiguity (since such a line could indicate either the actual scope of work that is part of the contract, or the available site area that the contractor may utilize to construct the project, etc.)
match lines enable a large object to be broken up into two or more pieces so it can be shown on a smaller sheet size than what would otherwise be required. It is also conceivable that a match line could be used for some other reason, but the effect is the same: the object is divided up and shown on separate sheets.
hidden lines are represented by thin dashed lines and are used to represent objects hidden (because they are behind) other objects. They are also used to represent objects above in plan (that are not exactly hidden, but would not otherwise appear). Not all things hidden (or above) should be represented: this is always based on a judgement that such objects need to be understood in the context of what is being shown.
property lines are shown as a line interrupted by a double dot (or short dash).

Dimensions

The idea is to indicate the size and position of everything that is to be built. Two repeat: indicate size and of all building elements. The usual strategy is to tie each object to something relatively fixed, such as structural column centerlines, or walls, or existing elements. These, in turn, are located to some absolute site feature, such as a property line (in plan) and a vertical datum point (in section).

The other basic strategy is to show each dimension only once, so that changes can be made without running into contradictory dimensions that one forgot to change. In an intelligent building model, on the other hand, such redundancy would not necessarily be a problem. In any case, this rule is violated with column grid lines (which are dimensioned on every floor plan) and in other circumstances where the redundant dimensions are judged to be more useful than dangerous.

Try to place dimensions on the outside of the drawing, on the top and right side of plans (but to the left of sections), where possible. Where parallel dimension lines occur, space then 3/8" apart.

Arrange dimensions in continuous strings, both for clarity, and to avoid errors.

Text size of dimensions should match the other text in notations.

Parallel strings of dimensions are useful, as they permit quick readings of small and large dimensions. To avoid extension lines crossing dimension lines, it is natural to place the smallest dimensions at the bottom, with the largest at the top. It is not always possible to avoid some line crossing, as column grid lines invariably cross the overall building dimension lines, but avoid unnecessary line crossing.

A typical plan dimensioning scheme is based on column centerlines, with an additional dimension at each end from the corner column centerline to the outside face of the building skin. Where the "skin" face is ambiguous, one needs a notation somewhere to clarify what the dimension actually signifies. Sometimes, the corner columns are dimensioned to their outside faces, in cases where those faces remain constant while the centerline moves (this happens more often with concrete than steel columns).

There are at least 3 options for dimensioning to walls and partitions:

Dimension consistently to the centerline;
Dimension to the face of the finish (e.g., the face of drywall in a drywall partition);
Dimension to the face of the stud, or other internal structuring unit (e.g., CMU or concrete covered with some other finish material).

There are different reasons for choosing one or another approach:

The contractor may prefer to work from a centerline, or from a face of stud;
The architectural concept may lend itself to a particular type of module that is reflected in the dimensioning scheme; so, for example, it may be simpler and clearer to have a string of dimensions reading "10'-0", "10'-0", "10'-0", etc. rather than "9'-7-1/2", "4-1/2"," "9'-7-1/2", "4-1/2"," etc.
Where a "clear" dimension, that is from finish to finish, is crucial (e.g., for code compliance), it may be prudent to use the "face of finish" scheme.

Note that where existing construction is part of the project, do not use dimension strings to indicate the existing dimensions; rather, just show the new construction in relation to the existing. Think of the intent of the drawings, which can never be to build what is already there. Where uncertainties are present, note VIF (meaning "verify in field").

Where different drawings show the same thing at different scales, attempt to avoid redundancy by showing the detailed dimensions only on detail drawings. Exceptions always are possible, if redundancy is justified by some overarching concern. See site plan for advice on datum for vertical (and horizontal) dimensioning.

Sections and elevations always have the same dimensioning scheme, on the right side if possible. This is an exception to the rule about redundancy that is justified by the need to relate each drawing to the grid of vertical dimensions. The "elevation mark" relating to the fixed datum point is generally shown on one floor (the same floor) only, and all other dimensions relate to it through dimension strings which pass through its extension line. There may well be instances where other "elevation marks" are useful (e.g., high point of roof), especially where establishing a datum clarifies what would otherwise might be confusing. Use judgement.
elevation mark and datum point

In general, leave a zone around the drawing for graphics and notations, outside of which the dimension lines are drawn. The column grid circles are the farthest items (in plans), just beyond the last dimension line.

Wall sections typically have at least two parallel dimension strings: the inner string for rough openings and top of slab (or subfloor); the outer string for floor-to-floor dimensions (i.e., top of slab or subfloor).

Ceiling height changes are awkward to show in section or elevation; they often show up in reflected ceiling plans, or finish schedule remarks.

Accuracy: try to avoid small fractional dimensions, especially those less than 1/16". Even that is pushing it a bit. Use the EQ EQ trick to subdivide areas, especially those with strange dimensions to begin with. The EQ dimension also has the advantage of displaying an intention, which is sometimes more important than the actual proposed dimension (which might end up disappearing within the reality of construction tolerances. The notion of using dimensioning to show your intention is important, and helps in ruling out (and ruling in) many dimensioning decisions:

dimensioning example
In the sketch above, assume that a cabinet (dotted line, NIC) must fit into the space shown. In that case, dimensioning the partition using "A" would not be as effective as dimensioning the same partition using "B."

In the sketch above, the EQ-EQ dimensioning trick is used, since it unambiguously indicates the intention. A dimensioning string with actual proposed dimensions is dangerous in this context, since any inaccuracies in the overall conditions might result in a noon-symmetrical result.

In the sketch above, the "ALIGN" notation is used to indicate that the two partitions should align. This is safer than showing independent dimension lines for each partition.

Use stable "datum points" for dimensioning. Do not tie some dimension string to a sloping roof beam, for example. Use the constant "top of parapet" or some other reference point.

Show dimensions to the side where the available space (for very short dimensions) cannot accommodate the notation.

NTS (not to scale) should be used in the drawing title where appropriate.

Use +/- after any dimension that is not intended to be precise (for example, because it includes some existing condition that cannot be accurately determined). Sometimes, the +/- note is combined with the VIF if your intervention or advice may be needed.