What Are Line Weights in Architectural Drawings?

The reader should see the cut first. In a floor plan, the walls the cutting plane passes through are the heaviest line. Doors, fixtures, and furniture beyond are drawn at a medium weight. Material patterns, such as hatching in a section, are light so they do not compete with the outline. Text, dimensions, and gridlines are fine so they sit behind the geometry.

Sections add one rule: the ground line and the profile of the cut are the heaviest of all, so the building’s outline against the earth reads at a glance. A section where every line is the same weight is technically complete and practically unreadable.

Line type is the pattern. Dashed lines show hidden elements, dash-dot lines show centerlines and property lines, and continuous lines show visible edges. Line weight is how thick each of those plots. A dashed line can be heavy or light, and the drawing conventions say which.

Mixing the two up leads to mistakes in a model. A property line that is the right type but plots at the weight of a wall will read as a building element, and a hidden line that plots heavy will compete with the visible geometry in front of it.

The National CAD Standard plotting guidelines define a ladder of plotted widths, such as 0.13, 0.18, 0.25, 0.35, 0.50, and 0.70 mm, continuing up to 2.00 mm. The ladder gives every office and consultant the same set of steps, so a plan from the architect and a plan from the structural engineer plot with a matching hierarchy.

The guidelines also warn about the finest widths. A line plotted at a very fine width can disappear when the set is reduced to half size, which is how many field sets are printed. Drawings plotted at full size on large sheets can read well on screen and fail on the half-size sheet in the superintendent’s hands.

In CAD, line weights are tied to layers or colors through a plot style table. In BIM tools, they are set by object category and by view, so the same wall can plot heavy in a plan and light in an elevation. A model with no consistent object style produces a set where the hierarchy changes from sheet to sheet.

Consultants who start from different templates are the usual cause. The coordination fix is to agree on the standard at kickoff and to check the first plotted sheets from each discipline together, before the set is far along.

A reviewer checks that the cut elements are visibly heaviest, that the annotation does not overwhelm the geometry, and that the sheet still reads after reduction. Plan reviewers and field crews both use half-size sets, so a drawing that cannot be read at that size fails in the field.

The check belongs on the plotted sheet, not on the model view. A hierarchy that looks right on a large monitor can break on paper, and the plotted PDF is what gets issued.