Blueprint symbols

About blueprint symbols

Blueprint symbols are the small drawings that stand for doors, windows, fixtures, outlets and materials on a building's plans. Most construction drawings are drawn at a small scale, so standard symbols stand in for the items and materials. On a floor plan, a symbol has the same general shape as the feature it stands for and shows any motion it makes, such as the swing of a door. Each symbol has its own page.

What a floor plan shows

A floor plan is a cross-section of a building, cut horizontally through every door and window opening, whatever its height above the floor. It shows the outside shape of the building; the arrangement, size and shape of the rooms; the walls and what they are made of; the doors and windows; utility installations; and the stairs. Distances on a plan are horizontal: they measure between two points without allowing for slopes in the ground.

Windows open at right angles to the page, so a plan can't show their motion, and window types are told apart by their lines instead. A 1920 textbook on blueprint reading notes that a window drawn with two lines instead of one is double hung.

The legend comes first

A drawing's legend, its key to the symbols, is the final word on what they mean there. NECA 100, the North American standard for electrical construction drawings, allows other symbols as long as their meaning is explained on the drawing or on a symbol legend sheet, and a Navy drafting course lets drafters devise their own symbols if notes or a legend explain them. Drawings also stay in use for years, so older plans can carry symbols that are no longer standard. A 1922 guide to reading architects' blueprints already advised putting a key to the symbols on the drawings.

Differences between drafters and standards

In the United States, the Navy's drafting course names the military drawing standards (MIL-STD) and the American National Standards Institute as the main sources for a symbol. MIL-STD-14A, the Defense Department's list of architectural symbols from 3 September 1954, allowed simpler symbols on smaller-scale drawings as long as the basic idea was kept. NECA 100 covers North American electrical symbols only, and leaves out those of the International Electrotechnical Commission (IEC).

Smaller details vary between drafters and agencies. A Dimension line may end in slashes or in filled arrowheads, and a North arrow can point to plan north, site north, grid north or magnetic north, depending on the drawing.