Trade drawing symbols

About trade drawing symbols

Trade drawings are the working drawings a building is built from. With the written specifications, they are the main source of information for the people who do the work, and they describe the structure, the site, the materials and how it is to be built.

A set usually has general drawings, the plans and elevations, and detail drawings, the sections and details, with separate plans for trades such as plumbing and electrical work. Because most are drawn at a small scale, they rely on standard symbols. This collection explains four sets of them: the blueprint symbols of architectural floor plans, and the welding symbols on drawings of welded work, the GD&T symbols on drawings of machined parts, which replace long written notes on a part’s form and position with symbols, and the P&ID symbols on the piping and instrumentation diagrams of process plants, which show how pipes, pumps, valves and instruments connect and are drawn to no scale.

Telling the two apart

A blueprint symbol is drawn where the thing it stands for will be. On a floor plan it has the same general shape as the feature, a door, a window or a fixture, and shows any motion it makes, such as the swing of a door. A welding symbol is assembled from standard parts: a reference line, an arrow connecting it to the joint to be welded, weld symbols for the kind of weld, and a tail when a specification or process needs naming.

Arrows appear in both. A dimension line may end in filled arrowheads or in slashes, and its ends mark the two points being measured. A welding symbol has one arrow, and it points to a joint.

Where a mark sits

A floor plan is a horizontal cut through the building, taken through every door and window opening whatever its height. On a welding symbol, a weld symbol below the reference line calls for a weld on the arrow side of the joint and one above the line for a weld on the other side, and the line is read from left to right whichever way the arrow points.

Who sets the symbols

The Navy’s Engineering Aid course names the military drawing standards (MIL-STD) and the American National Standards Institute as the main sources for architectural symbols, and NECA 100 covers North American electrical symbols. Welding symbols were standardized by the American Welding Society; drawings made to the international standard, ISO 2553, use a dual reference line, one continuous and one dashed. Many drawings stay in use for years, the Navy course adds, so older ones can carry symbols that are no longer standard.