Metric Scales

According to the American National Standards Institute (ANSI), the International System of Units (SI) linear unit commonly used on drawings is the millimeter. The American National Metric Council, in its publication American Metric Construction Handbook, recommends the following with reference to metric drawings:

1. Architectural working drawings are to be dimensioned in millimeters (mm) and meters (m).

2. Plot plans and site plans are to be dimensioned in meters (m) or possibly kilometers (km), depending upon the scale, with accuracy to only three decimal places.

Figure 4.5A A drawing showing scales in imperial (1/4" = 1'-0").

Figure 4.5A A drawing showing scales in imperial (1/4" = 1'-0").

Figure 4.5B A drawing showing scales in metric (1:50).

3. No periods are used after the unit symbols.

4. Scale on drawings is to be shown by a ratio (1:1, 1:10, 1:50, etc.).

The preferred method of metric dimensioning is called a soft conversion, in which common metric modules are used. Thus, 2-x-4 lumber is 40 x 90 mm using soft conversion. This method is far more convenient when drawing plans and measuring in construction. When measuring plywood thickness in met-

Figure 4.6A A drawing of a stair detail drawn to a scale of 1 1/2" = 1'-0".

ric units, 5/8 inch thick equals 17 mm and 3/4 inch thick equals 20 mm. Using the same method, the length and width of sheet of plywood change from 48 x 96 inches to 1200 x 2400 mm. In countries that have adopted the metric system, the dimensioning module is 100 mm, whereas in the United States architectural design and construction modules normally used are 12 or 16 inches. Thus, construction members that are spaced 24 inches on center (O.C.) in the United States translate into 600 mm O.C. spacing in Canada or the United Kingdom.

Figure 4.6B A drawing of a counter detail drawn to a metric scale of 1:10.

When reading metric dimensions on a drawing, all dimensions within dimension lines are normally in millimeters, and the millimeter symbol (mm) is omitted unless more than one dimension is quoted, in which case the symbol (mm) is included after the last dimension. Drawings produced in metric, such as floor plans, elevations, and sections, are normally drawn to a scale of 1:50 or 1:100, as opposed to the 1/4 inch = 1 foot, 0 inches or 1/8 inch = 1 foot, 0 inches scale used in the imperial system. Figure 4.5 shows examples of drawings drawn to 1:50 and 1/4 inch = 1 foot, 0 inches scales.

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