A speed square or rafter angle square is a triangular-shaped measuring tool used to draw straight lines on lumber to be cut, or to lay out angles for roofs, stairways, decks and other general construction. The tool is marked with degree gradations for fast layout and cutting of lumber without performing complex trigonometry calculations. It was invented in 1925 by Albert J. Swanson. He called it the "Speed Square," since it provides a short cut for measurements and calculations of angles.
Speed Square is a registered trademark of the Swanson Tool Co., Inc., which continues to sell it under that name, though the name itself has become a genericized trademark for all rafter angle squares. Modern Swanson squares can be differentiated from other manufacturer's rafter angle squares by their SPEED trademark and their trademarked diamond shape cutout at the 3½ inch mark, which assists in making birdsmouth cuts.
Generally, the tool comes in two sizes - the compact 7 inch version for portable use and the larger less portable 12 inch version for stationary work. They may be made of aluminum, steel, HDPE, and other composite materials.
A version of the 7 inch square is made by Stanley tools under their name "Pocket Square".[1]
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The tool is shaped in an isosceles triangle and is marked with the word "Pivot" at the right angle point, Degrees on its hypotenuse, Common and Hip/Val markings on its mid-section parallel to the hypothenuse, a ruler on one of the equal sides, and a fence on the other equal side. Some may have divots for fitting a writing utensil to mark lumber with. Genuine Swanson Speed Squares will also have a diamond shape cutout on the ruler side at 3½ in.
from 1 in. to 30 in.
Swanson Co. advertises that the Speed Square can be used as a, "Try Square, Miter Square, Protractor, Line Scriber, & Saw Guide". The original Swanson Speed Square is famous for its "Little Blue Book" that accompanies the tool. The pocket sized reference book describes uses and methods for maximizing the utility of the tool, but more importantly, includes charts listing rafter lengths for the width of the building (from 3 to 40 ft). Generic versions of the tool do not come with any instructions.
The tool is used to cut Common, Hip, Valley and Hip or Valley Jack rafters, layout stringer in stair fabrication, mark angles, determine angles, and to square off boards. It serves as a convenient straightedge and guide for circular saws (professionals recommend that the square be clamped when cutting splayed angles for safety).
Although there are slight differences in preparation, the usage of this tool is the same for measuring Degrees, Common, and Hip/Val.
The square consists of three parallel lines to its hypotenuse with markings on them. There is a degree line with increments of 1°, There are a common, and a hip-valley lines in increments of a one pitch to a 30 pitch. The intersection of the pivot point (located at the tip of the 90° angle on the square) and where the readings coincide on the edge of the board determines the pitch or angle. The 12 inch square can be used to lay out stairs.
This tool uses a 0 degree reference. This means when a board is squared off the tool reads 0°. The angle derived is actually a complementary angle. For example a 22.5° angle is actually 67.5°. The sum of the angles equals 90° (22.5° +67.5° = 90°). It is obvious from a visual check that where the instruments displays 22.5° is not 22.5°. Many of the new slide miters and miter boxes display both angles. Some of the new calculators have a 0° and a 90° references to do angular calculations. This can create much confusion if it is not clearly understood.