Right angles
Introduction
A common angle used in building is the right angle.
A right angle is an angle of 90°.
Builders use two common tools to check or mark 90° angles.
Try square
The try square forms a perfect right angle.
The angle between the two straight edges is exactly 90°.
Combination square
The combination square has two straight edges at exactly 90°.
It also includes two sides that have an angle of 45°.
Video – Using a square
The video shows a builder using a try square and a combination square. He shows how to mark out a piece of timber at:
- 90 degrees with the try square
- 90 degrees with the combination square
- 45 degrees with the combination square.
Press play to start the video
3, 4, 5 principle
All triangles that have side lengths with the ratio 3, 4 and 5 have a 90° angle between two of the sides.
To mark out a 90 angle over long distances, multiplying the length of the sides of the triangle but keep the ratio of 3, 4, 5.
An example
If you multiply the '3' side of the triangle by 4, making it 12 m long (3 X 4 = 12 m)
then you also multiply the other two sides by 4 (4 x 4 = 16 m, and 5 x 4 = 20 m).
The sides of the triangle still have a ratio of 3, 4, 5 and so have a 90° angle.
Video – 3, 4, 5 principle
The video shows a builder marking out a right angle on a building site using the 3, 4, 5 principle. First two pegs are placed four metres apart. Two tape measures are used from each peg. A third peg is place where the three and five metre points of the tapes intersect.
Press play to start the video
Summary
This is the end of the section on right angles.
Key points to remember are:
- a right angle is a 90° angle
- a 90° angle can be marked or checked using a square or the 3, 4, 5 principle.
You can review this section or choose another from the left hand menu.