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Competencies covered

MSFGN2001: Make measurements and calculations


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When two straight lines meet, they form an angle between them.

If the lines are walls in a square room, or the sides of a square box, they will form a right angle.

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Another way of referring to a right angle is to say it is 90° (degrees).

This is a reference to the amount of turn between the 2 lines.

That is, if you had a circle and drew a radius from the middle to the top, and then rotated it one quarter of a turn, you would have turned the radius through 90°.

One full turn around a circle is 360°.

This means that every angle formed between the two lines will be something less than that - for example, one quarter is 90°, half is 180°, three quarters is 270°.

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Setting out and marking lines at angles

The most common tools used by flooring installers to draw lines, set out angles and guide a utility knife while cutting are as follows.

Straight edge square, used for long lines at right angles to an edge.

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Carpenter's square, also called a builder's square, used for right angles and for measuring.

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Combination square, which allows you to set out 90° and 45° angles.

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Template tool, which can be used to transfer angles from the installation area onto the floor covering material.

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Using a level

Level means perfectly horizontal. A spirit level allows you to check that a surface or line is horizontal. It works on the principle that the bubble will find the highest point in a glass tube, because it is lighter than the surrounding fluid.

Since the tube is curved slightly with the highest point in the middle, the bubble floats exactly in the middle when the level is horizontal.

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You can also use a level to check whether a surface or line is plumb.

'Plumb' means perfectly vertical, and comes from a Latin word meaning 'lead'.

This is a reference to the plumb bob, which traditionally was always made of lead.

When a plumb bob is hung from a string, gravity draws the weight downwards, and the string forms a vertical line.

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Measuring diagonals

One way of checking to see whether a square or rectangular shape has right angled corners is to measure the diagonals.

This principle works because the opposite sides of a square or rectangle are always parallel - that is, the same distance apart at both ends. Therefore, if the corners are at 90°, or 'square', the two diagonals will be the same length.

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If the diagonals are not the same length, then the corners can't be square, even if the sides are still parallel.

It's worth keeping this in mind as a reminder that you can't simply measure the lengths of the sides to check that an item is square - this won't tell you whether the corners are at right angles.

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Using the 3, 4, 5 rule

Another way of checking a corner for square is to use the 3, 4, 5 rule.

This is an application of an old formula that Pythagoras, the ancient Greek philosopher, came up with over 2,500 years ago.

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Let's say you wanted to check whether the walls in the corner of a room were square, but it was a big open-plan room that didn't have opposite corners to measure.

The 3, 4, 5 rule states that if you measure 3 units along one wall and mark the point, and 4 units along the other wall and mark the point, the distance between the two points should be 5 units if the corner is square.

It doesn't matter what length a 'unit' is, as long as the proportions are 3, 4 and 5.

That is, your lengths could be 3 metres, 4 metres, 5 metres; or 3 feet, 4 feet, 5 feet; or 6, 8, 10 or any other multiple of 3, 4, 5.

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Learning activity

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The drawing below shows one corner of a room. The floor layer measures 1500 mm from the corner along one wall and marks the point. Then he marks 2000 mm from the corner along the other wall.

What length will the diagonal line be if the corner is square?

Enter your answer into the cell, and click on the 'Check your answer' button to see if you were right.

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