There will be times when the room you're measuring up is not square.

An angle is square if it is at 90 degrees - in other words, a right angle.

When a room is square, its opposite walls are parallel to each other, and the corners are all at 90 degrees.

The diagonal measurements are also the same, as you can see in the top drawing at right.

Note that saying a room is 'square' is not the same thing as saying it is 'a square'.

As you know, a square has 4 equal sides.

But in building terms, any rectangular room is 'square' when its walls meet at 90 degrees.

To check a room for square, all you need to do is measure the diagonals. If they're not the same, then the room isn't square.

Odd shaped rooms

Sometimes the room will be an unusual shape or have walls at odd angles.

In these cases, you need to use triangulation to draw it accurately to scale.

While you're on-site you should measure up all wall lengths, plus as many diagonals as you need to get a good picture of the walls' relative positions to each other.

Choose one wall as your 'reference wall', and use its corners as your starting point for the diagonals.

Mark all these measurements on your proportional sketch.

Once you get back to the office, you'll need to re-draw this sketch as a scale drawing.

Let's take the example shown at right and work through the process.

The first thing you need to do is get a scale rule, compass, pencil and blank sheet of paper.

Then follow the steps below.

1. Choose an appropriate scale for the size of paper you're using, leaving space for a margin around the outside so you can write notes in later.
   
   
2. Draw the reference wall to scale. In our case, the reference wall is A to B - 5940 mm.

3. Open the compass out to the distance between A and C on the scale rule - that is, the scaled equivalent of 5720 mm. Then put the point of the compass on A and draw an arc at the approximate position of C.
   
   
4. Open the compass out to the distance between B and C - 5180 mm. Then put the point of the compass on B and draw an arc that intersects the first arc.
   
   
5. Draw a line between B and C.

6. Go back to A and draw an arc at the approximate position of D, using the scaled measurement for 4200 mm.
   
   
7. Do the same for B to D - 6890 mm - so that the two arcs intersect.
   
   
8. Draw a line between A and D.

9. Draw a line between D and C
   
   
10. Check the lengths of your wall lines using the scale rule to make sure the proportions are accurate.