A scale drawing is an accurate representation of an area drawn onto paper or on a computer. For the estimator, a drawing provided by the customer, such as an architect’s plan of a new extension or an estate agent’s plan from the particulars of a house sale, can be really helpful. If the estimator can produce scale drawings of their measures, these can be used to help work out the estimate by making clear where seams may need to be included and to show how a pattern will run if there are alternatives available.
Modern computer-based estimating software always works to scale, but understanding what scales are and how to create such drawings, is a really helpful way to understand what these systems are doing and how to work with the plans they produce.
There isn’t a standard scale that must be used. Usually the scale is chosen so that the drawing will fit onto a standard size paper (A4, A2, etc). For domestic rooms this usually means 1:20, but for larger rooms it may need to be 1:50 and for whole buildings 1:100 or more. On a 1:20 scale drawing, every measurement is one twentieth of its real size. So if a wall is 4 metres long, the 1:20 scale size will be 200 mm (20 cm). If the drawing is 1:50 scale, it will be one fiftieth of its real size, so it will only be 80 mm (8 cm) long.
Working out the length of lines to put them into the drawing can be done in one of two ways. The ‘longhand’ way is to take the measurement and divide it by the scale size (using a calculator if this is easier, but mental arithmetic is quicker if the scale is a simple one). The other way is by using a scale rule. These may be flat or triangular and usually have a selection of different scales, so you simply find the scale you want to use, or which is used on the drawing. The rule then shows the actual measurements, already reduced by the ratio of the scale.
It’s important to remember that on a scale drawing any inaccuracy is exaggerated. For example, if the lines in the drawing are 0.5 mm wide, this equates to 10 mm at 1:20, 25 mm at 1:50 and 50 mm at 1:100. Always take this into account when reading from a drawing as that ‘thickness’ could mean the difference between flooring that fits and flooring that comes short.
In addition to this, be aware that when copied, drawings often go out of scale. They may state that they are 1:20 but enlargement or reduction may have changed the proportions. This can be checked by placing your rule against a line of known dimension and checking it is accurate to the scale. If it isn’t, there’s a simple formula to work out what the real scale of the drawing is:
Length of the line in real life (in mm) Length of the line as drawn (in mm)
This formula can also be used to create a drawing to any scale, so if you wanted your drawing to fill the page, you can simply divide the real length of the line you want to draw by the length you want it to fit into, but if this is not one of the standard scales on a scale rule, you may have to divide every measurement as you create the drawing, and anyone trying to read an odd scale will not thank you.
There are a number of conventions that are used when creating scale drawings and plans, especially by architects.
Actual dimensions, and any notes, are written so that they can be viewed when the drawing is the right way up, or when turned through 90 degrees. Dimensions and notes never go ‘round the clock’ so that they are upside down when the drawing is right way up. Where possible, dimensions and notes are placed outside the drawn area so it is easier to see.
Dimension lines are used to clarify what the dimensions refer to. These lines may have arrows or diagonal slashes at each end to show exact start and end points.
A reference line of an exact dimension may be shown alongside the scale so that the scale can be checked to ensure the drawing has not been altered.
Drawings will usually say ‘Do not scale’ to avoid liability if the dimensions are wrong or the actual area has been altered during construction works.
Whilst drawings can be very helpful when calculating rough prices, it is essential to carry out a site measure so that you are completely aware not only of the correct measurements, but also of any possible pitfalls the drawing does not show.
In the following article, we’ll look at what else an estimator needs to consider before working out the products and prices for the job.