Measure the length of the roof from one end to the other along the eaves. Write this figure down on a notepad.

Then measure the width of the gable end, or the hip end, and divide this number by two. Write down on the notepad the figure that you get after dividing by 2.

Now multiply together the two figures on your pad. Here’s an example;

The first figure you write down will be 10. The second figure will be 3 (6 divided by 2). Multiplying these two together gives 30 (10 x 3).

Now for the clever bit. To allow for the slope of the roof you need to multiply your roof area figure by the factor shown in the table below, depending upon the pitch of your roof.

In our example above, let’s assume we have a roof pitch of 45°. Simply multiply our initial figure of 30 by 1.5 to give an effective roof area of 45m2 (30 x 1.5).

Note: If you are unsure of the exact pitch of your roof, choose a pitch that is more likely to be too steep than too shallow – most domestic roofs are pitched somewhere between 35° and 45°.