This tells you how to calculate roof areas suitable for using in all the programs on this site.
It is the Horizontal area that is required, (not the sloping, actual area).
The effect of the roof slope is allowed for by entering the angle of the roof in the programs.

In the example above the total catchment entering the box gutter would be (A * B) + (C * D).
Should the vertical face on the left hand side (LHS) be quite big (say one storey), then the area to be added
to the box gutter catchment would be half the vertical area. ie (A * E)/2.

Any roof can usually be divided up into squares and triangles.
So, the total catchment area of this roof is = (A*B) + (C*E) + (D*E)/2


However if you have a PDF file/drawing of the roof plan, it is very easy to get the roof area.
check out the Video on how to do this.