Buildxact can easily measure the area of a raked wall, all you need to do is use the vertical M2 tool as per the video below, but when entering the height use the average height of the wall.
Using the image below, Average height = (H1 + H2) / 2
This method gives the same result as the manual way of calculating a raked wall. Please see below for both the manual and Buildxact formula to calculate the raked wall.
Manual way of doing raked wall area
Area of Shape1 = L* H1
Area of Shape2= (H2-H1)*.5*L
Shape 1 + Shape 2
= L * H1 + (H2-H1)*.5*L
= Length x Average height