Generally, gables are triangular, and, most often, isosceles triangles. As a consequence, we can calculate the approximate number of boards that will be necessary.
We have an isosceles triangle with a base of 20', and a height of 8'. The area of that triangle will be given by A = ½(base x height). In this case, the value for A will be 80ft2.
The area of each board will be given by A = length times width, which will be 8' x 3". But, 3" = 0.25', so we have 8' x 0.25' = 2ft2.
So, the area we need to cover is 80ft2 divided by 2ft2 per board, which equals 40 boards.
In an actual situation, full length boards will be installed first, and then other boards will be cut to fit in the remaining spaces. The process is less than exact, and there will be some waste. Additionally, since the gable is a triangle, and the boards are rectangles, inserts will have to be made to fill the gaps between the frame and the boards.
