Don't understand how the equation to calculate wall area in Thermal Model of a House is derived.

Good day,

Currently I am studying the Thermal Model of a House example in MATLAB, which is described below:

http://www.mathworks.com/help/simulink/examples/thermal-model-of-a-house.html#zmw57dd0e1255

The equation used to calculate wall area available in sldemo_househeat_data is written below:

wallArea = 2*lenHouse*htHouse + 2*widHouse*htHouse + ...
           2*(1/cos(pitRoof/2))*widHouse*lenHouse + ...
           tan(pitRoof)*widHouse - windowArea;

However, there are 2 terms that I don't understand with the equation above, which are:

1. 2*(1/cos(pitRoof/2))*widHouse*lenHouse
2. tan(pitRoof)*widHouse - windowArea

Do you have any idea how can we get/derrive thes terms? I have tried to derrive on my own, but so far I could not get it.

Thank you very much.