here is a short HowTo create objects like beams, wooden substructure in old houses and so on. This time we don't use the "Box".
the picture below shows the first step pretty clear: you create walls with the dimensions of the wanted structure, like 5 meters long and 20by20cm in thickness:
these "walls" can be exported as OBJ and then re-imported to the furniture library in SH3D, preferably in an own category to find it back easily. Make sure, you use proper dimensions and angle while importing!
macfrog