Complex sentence: They were all placed upside down, relative to the original direction of the tree, as the bottom of a tree is always wide than the top, and the greater width was needed to accommodate the joints with the beams that support the roof.
Split into simpler sentences: They were all placed upside down, relative to the original direction of the tree. The bottom of a tree is always wide than the top, and the greater width was needed to accommodate the joints with the beams that support the roof.