Possible meanings are 1) "beams" refers to large logs used to support everything above the walls and "rafters" refers to the large pieces of wood to which the roof is attached or 2) "beams" refers to the rafters and "rafters" refers to the strips attached to the beams, onto which the builders attached the roofing materials.
Cedars were large and strong trees. The word translated "firs" is a general term for trees like cedars but smaller. If cedar and fir trees are unknown in your area, you could use general terms for the tallest and strongest trees. (See: [[rc://en/ta/man/jit/translate-unknown]])