Its best-known use is clearing the rubble that blocks the underground passages under your home, the well, the vineyard and the warehouse. Each blockage takes one beam and 4 Iron Details, and the mine passage takes 4 Building Kits I instead of the details. It also goes into second-tier buildings: 6 for the Assembly bench II and the Smithy II, 4 for the second Kitchen and 2 for each second-tier barricade and archer tower.
Do not confuse it with the Reinforced Plank, the lighter piece the first barricades are built from.
Your first cleared passage opens the Factory. Clearing either blockage under your home, the first one at the vineyard or at the well, or the mine passage removes the darkness over the Factory and starts the conveyor work; the second ones at the vineyard and the well do not.
The vineyard blockages need only Building mastery 3; the others need 8.
The Builder sells beams from tier II, 10 in stock and 20 at tier III.
Studying one costs 10 Science and gives 30 red and 10 blue.
Oiled Beam ×1 
Iron Detail ×1 
8
Reinforced Beam ×1 
8