Precast building material

A precast building material comprising:

• a substrate formed in a parallelogram shape;
• a peripheral frame mounted around a peripheral side of the substrate so that an accommodation chamber is defined between the peripheral frame and the substrate, wherein the peripheral frame includes multiple first connection rails, and a respective first connection rail has at least one first slot defined thereon, wherein at least two sides of the peripheral frame are screwed with multiple second connection rails, and a respective second rail has at least one second slot defined thereon, the peripheral frame includes multiple first engagement portions fixed on the at least two sides thereof and multiple second engagement portions, wherein a respective second engagement portion mates with a respective first engagement portion, the respective first connection rail has multiple first locking orifices defined thereon, and the respective second connection rail has multiple second locking orifices formed thereon;
• a metal reinforcement unit accommodated in the accommodation chamber and welded with an inner wall of the accommodation chamber, wherein the metal reinforcement unit includes multiple rebars, and the multiple rebars are intersected with one another;
• and
• a concrete layer paving the accommodation chamber to cover the metal reinforcement unit, such that the concrete layer is connected with the peripheral frame and the metal reinforcement unit and is removable from the substrate.