A building material whereinside an accommodating space is formed is disclosed. The building material includes a first area whereon at least one protruding part and at least one sunken part are formed alternately in a first direction, and a second area whereon at least one protruding part and at least one sunken part are formed alternately in the first direction. The protruding part of the second area aligns with the corresponding sunken part of the first area in a second direction substantially perpendicular to the first direction, and the sunken part of the second area aligns with the corresponding protruding part of the first area in the second direction.