A back blocking device comprising an elongated member having a longitudinal axis and a first and second side. The first side of the elongated member has a first outer surface and a second outer surface that converge at the longitudinal axis. The first side has first raised portions and first recessed portions. The second side has a first inner surface and a second inner surface converging at the longitudinal axis, and second raised and recessed portions inverse to the first raised and recessed portions on the first side. First and second pieces of wallboard are positioned on either the first or second raised portions on either the first side or the second side and converge for attachment to the first or second side of the elongated member substantially near the longitudinal axis to form a butt joint.