The most prominent and long-lasting of the timber truss-roofed buildings from imperial Roman times was the Basilica Ulpia, the interior of which has been reconstructed by recent writers but with little agreement regarding the upper sections. The reconstruction that best passes the test of engineering viability is shown here to be the one published by James E. Packer in 1997, but Packer’s reconstruction is unconvincing with regard to the roof. The argument advanced now is that the roof was constructed with double trusses of the kind once found in the early Christian basilicas of Old St Peter’s and San Paolo fuori le Mura, and then recorded in the eighteenth century by Jean-Baptiste Rondelet, which were very similar in size to those used for the Basilica Ulpia. The spacing of roof trusses was determined by the nave column spacing. Engineering analysis determines that the trusses had sufficient capacity to support the roof and that the double-truss arrangement was critical to avoid overloading the stone architraves. The article speculates that doubling the Roman truss may have been a previous innovation. It argues that the later early Christian basilicas relied on the Roman double truss because of its proven performance, and that this continuity was recognised by Auguste Choisy and others who considered their use to be a reflection of earlier Roman practice.