Perhaps not one of the innumerable visitors to the familiar monument of the Herodeion has heard or even considered that it was the tallest building in the country until the era of the Hilton and Athens Tower, or that it must, even today, hold the world record for the largest span of a roof (50 m) made from natural timbers without intermediate supports. Despite the literary references to a roof, most archaeologists and architectural historians assert that spanning the enormous distance separating the walls of the Herodeion was simply impossible. And yet, even in its current ruined state, the building preserves structural details that show without doubt that a roof once existed, and at the same time indicate the basic arrangement of its trusses. The reconstruction proposed here by the author is based on all the surviving evidence from the building itself, and takes into consideration what is known about the great achievements of the Romans in this particular area, including bridge-making, the achievements of wooden construction in general prior to the industrial age, and what is known about the scientific knowledge of the ancients. The first part, in the form of a story, sets the historical framework and the protagonists of the project, the second contains the scientific documentation of the construction of the building and its roof, while the third part presents historic wooden bridges and roofs, a knowledge of which, apart for its intrinsic value, is invaluable for understanding the roof of the Herodeion.