Like any Mediterranean theatre of that time, the Theatre of Marcellus had no roof: the audience relied on a temporary awning for inclement weather. But 17th century Oxford was not ancient Rome, and the Theatre needed a permanent roof. The span of the D-shaped roof was over . However, no timbers existed that were long enough to cross that distance, and Wren dismissed the obvious solution of a Gothic roof. Instead, he decided to use the "geometrical flat floor" grid developed twenty years before by Oxford professor John Wallis.