4Over the columns coupled beams are laid of such height as the magnitude of the work may require. Their width must be equal to that of the hypotrachelium at the top of the column, and they are to be so coupled together with dovetailed dowels as to leave a space of two inches between them. For if they are laid touching each other, and the air does not play round them, they heat and soon rot. Above the beams and walls the mutuli project one fourth the height of the column. In front of these members are fixed, and over them the tympanum of the pediment, either of masonry or timber. Above the pediment the ridge-piece (columen), rafters (cantherii), and purlines (templa), are distributed so that the water may drip therefrom on three sides.
4Upon the columns lay the main beams, fastened together, to a height commensurate with the requirements of the size of the building. These beams fastened together should be laid so as to be equivalent in thickness to the necking at the top of a column, and should be fastened together by means of dowels and dove-tailed tenons in such a way that there shall be a space two fingers broad between them at the fastening. For if they touch one another, and so do not leave airholes and admit draughts of air to blow between them, they get heated and soon begin to rot.
Above the beams and walls let the mutules project to a distance equal to one quarter of the height of a column; along the front of them nail casings; above, build the tympanum of the pediment either in masonry or in wood. The pediment with its ridgepole, principal rafters, and purlines are to be built in such a way that the eaves shall be equivalent to one third of the completed roof.