‹‹‹ Vitr. 6.3.3 | Table of Contents | Vitr. 6.3.5 ›››
4Their height, to the underside of the beams, is to be one-fourth less than the length; the remaining fourth is assigned for the proportion of the lacunaria and roof above the beams. The width of the alæ, on the right and left, when the atrium is from thirty to forty feet long, is to be one third part thereof. From forty to fifty feet, the length must be divided into three parts and a half; of these, one is given to the alæ: but when the length is from fifty to sixty feet, a fourth part thereof is given to the alæ. From sixty to eighty feet, the length is divided into four parts and a half, of which one part is the width of the alæ. From eighty feet to one hundred, the length is divided into five parts, and one of them is the true width of the alæ. The lintel beams (trabes liminares) are placed at a height which will make the breadths and heights equal.
4Their height up to the girders should be one fourth less than their width, the rest being the proportion assigned to the ceiling and the roof above the girders.
The alae, to the right and left, should have a width equal to one third of the length of the atrium, when that is from thirty to forty feet long. From forty to fifty feet, divide the length by three and one half, and give the alae the result. When it is from fifty to sixty feet in length, devote one fourth of the length to the alae. From sixty to eighty feet, divide the length by four and one half and let the result be the width of the alae. From eighty feet to one hundred feet, the length divided into five parts will produce the right width for the alae. Their lintel beams should be placed high enough to make the height of the alae equal to their width.