As is widely known, the Ten Books on Architecture written by Marcus Vitruvius Pollio (c75BC- 15BC) is­ the only text on architectural principles to have survived from Classical Antiquity. He famously gave the prerequisites for good architecture as “Firmitas, Utilitas, Venustas” which is usually given in English as “Commodity, Firmness and Delight” (following The Elements of Architecture, 1624 by Sir Henry Wotton).

In Book V, Chapter 1 Vitruvius describes a Basilica which he says, “I erected and the building of which I superintended at Fanum Fortunae” (modern Fano).  Throughout the Ten Books, it is the only building Vitruvius claims to have designed. And it seems safe to conclude that it was the only one (or perhaps the only one of any significance) because the Ten Books are a sort of extended job application addressed to Emperor Augustus Vitruvius sets down all that he knows about design in the hope that he will be employed as the architect on some future project. Had he completed other buildings he would surely have said so, because, as an architect, it’s not so much what you know that counts, but how you apply that knowledge.

The Basilica is described in some detail and my interpretation of what Vitruvius says is shown on my drawings. (Vitruvius only writes, there are no drawings in the Ten Books.)

The first thing to note is the Basilica was a large building. Vitruvius says the columns are 50 Roman feet tall (about 47ft, 14.5m) with a base diameter of 5 Roman ft, approximately the same size as the columns in the temple of Castor and Pollux, or a little larger than those in the portico of the Pantheon. To give an English comparison, the columns in the portico of St Paul’s, London are 40ft tall. Given their size and the somewhat rustic, less grand, aspects of other parts of the design the columns are likely to have been made of rendered (plastered) brickwork, rather than stone.

The columns are Corinthian, and in Book IV, Chap 1 Vitruvius explains the origin of the Order (a delightful fiction) and gives rules for proportioning the capitals. He says the height of the capital should equal the base diameter of the column (5ft) and this is what I have drawn, also following Vitruvius’ rules for proportioning within the capital. (Most ancient and later Corinthian capitals would be a little taller than the base diameter.)

Classical columns diminish towards the top (i.e. they have entasis). Vitruvius gives rules for how much columns should diminish depending on their height, the taller they are, the less they diminish (Book III, Chap 3). Following his rules, the top diameter for the Basilica’s columns would be 4ft 4 ½ inches and this is what I’ve drawn. (However most later architects would make it 4ft 2 inches.)

With a base diameter of 5 ft and a height 50ft the overall proportions of the column are 1:10, these are canonic proportions for the Corinthian Order with which most architects would agree.

The most startling thing about the whole design is the curious way in which there are openings at high level around the column capitals – sort of clerestory windows. To modern eyes this looks entirely wrong, but Vitruvius is unambiguous on this point (Book V, Chap 1.6 & 1.7). However, its not unique, in some recent reconstructions a similar arrangement is shown on the basilica at Pompeii (albeit over the side aisles, not the main body).

Vitruvius gives the dimensions of the central part as 100 x 60 Roman ft and says there are 8 columns on the long side and 4 on the short. This results in uneven column spacing. He then says the roof gables form a “T” shape and implies the centre part of the long side is the same dimension as the short side. And this is what I have shown on my plan. I’ve taken the dimensions to the centres of the columns, this makes the central space a double cube. Such simple proportions always seem to be right.

The happy result of my setting out is the columns precisely follow Vitruvius’ rules for column spacing — intercolumnation (Book III, Chap 3): Systyle (2 diameters between columns) for those closely spaced on the long sides, and Diastyle (3 diameters between) for the short sides and the centre parts of the long sides.

Vitruvius warns that with the wide Diastyle spacing the architrave may break. There’s no risk of that in the Basilica because he says the architrave, and the whole entablature, are timber. Now that is unusual! The only other example he gives for the use of a timber entablature is for Tuscan temples. As far as I’m aware, Vitruvius’ Basilica is the only example in Classical Antiquity of a timber entablature used in conjunction with Corinthian columns.

Vitruvius describes timber blocks (4x4x3 Roman ft) in the frieze directly above the columns. Many have understood this to mean there are openings between, another set of clerestory windows. Surely this is unlikely? The amount of light grained would be minimal and such a feature would create a nuisance pigeon roost. There’s no reason why the spaces between wouldn’t be filled with timber boarding.

The timber blocks would undoubtedly be needed to ensure the colossal weight of the roof trusses is carried down to the columns. Also, the opposite: with a huge roof open on the underside, under certain conditions the uplift must have been formidable – a good connection, roof truss to column, would have been essential.

Usually the height of an entablature is between 1/5 and ¼ of the column height, So, for the 50ft columns of the Basilica it would be a height of something like 10ft to 12ft 6 inches. Vitruvius only gives heights for the architrave, 2ft, and the frieze, 3ft, he says nothing about the cornice. And a general Corinthian cornice is not described in the Ten Books. In fact, Vitruvius says Corinthian columns may have either an Ionic entablature or Doric, complete with triglyphs and mutules! (Book IV Chap 1) Generally in ancient examples the cornice is a little larger than the frieze, therefore on my drawings I’ve made it 3ft 6 inches, giving a total height for the entablature of 8ft 6 inches.

Assuming an entablature of reduced height was Vitruvius’ intention, it was a good decision. An entablature of more usual dimensions will look overwhelming when used above wide Diastyle column spacing, especially, as here, when part of an interior.

For the roof structure Vitruvius is silent. And there are no mentions of the roof structures, of any type, throughout the Ten Books. (Just one of several unfortunate omissions, for example there’s nothing on that most important Roman material: concrete.) So, what form was the Basilica roof? Vitruvius says the structure was exposed on the underside (another less grand counterpoint to the Corinthian columns). I’ve shown a series of king-post trusses, the universal common type. This would have been a challenge – there are only so many large trees, even 2,000 years ago. But whatever the truss form we know the Romans could have done it; for example, the basilica at Lepcis Magna had a span of 75 Roman ft (65ft, 20m) and that at Trier, 100 Roman ft (95ft, 29m).

The least clear part of Vitruvius’ description is the opening on the long side, “the two middle columns on that side are omitted, in order not to obstruct the view of the pronaos of the temple of Augustus (which is built at the middle of the side wall of the basilica, facing the middle of the forum and the temple of Jupiter) and also the tribunal which is in the former temple”. This is where drawn reconstructions vary most widely (see illustrations). It’s a mystery why the Temple of Augustus is described as “former”. Augustus would only recently have been deified. Could it be that something was hastily erected, or perhaps converted, to accommodate the new cult of the Emperor while something far more worthy was being constructed, and then, when it was completed, the first temple was incorporated as part of the Basilica?

Earlier I gave the less tall entablature as an instance of sound visual judgment. Other parts of the design are less successful. For example, Vitruvius says there are pilasters behind the main columns supporting the first-floor aisle and, in turn, the aisle roof. Structurally, they’re unnecessary and they compromise the soaring quality of the columns. It would have been better to leave them out.

Also, as my drawings show, the overall design is like a hybrid of two buildings: the grand central space, defined by the Corinthian columns, and the more humble side aisles (on two levels). A far more satisfactory solution would be to have the aisles as tall as the columns and all under the same roof.

Another example: I’ve tried to make the internal elevations elegant but there’s something decidedly awkward about the clerestory windows above the side aisle roofs. Was this an arrangement that Vitruvius thought a clever solution to the problem of getting light into the heart of the Basilica, but which others found less successful? And how easily could one have seen the Corinthian capitals? From the comparatively dark interior of the Basilica they would be seen more as silhouettes against a bright Italian sky.

It’s not unknown for an architect to design something to solve a practical problem but which then doesn’t quite work visually. But that’s what so enticing about architectural design: all good architects are striving to achieve the Vitruvian balance of Commodity, Firmness and Delight – Firmitas, Utitas, Venustas.