You’ve hit upon one of the major issues with the theory of “petrification” in ancient Greek architecture.

Petrification is the idea that the stone architecture of the Greeks was a direct translation of designs already perfected in wood. This theory comes to us from Marcus Vitruvius Pollio, an architect in Rome in the first century B.C.E. His De architectura libri decem (Ten Books on Architecture) is the only surviving complete architectural treatise from the ancient Western world. Vitruvius’s treatise explores all aspects of Roman architectural design and construction as well as the history of architecture in Greece and Etruscan Italy. His writing has been the source for much of what we believe about the architecture of ancient Greece and Rome, including the origins of the three main orders: Doric, Ionic, and Corinthian. As Vitruvius’ writing cites other texts now lost to us and it is the only comprehensive written evidence we have concerning the ancient architecture of Greece and Rome, it has long been accepted at face value.

On the subject of the triglyph of the Doric order, Vitruvius has this to say:

The craftsmen of old, building in some place or another, placed joists that protruded from the interior walls to the outer edges [of the buildings]. They built in between the joists and above them decorated the cornices and eaves with fine carpentry for a more attractive appearance. Subsequently they decided that these projecting joists should be cut off where they protruded beyond the plane of the walls, and because the result looked unattractive to them, they fitted plaques in front of the cuttings, which were shaped as triglyphs are made today, and they painted these with blue wax so that the cut ends of the joists would not offend the viewer (Figure 61). And thus the covered sections of the joists in Doric works began to take on the arrangement of the triglyphs and, between the joists, the metopes. [Vitruvius, De Architectura, Book 4, Chapter 2]

Vitruvius’ explanation for the origin of the triglyph was accepted when De architectura gained a new appreciation in the fifteenth and sixteenth centuries. Inspired by Vitruvius, Leon Battista Alberti, Sebastiano Serlio, and other Renaissance architects composed theoretical treatises of their own and employed aspects of ancient Greek and Roman architecture—including the orders—in their designs. Since most of Greece was under Ottoman occupation at the time and access to first-hand observation of ruins was limited (though, of course, examples of Greek architecture are to be found in southern Italy and elsewhere), architects of the Renaissance for the most part believed Vitruvius’ assertions.

It was only in the eighteenth century when figures like Julien-David Le Roy and James Stuart and Nicholas Revett were able to measure and document sites like the Acropolis in Athens and produce engraved reconstructions that were printed and circulated as bound volumes. At this time, amateur archeologists began to notice the contradictions between Vitruvius’ account and the built reality, bringing the theory of petrification into question and birthing an investigation into the origins of architecture, beginning with the “primitive hut”.

Works like Marc-Antoine Laugier’s 1753 Essai sur l’architecture grappled with the transition from the earliest and crudest forms of shelter to the stone architecture of the Greek orders. Such logical and methodical analysis reflected a trend in architecture toward structural rationalism—that is, the belief that the form of a building should be determined by its function and the materials and methods of its construction. In the nineteeenth century, structural rationalists like Eugène Viollet-le-Duc wrestled with reconciling the archeological evidence from ruins with the existing theory of petrification as proposed by Vitruvius. Among the most troublesome questions was the origin of the Doric order.

Viollet-le-Duc rejected Vitruvius’ assertion that the triglyph originated as a painted plaque covering the end of a beam. Instead, he asserted a structural origin for the decoration of the peripteral temples of ancient Greece. Viollet-le-Duc observed that wooden beams would not meet at a corner, yet Doric temples have triglyphs at the corners. Nor would beam ends protrude on the temple sides, yet the frieze of alternating triglyph and metope was continuous around the exterior of the Doric temples. The height of the interior ceiling, as well, did not correspond to supporting members at the level of the frieze. Viollet-le-Duc proposed instead that the triglyph was a structural element supporting the cornice and the roof. His systematic dismantling of De architectura stimulated both architects and archeologists to question the received wisdom of Vitruvius.

Throughout the twentieth century, several other alternative origins for the triglyph were proposed. These include the chamfered ends of exposed composite beams, supports for windows within the frieze in place of the metopes (something Vitruvius dismisses in his text), carved channels to redirect rainwater (when combined with the adjacent regula and guttae), the adoption and alteration of Egyptian designs, and the simple application of already existing decorative motifs, possibly drawn from representations of a tripod form often seen in vase painting or even the thighbones produced by ritual sacrifice.

Petrification remained a compelling theory, because certain aspects of ancient Greek stone architecture, like the subtle bulging (entasis) of columns, seem to bolster it. But the archeological evidence does not support a sudden shift from wood to stone as Vitruvius claimed. Recent scholarship by archeologists such as the late Barbara Barletta suggests the Greeks built in mud brick, terra cotta, wood, and stone simultaneously, using each material as appropriate and only recreating elements in a longer-lasting material when desired. The origin of the stone triglyph thus remains a mystery.

Sources:

Barletta, Barbara A. The Origins of the Greek Architectural Orders. Cambridge: Cambridge University Press, 2009.

Rykwert, Joseph. The Dancing Column: On Order in Architecture. Cambridge, Mass.: MIT Press, 1998.

Vitruvius: Ten Books on Architecture. Cambridge: Cambridge University Press, 2001.

Wilson Jones, Mark. Origins of Classical Architecture: Temples, Orders and Gifts to the Gods in Ancient Greece. New Haven: Yale University Press, 2014.