Rules · Light · Façades · Doors & windows
Raise the central nave above the lean-to roofs of the aisles and pierce the upper walls with a row of windows, the clerestory, so that the middle of a wide hall is lit from high on both sides.
nave height ≈ 1½–2 × nave width; one window over each bay of the arcade
The clerestory is the basilica’s essential device: without it a hall wider than a single span would be dark in the middle. Stepping the section (high nave, low aisles) lets the light fall onto the floor of the nave and onto its mosaics while the aisles stay in shadow, giving the church a luminous central axis.
The arrangement was inherited from the Roman civic basilica and, earlier still, from the Egyptian hypostyle (see Raise the central aisle and light it from above). Romanesque and Gothic builders kept it while replacing timber roofs with vaults; in the Gothic cathedral the clerestory grows until the upper wall becomes almost all glass.
In a bonded masonry wall the courses above an opening corbel over it of their own accord, so the lintel carries roughly the triangle of wall with sides rising at 60° from its ends; everything above that triangle arches onto the jambs.
load ≈ equilateral triangle on the span: W ≈ 0.43 · L² · t · ρ g
Over a doorway or window spanned by a lintel, turn an arch in the wall above so that the weight of the masonry is carried round the opening onto the jambs, leaving the lintel to carry only the small triangle of wall beneath the arch.
Gather the weight and thrust of the vaults onto a few points by ribs, carry them down piers and out across the aisles by flying buttresses to external buttress piers, and the wall between the supports carries almost nothing, so it can be opened into windows.
Divide the elevation of the nave into three storeys: a tall arcade opening into the aisles, a low triforium passage in the thickness of the wall against the aisle roof, and a clerestory of large windows reaching up into the vault.
Chartres: arcade ≈ clerestory, triforium a narrow band between
Proportion the cross-section of a church from the square: let the total height to the crown of the nave vault equal the total width, and take the heights of the aisles and the springing of the vaults at whole divisions of that square.
H = W · levels at k × (W ⁄ n)
Alternatively, fit the section to an equilateral triangle standing on the width, so that the height is √3⁄2 of the width, and take the levels at equal divisions of that height.
H = W · √3⁄2 ≈ 0.866 W · Milan: 96 braccia wide → 83.1, rounded to 84 = 6 × 14
Cite this page Chris Paton, “Light the nave from above the aisles”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/basilica-clerestory. Text, drawings and models licensed under CC BY-NC 4.0.