What is a bay in architecture?

A bay is the section of a building or facade that lies between two successive supports. Those supports can be columns, piers, posts, buttresses or portal frames. Because they repeat at regular intervals, a building reads as a row of equal compartments side by side: the bays. In Dutch and French the word is travée, from the Latin trabs, beam.

In churches and historic buildings

• In a church, a bay is usually the compartment between two piers of the nave, with the vault bay above it. Inside, the boundary is often marked by a transverse arch; outside, by a buttress.
• A nave is therefore often described by its number of bays: “a nave of five bays”.
• Sometimes the term refers only to the vault panel between two transverse arches.
• On historic facades, bays are also the vertical divisions with windows stacked above one another: a “five-bay facade” has five window axes. A rhythmic bay design alternates narrow and wide bays, or open and closed sections.

In modern construction

In contemporary building, a bay is usually the space between two columns or frames, for example in a warehouse, car park or office. The bay width or grid dimension (the distance between supports) determines:

• the span of beams and floors, and therefore their depth and cost;
• how freely the space can be laid out, for example for parking spaces or office units;
• the module of facade panels and windows.

Tip

When converting a barn, warehouse or shed, first count the bays and measure the bay width. New walls, doors and openings are easiest to line up with the column grid.

Related terms

• Vault bay
• Bay width
• Transverse arch
• Buttress
• Nave
• Column

Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.