A spandrel is the roughly triangular space between the outer curve of an arch and the rectangular frame surrounding it. In broader construction terms, the word also refers to the panel of wall between the top of one window and the bottom of the window on the floor above, particularly on the exterior of multi-story buildings. These two meanings cover most of what you’ll encounter when the term comes up in architecture, renovation, or building design.
The Classic Arch Spandrel
The original meaning of spandrel comes from arched construction. When an arch sits inside a rectangular wall or frame, the leftover spaces between the curve of the arch and the square corners above it form spandrels. If two arches sit side by side, the space between them (above where the arches curve away from each other) is also a spandrel.
You can see this clearly in Romanesque and Gothic architecture, where rows of arches line the walls of churches, bridges, and civic buildings. The spandrel areas aren’t wasted space. Builders and artists historically used them for decorative sculpture, mosaics, painted figures, or carved ornament. In many medieval churches, spandrels display angels, saints, or floral patterns. The spandrels of the Sistine Chapel ceiling, for instance, contain some of Michelangelo’s most recognizable painted figures.
On bridges, spandrels serve a structural purpose. A spandrel-arch bridge has solid walls filling the triangular areas between the arch and the road deck above. An open-spandrel bridge instead uses columns or smaller arches in that space to reduce weight while still transferring loads from the deck down through the main arch to the foundations.
Spandrel Panels on Modern Buildings
In contemporary construction, the term spandrel most often refers to the opaque section of a curtain wall or exterior facade between the vision glass of one floor and the vision glass of the floor above. On a glass-clad office tower, for example, spandrel panels are the bands of non-transparent material that hide the floor slabs, structural beams, mechanical systems, and fireproofing between stories.
Spandrel glass is one of the most common materials used for these panels. It looks like the rest of the building’s glass facade but is made opaque through ceramic frit (a baked-on coating on the back surface), an opaque film, or a painted backing. This gives the building a uniform, all-glass appearance from the outside while concealing the structure behind it. Spandrel panels can also be made from metal (aluminum or steel), stone, precast concrete, or composite materials, depending on the building’s design.
These panels do more than hide unsightly structure. Spandrel areas typically contain insulation and serve as part of the building envelope’s thermal barrier. Building codes generally require fire-resistant construction in spandrel zones to prevent flames from traveling up the exterior between floors, a concept known as floor-to-floor fire separation. The height requirement for this fire-rated spandrel zone varies by jurisdiction but commonly falls in the range of 3 feet (about 0.9 meters).
Spandrels in Everyday Residential Construction
The term appears in smaller-scale building too, though less frequently. In staircase construction, the triangular area beneath the stair stringer (the sloped structural member supporting the steps) and above the floor is sometimes called a spandrel. This is the space that gets converted into under-stair closets, shelving, or storage nooks in homes.
In timber-frame and brick construction, you might hear the term used for the panel of wall between window heads and sills on adjacent floors, just as in commercial buildings. Older brick buildings often feature decorative spandrel panels with contrasting brickwork, terra cotta tiles, or carved stone between their window openings.
Why Spandrels Matter in Building Design
Spandrels sit at the intersection of structure, aesthetics, and building performance. On a practical level, the spandrel zone in a curtain wall system is where much of the thermal and moisture detailing happens. Poor spandrel design leads to condensation problems, thermal bridging (where heat escapes through the structure), and visible differences in color or reflectivity between spandrel glass and vision glass that make a facade look patchy.
Architects pay close attention to spandrel proportions because they define the visual rhythm of a building’s exterior. A tall spandrel band between short window strips gives a building a heavy, horizontal look. A narrow spandrel between floor-to-ceiling glass creates a lighter, more transparent appearance. The ratio of vision glass to spandrel panel is one of the first decisions in facade design, and it directly affects both energy performance and daylighting inside the building.
From a structural standpoint, the spandrel beam (sometimes called a spandrel girder) is the edge beam at the perimeter of a floor slab that spans between columns along the building’s exterior. This beam carries the load of the curtain wall or exterior cladding and transfers it to the columns. In concrete construction, spandrel beams are subject to torsion (twisting forces) because the load hangs off one side, making their design a specific engineering consideration.
The Term Beyond Construction
You may also encounter “spandrel” in evolutionary biology, where biologists Stephen Jay Gould and Richard Lewontin borrowed the architectural term in a famous 1979 paper. They used the spandrels of the San Marco cathedral in Venice as a metaphor, arguing that some biological traits aren’t direct products of natural selection but are byproducts of other adaptations, just as architectural spandrels are byproducts of fitting arches into rectangular frames rather than spaces designed on purpose. The metaphor stuck, and “spandrel” is now standard vocabulary in evolutionary theory for traits that exist as side effects of other evolved features.