Arch Terminology, Types, and Structural Mechanics

What an Arch Does

An arch is a curved or angled structural assembly of masonry units that spans an opening and carries load without relying on a lintel or beam. Its genius lies in geometry: vertical loads from the wall and roof above are converted into compressive forces that travel outward and downward through the arch members and into the supporting walls or piers on each side. Because masonry is strong in compression and weak in tension, the arch exploits exactly the right property of the material. Understanding how this works—and being able to name every part of the arch—is foundational to NCCER Masonry Level 2 competency and to safe, code-compliant construction.

Anatomy of an Arch: Part by Part

Span is the horizontal distance measured between the two supports. Rise is the vertical distance from the springing line to the highest point of the intrados. The ratio of rise to span is one of the most useful numbers in arch work because it determines how much horizontal thrust the arch generates.

The springing line is the imaginary horizontal line connecting the two points where the arch curve begins. At each of those two points sits the skewback, the angled bearing surface cut or formed into the supporting masonry so that the first voussoir sits at the correct angle to begin the curve.

The individual wedge-shaped masonry units that make up the arch are called voussoirs. Their tapered shape—wider at the top edge and narrower at the bottom—allows them to lock together under load. The topmost voussoir, the one that locks the arch into compression, is the keystone. It is placed last during construction.

The curved underside of the arch that you see when you look up through the opening is the soffit, also called the intrados. The curved outer surface—the top or back of the arch—is the extrados. The zone between the springing line and the crown on each side is the haunch; this area carries significant shear and must be solidly supported until the mortar cures. The highest point of the arch at its centerline is the crown.

Finally, the abutment is the mass of masonry or concrete on each side that receives and resists the outward thrust the arch generates. Without adequate abutments, the arch will spread and collapse.

How Load Travels Through an Arch

When dead load (the weight of the wall above and any floor or roof load it supports) bears down on the arch, it cannot simply fall straight down because the voussoirs are wedged together. Instead, the load follows the line of thrust, a curved path of compression that runs through the arch body. The load breaks into two components at any given voussoir joint: one component is directed along the joint face (shear), and the larger component is directed perpendicular to the joint face (compression). Ideally, the line of thrust stays within the middle third of the arch thickness so that no joint opens on either face—the same middle-third rule you will apply to retaining walls and footings later in the program.

At the skewback, the thrust vector has both a downward component (vertical reaction) and an outward horizontal component. The abutment must be heavy and wide enough to resist that horizontal thrust without sliding or overturning. The flatter the arch (low rise-to-span ratio), the larger the horizontal thrust for the same load—a fact that directly affects how you size abutments and select arch type for a given opening.

Common Arch Types

Semicircular arch: Rise equals exactly half the span. The intrados is a perfect half-circle. Generates moderate horizontal thrust; strong and visually classical. Common in brick facades, fireplaces, and historic restoration work.

Segmental arch: Rise is less than half the span—the intrados is an arc of a circle rather than a half-circle. Flatter profile means higher horizontal thrust than a semicircular arch for the same span; requires robust abutments. Very common in residential and commercial window and door openings.

Jack arch (flat arch): Has virtually no rise; the intrados is nearly horizontal. Voussoirs are cut with only slight taper (camber is typically about 1/8 inch per foot of span, centered at midspan). Generates very high horizontal thrust. Used over narrow openings where headroom is limited. Requires steel angle backup or a relieving arch above in modern construction.

Parabolic arch: Intrados follows a parabolic curve. When load is uniformly distributed, a parabolic arch develops almost pure compression throughout with minimal bending—structurally efficient. Seen in bridges and large-span structures.

Gothic (pointed) arch: Two circular arcs meet at a point at the crown. The pointed geometry allows builders to vary rise independently of span by adjusting the radius of each arc. Generates less horizontal thrust than a semicircular arch of equal span, which made Gothic cathedrals possible with thinner walls. Still used in ecclesiastical and decorative masonry.

Multicentered arch: Constructed from three or more arcs of different radii, producing a flattened elliptical appearance. Common in larger decorative openings where a segmental arch would look too low and a semicircular arch would look too tall.

Safety Notes

An arch under construction is not self-supporting until the keystone is set and mortar has reached adequate strength. Always keep temporary formwork (the centering) in place until mortar has cured per the project specifications—removing centering too early is a leading cause of arch collapse on job sites. Never stand directly under unsupported arch centering or a partially built arch. Wear your hard hat, safety glasses, and steel-toed boots at all times in the work area. When mixing or cutting masonry units for voussoirs, add a dust mask rated for silica (N95 minimum) because cutting brick or stone releases respirable crystalline silica, a serious long-term lung hazard regulated under OSHA's silica standard.