Building components · Wall Leaves & Backing
Voussoir
Explains the voussoir as the single unit within an arch, and why the wedge shape can come from the unit or from the joints between units.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a voussoir is
A voussoir is one unit of an arch ring. Its faces are set so that they radiate from the centre of the arch's curve, which means each unit is narrower at the inner face than at the outer one and is therefore wedged between the units either side of it.
That wedging is what allows an arch to work. No voussoir does anything on its own; it is held by the units next to it and it holds them in turn, and the ring only becomes a structure when every unit is in place and the thrust can travel around the curve. The wedge may be cut into the unit itself or produced by tapering the joints between parallel-sided units, and both arrangements are met.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not the arch. The arch is the whole spanning form; a voussoir is one unit within it and has no arching action by itself.
- It is not a springer. The springer sits at the base of the ring, where the curve leaves its support and the thrust passes into the wall; voussoirs are the repeating units between the springings and the crown.
- It is not simply a tapered brick or stone. The taper may be cut into the unit or created by the joints between untapered units, and the two are different pieces of work with different consequences for how the joints behave.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- arch stone
- arch brick
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Its neighbours, since a voussoir is only held in place by being wedged between the units either side of it.
- The joints on both of its radiating faces, because the thrust crosses the ring through those joints rather than through the units alone.
- The completeness of the ring, as an incomplete arch offers a voussoir nothing to bear against.
- Whether the wedge is cut or formed by the jointing, which changes how much of the bearing is unit and how much is mortar.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Thrust passes from voussoir to voussoir across the radiating joints, so what happens at those joints is what happens to the arch.
A single voussoir dropping or rotating is commonly a sign that the ring has lost its geometry rather than that the unit itself has failed.
Where the taper is formed by the joints, there is more mortar at the outer face than at the inner one, and that outer mortar is the part exposed to weather.
Voussoirs cannot be placed one at a time as a self-supporting sequence; they rely on temporary support until the ring is closed.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Clarifications
Commonly misunderstood
- That a loose or dropped voussoir can be replaced in isolation. The unit is held by the ring around it, and what allowed it to move is usually a question about the ring rather than about the unit.
- That every arch is built from specially shaped units. Many are built from ordinary units with the wedge produced entirely by the joints, which looks similar from a distance and behaves differently up close.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Are the units in this ring cut to a wedge, or is the taper formed by the joints?
- Has any unit in the ring dropped or rotated, and what does that indicate about the arch as a whole?
- What condition are the radiating joints in, given the thrust passes through them?
- What temporary support would the ring need if any unit were to be removed?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- This entry describes a unit's position within an arch. It states no unit size, no taper, no joint width and no load capacity, and none can be inferred from it.
- Whether a particular voussoir has moved, and what should be done about it, is a structural judgement made on site and not a question a reference entry can answer.
Wall Leaves & Backing
The distinct vertical layers that make up a wall — outer and inner leaves, the cavity between them, and the backing that supports what is fixed to the face.
Browse all wall leaves & backing entries →