What is a round arch?
A round arch is an arch whose curve is exactly a semicircle: the centre of the circle sits on the springing line, so the rise is always precisely half the span. It is the simplest and oldest arch form in Western building, developed by the Romans and the defining feature of Romanesque architecture.
Geometry
The round arch is struck from a single centre. Radius = half the span. An opening of 1.20 metres therefore takes an arch of 600 mm radius and 600 mm rise. That fixes the form completely — with a round arch nothing is adjustable except the span. This is the key difference from the segmental arch (flatter, centre below the springing line), the basket arch (several centres) and the pointed arch (two centres).
| Round arch | Exactly a semicircle; centre on the springing line, rise always half the span |
|---|---|
| Segmental arch | Flatter; centre below the springing line |
| Basket arch | Several centres |
| Pointed arch | Two centres |
How the forces run
In an arch the load is carried almost entirely in compression, through the depth of the arch down to the springings. At the base this produces horizontal thrust, which has to be resisted by:
- mass beside the opening — a wide pier; in Romanesque churches the thick wall did this by itself, and
- for a run of arches, a tie rod or tie anchor, otherwise the end arch pushes the wall outwards.
Over the same span a round arch is always stronger than a segmental one, because the load path is steeper and the thrust smaller.
Building a round arch — in practice
- Make the centring from plywood to the correct radius, less the mortar bed. Set it on props you can lower later.
- Set the springer on the springing line, exactly level on both sides.
- Lay the bricks out dry on the centring and count towards the middle. An odd number gives a single keystone at the crown, which is best both structurally and visually.
- The joints taper. Never widen the outer joint to make things fit — you lose the compression arch. Cut the bricks instead if the spacing does not work out.
- Build up symmetrically from both springings, so the centring is loaded evenly.
- Leave the centring in place until the mortar has gained enough strength — depending on mortar and temperature, several days to well over a week. Striking too early is the most common mistake: the arch settles and the crown cracks.
Application
Round arches appear in Roman aqueducts and gateways, in Romanesque and Romanesque Revival churches, in Renaissance arcades, and on a small scale everywhere: over garden gates, in cellars, in vaulted passages and in brick façades above windows. To insert a round arch into an existing wall, prop the opening first and build the arch afterwards — never the other way round.
Related terms
- Arch
- Segmental arch
- Basket arch
- Springer
- Keystone
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