Christian Lassure

version française

In dry stone construction, the vault of voussoirs is sometimes encountered. A vault of voussoirs is one of wedge-shaped stones whose oblique beds  – the sides by which they rest on the neighbouring voussoirs – converge towards the same point.

The vault of mortarless stone voussoirs is not a vault made of cut and matched voussoirs, the construction of which is beyond the skills of dry stone builders as it requires stereotomy studies, it is a vault made from either raw or hammer-corrected slabs, or rough-hewn and roughly matched rubble stones, placed on a temporary truss.

The mortarless vault of stone voussoirs falls into two different types: the barrel vault and the dome-shaped keystone vault.

In the making of a barrel vault using parallel courses of slabs, a wooden truss – made of contiguous boards supported by spacers and wall plates – intervenes. It is placed on corbels or on a scarcement made at the springing lines of the vault and it is moved longitudinally on the support to achieve other lengths. The vault intrados then follows the shape of the truss, the joints of the sectional beds are aligned and parallel to the ridge line.

Lancet-arched dry stone vault in a sheepshelter at Les Fraches du Contadour in Redortiers, Alpes-de-Haute-Provence. © Dominique Repérant.

For the making of a dome-shaped keystone vault, where the voussoirs are either rough or hammer-rectified slabs, or rubble stones with their intrados summarily dressed with a chisel, a hemispherical truss is assembled, the spacers of which are embedded in the top of the vertical wall. Once the vault is completed, the centring is dismantled. The locking system is a keystone here. In the intrados of the domed keystone vault, the joints of the beds are concentric.

Dome-shaped vault made of split cobbles at Saint-Pargoire, Hérault. A grouping of three split cobbles is used as a substitute for the keystone. © Gilles Fichou.

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Christian Lassure
The vault of mortarless stone voussoirs

May 9th, 2023


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