Restoration of Notre-Dame Cathedral, four years after the fire in 2019 (Paris, May 2023).
© BRGM - Aude Rouger-Loiseau
Following the fire in April 2019, Notre-Dame Cathedral in Paris required major restoration work. In particular, huge quantities of ‘new’ stones were needed to replace the damaged originals.
Project managers faced a major challenge: to secure a supply of suitable replacement stones in sufficient quantities. This was a priority. The first crucial question was to establish where to find the stones.
Working closely with the public entity Rebâtir Notre-Dame de Paris (RNDP), the architects and the Laboratoire de recherche des monuments historiques (LRMH), BRGM found solutions to deliver a secure supply of stones in a very short time.
The difficulty of finding replacement stones for historic monuments
In France, the number of building stone quarries has fallen steadily since the end of the First World War. Today, fewer than 500 quarries remain in operation. Of this total, only half extract limestone, the other half are of granite, sandstone and other rocks.
As a result, it is increasingly difficult to find ‘good’ replacement stone – i.e. stone that is similar to the original from a physical, mechanical and visual standpoint, to restore historic monuments and other old buildings.
A site of exceptional requirements
The fire at Notre-Dame Cathedral in Paris in April 2019 caused extensive damage to the original stonework. As a result, restoration work required a colossal quantity of replacement stone: around 1,000 cubic metres (m3) of ‘finished’ blocks, compared with a few dozen m3 at the most, for routine maintenance and restoration work sites.
This extraordinary quantity of ‘new’ stone had to be made available within a particularly short time and as part of a particularly tight schedule, given the scale of the work to be carried out. Furthermore, the replacement stone needed to have precise physical, mechanical and visual characteristics.
Cutting face of a limestone quarry.
© BRGM - S. Andrieu
Nine quarries under the microscope
To meet this supply-related challenge, the first step for BRGM between 2020 and 2021 was to assess the capacity of the nine Lutetian limestone quarries still in activity to supply stone compatible with the original stone of the cathedral.
As part of this assessment, BRGM geologists conducted field investigations of each quarry. Laboratory analyses were also carried out. The results were published in the form of a methodological guide to the selection of stones for the restoration of Notre-Dame Cathedral, together with a collection of reference samples for architects.
The biggest requirement by far for the Notre-Dame project was for hard stone, in the form of around 750 m3 of cut blocks. Only one quarry was shown to have a bed of rock of sufficient height, Croix-Huyart at Bonneuil-en-Valois outside Paris.
Securing and managing the stones supplied
In mid-2021, BRGM and the public entity RNDP quickly set up a second initiative to secure a supply of hard stone from the Croix-Huyart site. A rigorous and particularly innovative procedure was drawn up and then applied by BRGM, based on quality control, conformity and traceability of the stone.
Between March 2022 and March 2024, BRGM conducted some 90 inspections.
Today, in Notre-Dame Cathedral, the ‘new’ stones sit side-by-side with the stones laid several centuries ago, but visitors are unable to distinguish one from the other with the naked eye.
