Rebuilding Notre Dame - Cathedral Preservation & Disaster Management

Hosted by Surrey Churches Preservation Trust, January 2025

Speakers: Johnathan Deeming & Lian Harter, Purcell Architects

How do we react when a historical monument goes up in flames? How does this differ across cultures and what can we learn from what has gone before?

What Happened at Notre Dame?

Notre Dame is one of 89 cathedrals across France, and arguably the most famous. When it was first constructed, Notre Dame used innovative building techniques which made the vault serveries only 150mm thick, 200mm thinner than at contemporary Saens Cathedral. The roof was clamped to the top of the walls with a series of iron clamps. The roof structure, colloquially known as “le forêt”, was reconstructed 30 years after it was first built using coppiced oak that was around 60 – 80 years old.

It is still unclear what the exact root cause of the 2019 Notre Dame fire was, as there were multiple failures at raising the alarm, and the cathedral was ill-equipped to handle a fire. Unlike British cathedrals, French cathedrals typically do not have risers to pump water to high level, and there were no fire breaks to stop the flames from spreading. In the UK, dry risers have been mandatory since the 1984 fire at York Minster.

Even with this catastrophe, the ground effort which was mobilised saved countless historic artefacts.

Notre Dame measures 128 metres in length, 35 metres tall, with the flèche or tower at 96 metres high. The flèche was an addition by French architect and historical champion of conservation Viollet-le-Duc, who redesigned the cathedral in the 19th Century, and was undergoing restoration works during April 2019.

The fire started within the flèche, and within minutes was a huge blaze. It then progressed eastwards and westwards across both transepts, with the intense heat from the fire melting the lead roof, and the evaporated lead causing severe lead poisoning in the local area. The black smoke from the fire reached both of the towers, and vaults in both naves collapsed.

The scaffolding itself was totally distorted from the heat of the inferno, but it may have actually saved the cathedral from further destruction by catching the flèche as it fell.

After the fire had been contained, the interior of the cathedral had been covered in the burnt remains of the collapsed vaults and roof timbers, however although the vaults fell, the arches themselves remained intact. The masonry also sustained water damage from the jets used to control the blaze, causing staining to the stonework.

Looking to the Past - What have been previous conservation approaches in France?

One thing to note is unlike British conservation efforts, French rebuilding is not afraid of innovation, as shown by the examples we went through.

Chronologically:

Rouen Cathedral – (lovingly described as “The dream of a metalworker’s delirium” by Rouen-born author Gustave Flaubert) – in 1823, lightning struck the cathedral, setting it ablaze. The reconstruction used innovative materials, but took 50 years to rebuild.

Chartres Cathedral's wrought-iron frame (Photo by StuartLondon, CC BY 3.0)

Chartres – fire caused by unattended hot works in the 1830’s. Reconstruction used a wrought-iron frame to speed up the rebuilding process to avoid what had happened at Rouen, and the metal trusses were vaulted and left exposed internally.

Saint-Denis – also lightning – used cast iron for the replacement truss, and copper roof ribs & rods.

Metz – the roof was destroyed by stray fireworks in 1877, reconstructed in the 1880s. The architect raised the roof height with a metal frame, raising its height by 3.5 metres and its pitch to 60º.

Reims – suffered war damage, rebuilt between 1923 and 1940 using concrete based on the previous short timbers. The lighter load of the concrete reduced the stress on the stone below.

Soissons – regularly damaged across the 15th to 20th centuries, Soissons Cathedral was restored in 1927-1928 using pre-cast concrete, which made for a much faster restoration and freed the structural stability of the walls from the weight of the roof.

How to Rebuild Notre Dame?

2 days after the Notre Dame fire, the French government declared rebuilding the cathedral a national emergency, and appointed military leaders to secure funding and manage the rebuilding effort. President Macron gave a deadline of the 2024 Paris Olympics for the reconstruction to be completed.

Straight after the fire, the reconstruction teams got to work:

CLT formers being installed at Notre Dame

1. Gables netted and propped up, to prevent further collapses

2. Netting installed internally to catch falling debris

3. Columns splinted to provide structural support

4. New pine beams and support decking (built “Meccano style”)

5. Stained glass removed from Chancel clerestory; windows covered. Scuppers installed to catch water and prevent further damage

6. CLT formers installed under flying buttresses to provide support – each former weighed 8 tons.

7. Temporary roof constructed, before the removal of the initial damaged scaffolding.

Clearing the cathedral took 2 years, as site clearance posed a major opportunity for archaeology, particularly with a building of such monumental heritage.

The archaeology team set common sorting criteria to be able to sort an item in less than 5 minutes, and the survey set up a cable-camera (like in the Formula 1 pit lane, and other sporting events too I guess) which recorded as much data as possible. This technique allowed the team to use more than 95% of the data it gathered. However, that wasn’t the only data available – in 2015, two architecture students had scanned the cathedral for their thesis work, and this scan was used in 2023 to create a multi-layered drawing of what was there before the fire.

The initial goal in the reconsruction had been to preserve as much stone as possible, those in charge citing a “moral obligation” to return fallen stones to their original places. 79 stones had fallen as a result of the fire, of which only 15 were in a usable condition. Making things more challenging, all 15 stones were different thicknesses and did not follow any standard dimensions. Dimensional assessment was impossible, but the reconstruction team noticed notches in the stone which showed where they had been placed and how they had been aligned, so with the aid of a computer model, they were able to locate the stones’ original positions to 80% accuracy. 80% was not precise enough however, so the architect decided that all stones were to be fully replaced.

Compare and Contrast: Canterbury Cathedral, Kent, UK

The restoration at Canterbury Cathedral has been a 10-year project, funded by the Heritage Lottery, and focused on the nave roof and the western towers – around 1/3rd of the Cathedral.

Planning consent took 2 years, with most of the data and defect diagnosis taking place within around 8-12 months of that time. Purcell, the architects behind the restoration, undertook a stone-by-stone defect analysis using laser and drone scanning to build up their survey data, and found that while Canterbury is mostly constructed from French limestone, much of it is repair work from various other stone sources.

Each buttress was individually surveyed, and all were found to be in poor condition. The cathedral’s dry risers were also surveyed and modernised.

Purcell also undertook continuous structural monitoring and tilt monitoring, to measure how much the structure moved over a period of years and what the primary causes of these movements were – whether cyclical (seasonal) movement or progressive movement (structural failure).

On the Western towers, the team could not do a stone-by-stone analysis so got to abseil down the front face of the towers to survey instead – (side note, I would love this job, how do I apply.)

The nave’s vaults were also surveyed, a process which was undertaken across 3 days and showed complicated crack patterns across the nave caused by the stress of the towers.

The continual stressors were identified as caused by groundwater drainage fluctuations, rather than just building defects – Purcell’s aim was to deal with the root cause rather than just fixing the symptoms.

At this point, Canterbury cathedral was served by a medieval drain, which had been sufficient… in the Medieval times, before other buildings had been constructed around the cathedral grounds… The drainage was no longer fit for purpose and the site needed a relief drain. The relief drain was initially meant to run under the road next to the cathedral, to disturb as few burial sites as possible, however this space had already been completely filled by other services, so they had no choice but to run through the lay cemetery.

The speakers highlighted the vital importance of having written records to refer to – at Canterbury, they were able to use a still-existing 11th Century drawing of the drainage plans!

Takeaways

There were several great questions posed at the end of the lecture, but the most pertinent question was - How do we bring together knowledge to protect our buildings in the future?

For example at Canterbury Cathedral, it’s amazing that a drainage diagram still exists from the 11th Century - but what of photographs taken in say 1992 that are stored on now-obsolete file types? How much data are we at risk of losing if everything is saved on computers?

At Notre Dame, they were able to use a scan from 2015 to reconstruct what the cathedral had been like before the fire - there had been a laser scan completed in 2010, but this data was practically unusable by 2020. What would have happened if the 2015 scan hadn't been run?

Digital data degrades so much faster than physical equivalents, and we do not have the technological processes in place to store it for very long yet. The architect(s) in charge must ensure material is archived, however even modern print decays quickly. At this point someone from the audience piped up: “Maybe you could use vellum!”