The original builders of the Notre Dame in Paris built the structure with an inability to fight a roof fire in medieval times in mind. 

The detailed structural engineering implications of the Notre Dame fire cannot be determined yet but at a strategic level I believe there is a good chance that things will turn out to be a lot simpler than the worst predictions currently appearing in the media.  

The original builders delivered a robust, incombustible stone box with a tight-fitting stone lid (the vaulted ceiling). The roof is a separate, relatively light-weight, combustible timber structure supporting lead sheets and gutters to keep the wind and rain out.   

In medieval times there was no means of delivering large quantities of water to fight a roof fire, so it was accepted that if a fire started, it would last until the timbers had burned away. The interior of the building would be protected from the fire by the stone vault. That is what happened at Notre Dame, except where large chunks of the fleche fell away from the footprint of the fleche itself and pierced the vault. The fleche was a 19th-century addition that those who built the vaults would not have anticipated.

The “simple” strategy for a way forward is:

  • Build a completely new roof, following the profile of the old, and supported at the same points, (but using new incombustible materials)
  • Reconstruct the missing and severely damaged sections of the vault, in stone, copying the original geometry and constructional details
  • Treat the reinstatement of the fleche as a completely separate issue, which can be debated and resolved quite independently of the work to get the basic structure restored and the Cathedral functioning again 

That’s what the original builders would have expected.

The unintended loss of any building has serious economic implication and additional emotional and cultural implications. This tragic incident has triggered all sorts of questions about why it happened and what could have been done to prevent it.  

To comment on that would be quite inappropriate until all the facts are known. In general, the management of risk during construction has received much welcome attention in the last 30 years. Fire is a significant risk, but normal good practice, if properly applied, now reduces that (and other significant risks) to an acceptable level. 

Inevitably at times like this people look to previous apparently similar events and ask if the right lessons have been learned and applied. In the UK that means looking again at the fires which occurred at Hampton Court, York Minster and Windsor Castle. 

These had very different causes, but common to all is that the fires were not discovered early enough, and, at Hampton Court and Windsor Castle, the construction of the buildings allowed fire to spread unseen from one area to another via cavities in floors and walls. It seems that these risks were recognised and there was a fire detection system at Notre Dame. An alarm sounded, but the fire was not discovered immediately. 

One of the most curious suggestions to be triggered by the Notre Dame fire is that the 19th-century Palace of Westminster, where major refurbishment is planned, is similarly at risk. That can’t be true; the roof of the Palace of Westminster are entirely incombustible. That was part of the brief for the architects when the Palace was re-built in the 1840s… following a fire. 

Robert Bowles is a conservation accredited structural engineer from the Institution of Structural Engineers