Rebuilding the Cathedral of Notre Dame

James Simpson OBE Hon.DSc FRIAS, Chairman of the Building Limes Forum, discusses the rebuilding of Notre Dame after the fire, highlighting the importance of the skilled trades and welcoming the opportunity the project creates for in depth training and experience at all levels. The fire was a disaster, but the rebuilding may also be a once in a generation opportunity, particularly for the design and use of lime mortar in the rebuilding of the vault.      

The fire at Notre Dame was a dreadful disaster. One must be grateful that the fire brigade prevented the roof fire from spreading into the western towers, whose collapse would have caused massive destruction, and that there was no loss of life. As always in these situations, however, there can be benefits as well as disbenefits. Christopher Wren’s St Paul’s would not have been built if its predecessor had not been destroyed in Fire of London of 1665. The rebuilding of St Paul’s and of all the city churches created a generation of great craftsmen, of which Grinling Gibbons was only the best known.

Mediaeval cathedrals were designed to withstand fires. It is the roof timbers which are most vulnerable and the main part of the building is usually protected by stone vaults: an example of early fireproof construction. It was only the crashing down of the 19thC fleche over the crossing which broke the vault at Notre Dame. Nonetheless, the cathedral is eminently restorable: there will be many ways of approaching such a rebuilding project and the debate has already begun. The repair of surviving but damaged stonework will be relatively straightforward and will cost money, but it will also create good work and training opportunities for stonemasons. The actual rebuilding of the lost parts can be broken down into three main elements, each of which can be approached in a different way: firstly the central fleche or spire, secondly the roof and thirdly the vault.

The fleche, which was a 19thC timber structure designed by the great architect Violet le Duc. It is no more, it is a highly visible freestanding element and there would seem to be no overriding reason for recreating in it exactly as it was: there is the opportunity for creating a 21stC structure as wonderful as Violet’s was in its day. The roof is different. If the expectation is that the external appearance should look as it did before the fire, to match the other roofs, the internal structure is another matter and ideas have already been floated for using the space. Such re-use could be said to follow the precedent of the Undercroft at York Minster, where the need for new foundations beneath the central tower in the 1960s created the quite unexpected opportunity for a new museum. The roof will probably entail a combination of traditional and contemporary construction methods. Oak for new structural timbers could be hard to source and the loss of trees might be undesirable in any case. The leadwork, on the other hand, would be good work for traditional plumbers.

The third challenge will be the rebuilding of the vault and it is here that the importance of complete integration with the original masonry structure will be most important. It will also be the greatest opportunity in a generation for the masons, the engineers and the architects involved. There would be an element of ‘experimental archaeology’: there is no better way of learning how such a thing was done than by doing it. The classic French cathedral structure is a three dimensional stone framework, in which the flying buttresses balance the outward thrust of the vaults: since neither stone nor mortar has tensile strength, everything must be in compression. In the Middle Ages, the Master Masons learned empirically and knew from experience how to build vaults. In the 21stC, the structural engineers, other professionals and craftsmen will have to plan and work together, to know in advance and as the work progresses that everything works. When the job is done, the record of the process will be there for all time: or at least until the next disaster.

There will be much work and great opportunities for education and training in the process which will be of interest to art and architectural historians, building professionals, craftspeople, students, children and others. In the wider context, the design and use of the mortar may seem fairly insignificant: but it is not! It is easy to forget that walls, arches and vaults were made of stone and lime mortar and that, contrary to popular understanding, lime mortared walls are stronger and more robust than walls built dry. This is because lime mortar distributes the load, so that every part takes its share and that there is no point at which the force becomes excessive. It is for this reason that the mortar must not, like Portland cement and other strong mortars, ‘stick the stones together like glue’ and create tensile stresses. The new vault at Notre Dame must be as good as the old and must last for ever: which is a long time! The design and the workmanship in the use of the lime mortar will be crucial.

The French approach to trade apprenticeships, through traditional organisations like the ‘Compagnons du Devoir’ is, like that of the Germans, much, much better than the UK’s and there is much to be learned on this side of the Channel. Notre Dame could provide the opportunity.

The Building Limes Forum is the centre of knowledge and expertise in the UK and beyond on the design and use of lime mortars and plasters: the advice of its members is available and the work at Notre Dame will be followed closely: it will doubtless be the subject of many a case study. The fire at Notre Dame was a great disaster, but much good can still come of it!  

Find out more about the Building Limes Forum at their conference, 'Traditional Mortars Symposium', supported by  Historic England and Historic Environment Scotland, is taking place in the hall of St Anthony York and York Minster Stoneyard on 10, 11 and 12th June 2019. 

The Traditional Mortars Symposium aims to bring together current thinking and knowledge about traditional mortars, presenting recent research outcomes, revised heritage agency guidance and case studies from around the UK, Ireland, Spain and Canada. Speakers will draw upon long and more recent experience using traditional mortars for the repair and conservation of historic fabric, sharing their insights and knowledge in theory and in practice. 

https://www.buildinglimesforum.org.uk/