Mediaeval roof design, to be specific. Not something we’d normally concern ourselves with until a fine example went up in flames in Paris this week.
Notre Dame de Paris, like many another building of its age, had what the French call a charpente, literally “carpentry” and I can’t quite think of an English word - rafters maybe? - clad in lead sheets. Lead was used because it’s soft and easily worked and doesn’t rust. On the other hand it’s toxic, but no health and safety then and that was just another of the many hazards facing builders at the time. The roof was one of few surviving mediaeval roofs and was such because most of the others had burnt down, some more than once.
Overall the layout was like this:
It’s the traditional cross shape, with the choir (the shorter of the longitudinal sections) at about 10 o’clock and facing east, the nave extending to the four o’clock position and two transverse sections, the north and south transepts. The spire was at the intersection of all of these elements. All of these have a vaulted ceiling with the charpente over that.
From inside it looked like this. Looking at it, I’m not sure if they ever used nails.
The roof was exhaustively surveyed, for the first and last time, in 2014 by architect Cédric Trentesaux as part of his training and he wrote later “At first, you didn’t see the details. It was with time that we got our eyes in and we learned to see. Although it was never intended that the rafters be either visible or visitable, the greatest care was taken with the smallest detail. And technically, it was for the time the equivalent of Formula 1. They had to test there things which had never been tried elsewhere, to innovate so as to stretch the boundaries, they were forerunners”.
As it was a very obvious fire hazard it had walkways so you could easily inspect it and a fire detection system. The latter first sounded the alarm at about 6.20 pm but sadly located the fire incorrectly. Somebody went to have a look and found nothing amiss until 6.43 when the fire reached another detection zone by which time it had taken hold and was well beyond extinguishing.
The entire roof is obviously very old and some years ago research was done to see just how old. Samples were drilled out of some of the timbers (the French call this “carroting” from the shape of the sample) and the resulting bunch of carrots subjected to dendrochronological dating which found that the majority of samples were from trees cut in 1226. A few outliers were from about 1159 and are thought to be survivors from an earlier building.
In the immediate aftermath of the fire it was claimed that France no longer possessed oak trees of sufficient size to rebuild the roof to its original design. The French timber industry begged to differ and produced facts and figures. The original roof was made from about 1,300 oaks (so many it was called “the forest”) and their average age at cutting (that dendrochronology again) was about 100 to 150 years. The approximate volume of timber was about 3,000 m3. To put that into context, private producers in France sell about a million cubic metres of oak a year. Oak timber cut to size costs about €800 to €1,000 per m3 so say €3m before final adjustments and installation. Offers have already been made to donate the timber.
The only remaining problem would be finding craftspeople to build the roof.
Alternatively, you could go modern. Reims cathedral was burnt out during WW1 and was rebuilt using a concrete charpente and Chartres, burnt in the 19th century, was done in steel. Looks exactly the same from outside and it’s not going to burn. Perhaps surprisingly, both steel and concrete give you a lighter roof. If you want be really fancy, you could use titanium.
Me, I’d go the modern route and avoid the next incéndie but it’s not my money.