A subject rarely seen

"Cé hé sin" (michael-m-mouse) 04/18/2019 at 16:15 • Filed to: Mediaeval roofs

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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.

Reims

Chartres

Me, I’d go the modern route and avoid the next incéndie but it’s not my money.

For Sweden > Cé hé sin 04/18/2019 at 16:21

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Rafters is the right English word. The original design is fantastic, but I would be shocked if the new roof isn’t steel. I would like to see a portion of timber-framed roof at ground level as an exhibit.

Also, would a copper roof be sacrilege ?

Amoore100 > Cé hé sin 04/18/2019 at 16:36

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I love this aesthetic, i t’s almost steampunk airship hanger

Ash78, voting early and often > For Sweden 04/18/2019 at 16:36

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London 1666, Never Forget.

ttyymmnn > Amoore100 04/18/2019 at 16:43

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I was thinking more like this. It’s the inside of the Hindenburg .

ttyymmnn > Cé hé sin 04/18/2019 at 16:46

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Great write up. Thanks.

Are you sure about that 1,300 number? CNN added a zero:

To kick off the project, workers cleared 21 hectares of oak. Each beam of the intricate wooden cross-work was drawn from a different tree: estimated at 13,000 trees in total. To reach the heights the carpenters needed to build the structure, those trees would likely have been 300 or 400 years old, meaning they would have sprouted out of the ground in the eighth or ninth centuries. ( CNN )

ttyymmnn > For Sweden 04/18/2019 at 16:46

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Too Danish.

For Sweden > ttyymmnn 04/18/2019 at 16:46

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But I want to French to suffer just a little bit

punkgoose17 > For Sweden 04/18/2019 at 16:49

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Honestly I vote for concrete the steel would rust eventually.

My bird IS the word > Cé hé sin 04/18/2019 at 16:50

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Cool, ancient archetecture. I say go modern as well, if the original roof lasted thousands of years this roof should last twice as long.

ttyymmnn > Cé hé sin 04/18/2019 at 16:52

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Does the roof provide any structural support beyond holding up the roof? I would think that most of the load would be straight down and pushing outward, hence the flying buttresses. If not, you could go even go with something like aluminum.

Amoore100 > ttyymmnn 04/18/2019 at 16:55

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That’s it! I knew it was something airship related...

Cé hé sin > For Sweden 04/18/2019 at 16:55

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They might not like the patina though, because tradition .

I’ve googled lead roofs and to my surprise you can still have one installed . Costs about the same as copper, both being very expensive. One of the downsides is theft...

Sur prisingly copper is expected to last longer.

ttyymmnn > Amoore100 04/18/2019 at 17:01

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But your hangar analogy wasn’t all that far off. This is Goodyear’s blimp hangar in Akron. Very similar construction techniques, I would think.

facw > For Sweden 04/18/2019 at 17:02

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I think copper would look pretty good. There are at least few Gothic cathedrals that have gone that route, for example Chartes and Metz:

Cé hé sin > ttyymmnn 04/18/2019 at 17:02

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It does to a limited extent. One of the issues facing Notre D ame now is the gable ends of the transepts which are now exposed to the wind and in at least one case is being dismantled. Other than that I think the whole structure just sits on the walls. I can think of several ruined and now roofless buildings, generally old castles, which are still standing.

Cé hé sin > punkgoose17 04/18/2019 at 17:05

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The rebar in the concrete will do just the same though. It rusts, expands and cracks the concrete.

Either way though the roof covering should keep the structure dry though.

ttyymmnn > Cé hé sin 04/18/2019 at 17:11

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Here’s a nice illustration from the NY Times .

Cé hé sin > ttyymmnn 04/18/2019 at 17:13

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They also increased the age of the trees!

The source I have (in French) is here . There has been an offer of 1,300 trees so I guess they got the figure from somewhere.

ttyymmnn > Cé hé sin 04/18/2019 at 17:14

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Metric system?

/s

facw > Cé hé sin 04/18/2019 at 17:47

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I’m a bit surprised that there hasn’t been more uptake in plastic/fiberglass/composite rebar, given that glaring weakness with traditional reinforced concrete.

RacinBob > Cé hé sin 04/18/2019 at 17:58

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Terrific article. I learned some stuff and you answered a lot of questions. 

Cé hé sin > facw 04/18/2019 at 18:40

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It’s certainly used but has issues of its own - it’s not as strong.

The Compromiser > Cé hé sin 04/18/2019 at 18:59

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I actually knew a lot more than average about medieval roofing techniques . I’ve seen too many shows, and too many books on the subject. This is like sad porn for me.

ranwhenparked > punkgoose17 04/18/2019 at 20:24

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The steel rebar in the concrete might, too. Henry Mercer’s castles are many centuries younger than Notre Dame, and they’ve had all sorts of conservation problems over the decades. 

punkgoose17 > ranwhenparked 04/19/2019 at 10:29

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True. The rebar rusting did cross my mind.

gmporschenut also a fan of hondas > Cé hé sin 04/21/2019 at 00:15

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If going modern, go the full 9 yards