Restoring a complex roof frame

Supporting the roof of the Royal Chapel – the younger sister of the Sainte Chapelle in Paris – is a magnificent but badly water-damaged oak frame. At stake in this restoration effort are the highly interactive forces at play above the precious frescoes.

128 trusses and half-trusses

Number of trusses and half-trusses in the frame to be restored

130 linear metres

Length of purlins to be replaced

48 m3

Volume of wood to be installed/replaced

An imposing structure

Standing firmly on a solid substructure, the top of the Royal Chapel soars from its arched buttresses towards the horizon. Once the heavily adorned roof is taken out of the equation, something else comes into play, whereby the density of the wood battles with a variety of invisible forces. This non-standard structure comprises trusses, beams, rafters and other reinforcements arranged over and around each other almost ten metres – or three levels – up, and supporting the weight of its lead adornments, calculated to weight more than 400 tonnes. It extends across the priceless paintings of the building’s internal vault, a sky covered in the iconography of the Holy Trinity.

It could all have been so much easier had a lantern not been added to the Chapel roof, thus adding another 20 metres to its height. The central part, topped by a cross, caused problems right from the time it was built. It turned out to be too heavy and was destroyed in 1764. False hip rafters, designed to draw the eye to the peak, instead pull the roof back to its four corners. This sawtooth structure exacerbated the ingress of water, which has, over time, caused many of the elements to decay, to the point where they now crumble into dust in the hand.

Scale of the damage

The lower part of the structure was concealed by enormous stone gutters. For the first time since the Chapel was built, these have been removed, revealing the full scale of the damage. Piece-by-piece analysis, some of it undertaken in the laboratory, has determined that much of the damage has been caused by rainwater coming in at a point that is particularly exposed to the wind. Ninety per cent of the external purlins encircling the top of the walls and on which the bulk of the frame rests had to be replaced. Underneath, the stanchions supporting the trusses that run from one side of the building to the other had deteriorated significantly. Above, more than a third of the hammer-beams, which act as load struts for the rafters, turned out to be defective. So there was a whole network of wood whose every element had to be checked meticulously. A noteworthy remnant of this completely unique assemblage is the still-intact arrangement formed by the false beams and braces that supported the old lantern.

One of the carpenters on the project drew attention to the fact that the rafters in this frame consist of a single piece of wood, from the foot of the roof to the peak, so the lengths and sections involved are highly unusual. This is the first time in his long career that he has worked on such items: “You have to be able to find trees that tall! And even then, how did they get them up there?!” Moreover, he points out, at the time they were installed, the wood would still have been green and therefore heavier… Stéphane Masi, the director of the project to restore the Chapel, draws an analogy between the game of Mikado (pick-up sticks) and this monumental structure, criss-crossed as it is by contradictory forces, with initial studies revealing it was actually oversized. Some of the alterations to it were caused by the plethora of assemblages.

A frame suspended temporarily above the vault

Owing to weaknesses at certain points, the frame was sagging and coming apart dangerously, making it impossible to work safely in such an unstable situation. Large metal girders were brought in to surround and hold firm some of the trusses, which, with the help of jacks, were raised ever so slightly, by barely a millimetre. This has relieved the most stressed parts, allowing the elements that require attention to be removed. Indeed, all of the stanchions have been changed, with “grafts” replacing the most defective parts. These have been meticulously integrated, their outlines clearly visible on the new oak and contrasting with the surface of the older construction.

Working to the protocols stipulated by the project leadership, the carpenters are halting the slow deterioration of this oak masterpiece. They started at the bottom and are progressing slowly but surely, one level at a time. A temporary floor was installed to enable them to work easily and safely. Each one focuses on their own tasks and the air is fragrant with the smell of freshly cut wood. The extreme stress on the gigantic elements supporting the frame does not bother the men, who slip between them with ease, sometimes jumping from one girder to another, happy to be contributing to restoring its full glory.