Notre Dame should use Transverse Laminated Timber and Glass, not Oak and Lead, in Rebuilding its Roof #NotreDame

T what extent should the rebuilding of Notre Dame’s Roof Trusses attempt to replicate blindly 12th century materials or to what extent should we make use of more modern techniques that mimic the original design’s surface appearance?

We are lucky that due to laser scanning we now precisely its appearance.  Violet le Duc in the 19th Century had no such advantage.  The stripping of gargoyles and statutory in the French revolution had no records kept in many cases.

To restore a building is not to maintain it, repair it or remake it: it is to re-establish it in a complete state which may never have existed at any given moment…The restoration had to involve not just the appearance of the monument, or the effect that it produced, but also its structure; it had to use the most efficient means to assure the long life of the building, including using more solid materials, used more wisely

Today he is sometimes accused of overrestoration.   Had he not restored Carcassone and had English Heritage been today in charge of restoration it would have been restored as a stabilised ruin even if that meant many towers being unsuitable.

But a stabilised ruin is no option for Notre Dame as it was for example at Coventry Cathedral.  It is both a national symbol, where Napoleon was crowned emperor, and an outstanding example of Gothic architecture, which France invented and le-duc commented was Frances national style. The Notre Dame we saw before the fire was a recreated of a severely damaged cathedral stripped of its statuary during the revolution and having lost its previous 13th century spire as it was unstable in the wind.  Le-duc essentially recreated a Gothic of the imagination, or more precisely of Vogtor Hugos imagination, as his novel the Hunchback of Notre Dame was an extended guidebook to how it appeared in the middle ages.  Led Duc rebuilt a larger and more stable spire.

Looking up at the trusses and ceiling of a Gothic cathedral they appear as stone, when in fact you see plaster beneath an oak beam structure topped with many tonnes of lead.  As we found at York and Windsor you cannot always replace like for like as the original masonry structure may have expanded with the heat and lack of a binding roof.

As Oak cannot be seen, is heavy, not fire resistant, and environmentally damaging.  Where are the 1,300 primeval mature oaks over 52 acres?   Certainly no longer France, and even if a patch were found deep in eastern Europe somewhere why not leave them in place.  Similarly lead.  Now we have far cheaper and lighter composite roofing materials, which no one would ever know the difference in seeing them from the ground.

Having just burned down why rebuild a roof in a location where it is hard to fight fire in a highly compustable material?

A more modern and cheaper, lighter and stronger material is TLT transverse laminated timer.  It chars not burns.  Studies support the counter-intuitive idea that charring produces an insulating layer that actually slows pyrolysis, making it advance predictably and sparing enough wood to pass two-hour fire-resistance tests. Being cpoisted from softwood it retains moisture and fire resistence, whereas 9 century old oak is tinder dry.  .  TLT, made of laminates of softwood and epoxy it is almost as strong as steel and far lighter.

Pieces of sawn timber are graded for strength, before being glued together
under pressure with the grain in the laminates running parallel to the
longitudinal axis of the section. Strength-reducing defects such as knots,
splits and sloping grain are randomly distributed throughout the component
allowing it to be designed to higher stresses than solid timber of the
same grade.

Use of modern materials raises the possibility of striking architectural interventions.  Many a gothic cathedral prior to electric lighting had stained glass windows replaced with clear glass.  Using TLT a roof can in effect be engineered as a curtain wall, a fully glass roof stairing up to heaven.

Macron wants the restoration done in 5 years.  Le Duc took 25.  Often the key constraint is craftsman.  But if your not using oak you dont need master craftsman.  You can and should build the wood in a factory.  A roof a parametric structure designed in Revit with a computer structural and fire resistance model.

One thought on “Notre Dame should use Transverse Laminated Timber and Glass, not Oak and Lead, in Rebuilding its Roof #NotreDame

  1. I remember visiting Reims cathedral a few years ago and being amazed to see a beautiful concrete beamed roof. It was bombed in WW1 and they took the sensible and the then quite novel option of rebuilding in concrete.

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