[theoh]

Of course fire prevention measures can and should be taken in old buildings. Installing unobtrusive systems with the goal of protecting an authentic structure is routine, as is finding any and all possible ways of mitigating fire hazards without changing the character of the building.

This is a totally separate issue from the bizarre hylomorphic idea of replacing limestone and marble with granite(?!) and porcelain.

Here's an example of unseen functional reinforcement of an historic roof with a modern steel truss: https://www.julianharraparchitects.co.uk/st-annes-limehouse. This is a case of pragmatism and economy. It's a long way from completely replacing the roof structure with a modern solution.

The idea that is going around of rebuilding the roof of Notre Dame with diamond/Kryptonite/carbon fiber as a way of trumpeting how advanced we are is annoying. The consensus these days about restoration is that it should be about making humble, minimally invasive, economical changes. Just put things back in order. Put in a sprinkler system. This is not an occasion for making a tacky gesture that will only reveal how mediocre 21st C architecture is in comparison to medieval Gothic.

[burfog]

That steel truss is not a long way from completely replacing the roof structure with a modern solution. If anything it is more a departure from the original, which stood alone without help. It's not even an option for Notre Dame, because there is no longer any original wood or lead that one might want to save.

For a truly authentic roof appearance, only the lead surface matters. Lead is actually an awful substance to use though. It had cracked, letting in water to rot the wood. Swapping out that metal for one that is lighter and more durable would be sensible. I don't think it would be sacrilege to pick a better metal. Suggestions: phosphor bronze, surgical stainless steel, nickel, titanium, titanium nitride, titanium aluminum nitride, gold leaf over aluminium oxynitride, Monel, Inconel, cupronickel. Some examples here: http://finemetalrooftech.com/products/metal-shingles/

Limestone gargoyles only last a century. They aren't even old! They disintegrate due to the water flowing through them. The other grotesques, such as the chimera, were also pretty well disintegrated. Limestone is fundamentally terrible for long-term durability. The stone chemically dissolves, causing statues to turn crumbly and look melted. Glassy materials are far superior. All desired colors are available, so matching the original color is possible. You can go natural (white granite, black granite, etc.) or artificial.

There is nothing tacky about building with durable materials. Tacky would be like my house, with stucco over styrofoam, a very common construction method in the USA. Granite is the good stuff. Limestone is junk, almost as bad as my stucco over styrofoam.