[blahedo]

Most of the "entries" that people are making for designs for the roof/spire replacement, and many of the posts on here, suggest that many people think that when you were looking up in the pre-fire Notre Dame, or any other church building of similar construction, what you were/are seeing is the underside of the roof.

It isn't!

What you are seeing is stone vaulting, essentially a great big three-dimensional arch, related to a hemispherical dome in somewhat the same way as a gothic arch is related to a Romanesque rounded arch. It may or may not be painted or plastered or frescoed (ND's wasn't, I think) but the actual "ceiling" of the church—and, crucially, the structural part of what's over your head—was/is stone. The roof in a modern building is often structurally keeping the tops of the walls at a fixed distance as well as holding itself up, but in a stone cathedral, the roof was an extra layer over the top of the stonework, primarily to shed rainwater.

All of which is to say, the loss of the wooden roof structure is a lot less threatening than the loss of (some of) the vaulting; and replacing the roof with glass or stained glass would be utterly invisible from inside the church, whose ceiling would still be the stone vaulting.

[Kyrio]

Yes, I think that's something that isn't always made clear. The roof is simply used to prevent water from falling on the stone vaults; when a fire destroyed the roof and timber frame in Reims during World War I, the vaults resisted and only collapsed later because of rainfall.

[robbrown451]

I think most people understand that the stone vaults are what is seen from the inside. At least most people who have given it a tiny bit of thought or looked at any pictures. Most, but not all, of the stone vaulting remains intact, although some of it may be weakened.

While they may comprise the bulk of the structure, I don't think it is correct to say suggest that the roof and roof supports (all that timber) is merely to keep the rain off. It plays a significant role in the structure as well. Sure, without an earthquake or strong wind the valuting is strong enough, but with the roof supports gone the building is substantially less able to stand up to extreme conditions.

As for replacing the roof with something else, it would have to be done where it contributes to the structure, not just sits on top. Glass can actually be structural, in fact. I like some of the new ideas, and don't think the ones I've seen are unfeasible, but they would simply (or not so simply) be done with a whole lot of regard for adding to the structure.

[blahedo]

I think your middle paragraph is much more true for modern (last 100-150 years) buildings than older ones, though I'm not an architectural engineer.

When you say "glass can actually be structural", what meaning of "structural" are you using?