There are some videos online that appear to show flashes of light on top of notre dame cathedral prior to the fire.
However, the roof of the structure was metal.
Does this video show someone starting the fire?
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The video in question comes from this webcam: Viewsurf: Cathédrale Notre Dame. This webcam only takes one 60-second video every hour, at 5 minutes after the hour (17:05, 18:05 etc). Here's a Youtube video which shows snippets recorded at 17:05, 18:05, 19:05 etc.: Viewsurf Notre Dame Webcam: 20190415 - 17:05 to 21:05. The flash occurs at 0:23. It's pretty hard to notice; you have to know where to look (somewhere where those small spikes end).
Now, this video shows that the flash occurred at 17:05, and as per Wikipedia, the fire started at 18:50 (which this video confirms because you can see that there's no fire at 18:05 and then there's fire at 19:05).
What I find strange about this is that it would take almost 2 hours for the fire to become noticeable to anybody, if we assume that the flash was actually the very start of the fire. I'm no fire expert, but it just seems too much time for the fire to develop (in the video even at 18:05, i.e. one hour after the flash, there's still nothing visible).
Therefore I'd bet that was most likely a reflection from the sun. This is also corroborated by the fact that the sun is behind our back in that video (not so visible at 17:05, but look at the shadows of the spikes at 18:05. It seems the shadows are moving from left to right). A more precise determination of the webcam position and the position of the sun at 17:05 would probably confirm that.