However, the important thing is that Coinplate will survive a house fire, even if it's abnormally hot. House fires burn at around 600°C/1100°F (at max can get up to 850°C/1500°F), and it survived a much higher temp of 1093°C / 2000°F in that test quite fine.
[...]
My finger must have slipped, I meant 1.1kg. Yeah, that's how much our backups weigh. The weight of each is between 1.13-1.14 kg.
That's some serious density
and heat resistance!
While I'm still a big fan of the 'DIY washer method' (there's no
metal seed phrase backup, remotely as cheap) since a coinplate
does not cost $300 USD, I may consider it in the future for a very 'heavy' Bitcoin wallet.
Though if just your own house burns down (not a huge apocalyptic sized fire), you should have redundant seed phrase backups elsewhere.
Buried under the basement / in the cement sounds like a good idea.