We have reserves set aside for replacing cars, roof, etc. We're unlikely to have to do any of them more than once (due to our ages).
If you don't do that, then your budget should include funds that are being set aside for this purpose in the future.
Example:
Let's posit that a new roof will cost you $6000 and will last 20 years. That's $6000 / 20 = $300 a year.
Let's say that your FIRE income is $30,000.
$300 is 1.0% of $30,000. You should be setting aside 1% of your annual income for a new roof.
Note that I said 1%, not $300. Your income should be adjusting for inflation and the cost of a roof will, also.
Repeat for all the likely expensive things that can go wrong, then add in a dammit factor to account for the things you forgot about.
Hope that helps!