When I was doing napkin math it seemed pretty surprising to me that, for example, a 30 year roof replacement at an estimated $30,000 amortized over 30 years yields 1000/year, or $83 per month.

Now consider a set of tires for a car, replaced every 6 years, at $1200. That amortizes to $16 per month.

The replacement tires is only 4% of the cost of a roof replacement (1200/30000) but the amortized monthly cost of the tires is nearly 20% of the monthly cost of the roof replacement (16/83).

Long story short, I got increasing anxiety doing this math and watching my expected savings income evaporate over the long term as these amounts added up.