How do you calculate CAPEX cost for an aging roof (or other big ticket item)?

[Jeff L.]

Let's say you're looking at a property and the roof (or furnace, or whatever) has 7 years left on it. A new roof will cost $3000.

What formula do you use to calculate that expense into your property analysis or cash on cash return?

Do you add the full $3000 to your rehab costs? Is it $3000 divided over some time period?