Definition of total ROI

[Alan Roebuck]

Got a question about calculating total ROI. We have a rental property and after owning it for about a year we put around 35K into major renovations. That was two summers age. When calculating total ROI you divide by the cash basis, but if we calculate the ROI year after year, do we always divide by the same denominator (down payment plus later renovations) every year, or do we look at the major renovations as a one-time event and go back to dividing by just the down payment a year after we made the renovations?

Or am I not asking the right question?  Any insight here would be appreciated.