Live in the smallest unit and manage the remaining 3.

For example, if a new roof costs $5000, and the current roof is in good shape and has 10 years of remaining life; then what is monthly cost that should be allocated toward roofing.

@Jon Holdman good explanation in plain English, there is no disagreement I see, my statement refers to several risks, market risk, credit risks, repayment, collateral and so on, the interest rate charged is based on the risks assumed, including the risk of loss from any aspect.On the hard money lender side, they mostly set rates to the market, as much as they can charge and remain competitive, while they do some underwriting they seldom go  through any risk analysis, one reason the loan pool is usually too small to bother with it. 

In finance and accounting we look at a "Quick Ratio" (also known as the Acid Test) this shows the ability of a company to meet obligations, but this only looks at current assets or liquid assets and current liabilities, real estate is not a liquid asset.So we move to other ratios like "Current Ratio" and "Cash Ratio" these to help to illustrate the ability of a company to cover current debts.As to mortgages or long term debt, coverage is difficult to predict due to uncertainty over longer periods, but by looking at the current coverage and cash available we can see what funds remain for these other obligations.Most everyone is familiar with the debt to income ratio we suffer through at a loan application, say 24/36, meaning that you should not pay more than 24% of your income to current liabilities, revolving accounts and car loans, the 36% includes the 24% to establish your ability to cover the mortgage, total debts should not exceed 36% in this instance.

Almost $165 billion of mortgages for skyscrapers, shopping malls and hotels are due this year.While financial-market conditions “have improved considerably in recent months,†the markets for ABS and CMBS “are still impaired and seem likely to remain so for some time,†the Fed and Treasury said.The central bank said it doesn’t intend to make other types of collateral eligible for the program, indicating officials rejected adding residential mortgage-backed securities after considering such a move for several months.

YOU set in a wheelchair for a whole day and see how things for normal people dont work for chair bound people.I would think you would want these ladies as happy as possible for there remaining years,,they wont move,,,do the work.And when they pass on and it is available,,advertise as wheelchair friendly and watch the phone ring.

Meanwhile, your deductions all remain passive. 

I agree with @Douglas Vogel that underwriting should remain conservative and disciplined - beating your own projections should be the gravy.  

Until we (as a society) have effective vaccines, treatments, and some kind of immunity/immunization, those people in high risk groups will remain mostly isolated.

Now with regards to him not finishing the job and/or correction of unfinished work, get someone to give you a price to finish/correct the remaining work, and deduct it to his contract.