It had Cedar shingles, 2 layers of asphalt plus it required all new sheeting.It was the best $10,500 we spent & after 7 years of renting we sold the property for a $25k profit & hold the note @ 12%. 

We cash flow 300 to 500a door and I'm still shingling roofs.

Then, debt to income ratios for Fannie & Freddie requirements for loan approval which are just computerized algorithms.

lets use an asphalt shingle roof as an example that we will use 15 year life (in Texas I would think this is fairly accurate). 

I don't expect to have any outside work for AT LEAST the next two months, so next week I'll be up on a roof doing the "delightful" job of removing shingles on one of my rentals, followed by a bunch of painting & gardening.

Now the AC unit was 1 year old at that point, and I had just had the roof redone in architectural shingles that spring.

Asphalt shingles?

If you are doing multiple properties you can carry materials from one site to another (shingles for example).

A metal or flat roof is going to cost more than a truss roof with asphalt shingles. 

Some places will not insure a home if,as an example, it has two layers of shingles on the roof.I would be up front with the lender and up front with your insurance agent BEFORE you even commit to a formal credit check on a loan to get answers.Fannie Mae also has some really strange rules you might have to abide by...talk with your lender.