How and when do you calculate repairs and expenses ?

[Jorge Sosa]

Hello BP community, I'm new to real estate investing and I'm thinking of getting my first rental property this year. I've been already networking with people, studying, and doing my due diligence to inform me well of what work and what doesn't in my area.
One question that I've had in the process is the following: How do you calculate expenses and repairs on a property? at what point? What I mean is, let's say you see a house in the market that you like. You go and see it, if you are not an expert how can you tell if the roof needs to be replaced or if there are foundation issues? or how long will these things last so you can calculate expenses accurately? Do you bring a contractor with you when you tour the house so you have a better idea? or are these kinds of things discovered during the inspection? This would seem late because at that point you would already have to have an idea on expenses right?. So I'm a little bit confused about this part of the process. 

Any clarification would be appreciated!