How to estimate rehab properly

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Hello, I found a couple of rehab properties, I know the comps and market. I am having a little trouble estimating the rehab and the process. Can I send a general contractor (non specific) out there and they can tell me the exact rehab that needs to be done (price, scope of work and labor)? Or do I need to send a specific individual contractor for each part of the house? For an example, send a roofer to look at the roof, a painter to look at painting, etc. 

What tools or suggestions do you have to guide this process?