When you need to chose a roofing contractor for a home roof installation you will need to find one first. Check with neighbors or friends for their recommendations or go online for roofing contractor names. However you select the ones you call, you will need to set up an appointment for them to look at your roof.
Now you have three names who think you may do a good job on your replacement home roof installation. When you call them you will need to tell them what you have in mind. If you want to something radical like install a green roof on your home then you will also need to ask some questions like how much experience they have had doing this type of application. They may choose to send you a roofing questionnaire via email. If they do this then you can fill it out and in that manner tell what your have in mind.
Once you have three names of roofing contractors and the appointments are set you can sit back and relax until they make their visit. They will get up on the roof and walk it. And measure it. They may test some shingles.
They will look at the flashing and gutters and fascia and other roof parts for damage or mold or moss or other problems like broken flashing or cracked sealants if they can spot them.
Note: If they do not go up on the roof and simply measure it from the ground, which they can do, as they can figure the pitch and other components of a roofing calculation, then you may want to call another roofing contractor. You should do this because that contractor has no idea what other problems may be on that roof. Still get the estimate from them but look at it very closely.
When you have the three estimates you then need to compare. Make sure you are looking at similar roofing materials and any options so you can figure out if the estimates are for the same roof. If they appear to be different then call and ask questions. It is too late to ask questions after the work has started.
When you have compared the estimates and spoke with each contractor then find out just how long they estimate the job will take. Shorter is better. You are now on your way to a home roof installation.