As a remodeling contractor, I deal with roofers and bad roof jobs all the time. After foundation repairmen, roofers are the most dishonest people in the industry. It's very hard to find one that doesn't take advantage of clients, or cuts corners to get the job done as fast as possible.
First, talk to at least five different roofers before hiring anybody. The bigger companies rely on super friendly sales people to get you to sign a contract. It's proven that most people sign with the person they like the best. Some times you need to talk to more then five. There are a lot of roofers out there, which means there are a lot of crooks out there.
If you go with metal, be sure to get the thickest metal that you can. There is some stuff out there that is as thin as a soda can, and there is some out there that will still be keeping out water 100 years from now. Be sure that all your bids are for the same thickness of metal. And then make sure that's what they are installing when they start the job!!!
When getting the bid, make sure that everything is removed down to the decking. Some will want to leave the old tar paper on and then cover over it. The decking has to be inspected and repaired where needed. Know what type of decking you have. Solid pine 1x6's and other sized is very common in older homes. It's always full of voids that need to be fixed. There is always some boards that have broken and need to be replaced. If it's plywood, make sure it can be used again. Plywood likes to blow out on the underside and a lot of the time, it's a one time use material. It needs to have another layer put over it if it's really bad, or the bad pieces need to be replaced. OSB can usually handle a second or third roof pretty easily since the nails only poke throw it and don't tear it apart like they do to plywood. It should still be inspected because if there was a leak anywhere, it's the worse at falling apart.
All of the flashing and vents need to be removed and replaced. A ridge vent is by far better then any other type of vent. The higher the vent is in the roof, the more air it moves and the dryer your attic will be.
If going with metal, be sure to double check with the manufacturer on what type of underlayment is allowed under the metal. There are options and the only way to get your warrantee is to use what they say has to be used. Do not rely on the installer to use the approved material. Same thing with shingles, but they are usually more easy going and just about any standard underlayment is fine with them.
If using shingles, make sure that the nails are at least an inch and a half. I personally use 1 3/4 inch nails. The nail has to go all the way though the decking to hold the shingle in place. It's the shaft of the nail that holds it in, and if they use short nails to save a buck, the shingles will fall off in a wind storm. Staples are even worse and will tear the shingles. Never, EVER allow staples to be used on shingles. Also be sure to look at the package to see how the nails are to be installed, and how many. Sadly, most install crews don't speak or read English and they are told to use as few nails as possible. It's a small thing, but these guys are crooks and if they can do the job with one less box of nails, they will.
Valleys, vents and chimney flashing are where most leaks start. Skylights should be thrown away and sealed up. They always leak, it's just a matter of when. Depending on what you use, be sure that a proper valley is installed before the shingles or metal are installed. Be sure to have this in writing on the bid!!!
The bid needs to have everything in writing. What they like to do is give you a price. Lets say it's $20,000 to do the job, but it doesn't actually say new flashing, new vents or 1 1/2 inch nails. Or anything else that I mentioned. They just give you a price and it's better then the other guys, and the salesman is super nice. Then when the crew shows up, they do what they want and you never know what you are getting. Five or ten years from now, the roof fails. They blame something else for the problems and you are stuck with a bad room that will need replacing again. Crooks are great at not fixing what they do wrong. Get every single step and material used in writing, and be sure to inspect everything that they do. I've been hired to inspect roofs as they are being installed because this is such a common issue. Even with me there, I've caught all sorts of issues that they tried to cover up.
Lastly, do not feel pressured to hire anybody right away. Time and research is your friend. Only commit when you are 100% sure that you have found the best company to do this.
