If your home's roof is very old and you know it needs replacing, you may call a contractor and simply have him or her install the most affordable option for you and not give much added thought to their work. However, it's good to discuss your options for a new roof with your contractor and ask a few questions about the material and the job itself, so you know you'll be happy with your new roof and your home overall.
Ask if you can have a metal roof put over your old roofing materials
A metal roof can often be installed right over your old roofing tiles and shingles. This can save you the money of a tear-off and also reduce the waste and mess involved with a new roof. Note that this might not be an option for every home; while metal is very lightweight, this added layer might mean too much weight for a roof frame, especially for an older house that isn't strong enough to hold that weight. You might also have some added layering of roofing materials under your tiles already, and building codes may allow you to add only so many additional layers.
Note if they can upgrade your home's insulation
Many roofers are experienced in adding insulation to a home, as they may work on new construction projects as well as renovations, and may need to add that insulation before a roof can be installed. If your old roof needs to be torn up for repairs or for a new roof to be added, you might ask if they can upgrade your home's insulation at the same time; they might give you a discount on having it done on the same project, and new insulation might cut down on your heating and cooling bills.
Always ask their recommendations for other materials
A roof material should never be chosen by how cheap it is, as investing in a quality roof replacement will often be cheaper in the long run than having to consistently repair or replace those cheap tiles. Ask your roofing contractor if your roof needs replacement sooner than it should because the tiles are not right for your climate; they may be getting dry and brittle in the sun or not allowing snow and ice to melt easily off the roof. If they can't put metal over your old roof and you'll be getting new tiles, be sure to note their recommendations and anything they noticed about your roof and its damage during their first inspection, so you can make the best choice as to new roofing materials.