New roofs need to keep a house protected from the outdoor weather and be sturdy and strong. Do away with that old chimney on top of the roof and replace it will a ground chimney for venting the furnace outside. Modern improvements to your home should always include doing away with the chimney and choosing the alternative method for venting.
Not only does this make reproofing the house easier, but also is more attractive to view. The roof needs to have new shingles about every twenty to thirty years, depending on the type of shingle you use, what the rating is for length of time. Some roofs hold up well, depending on the weather conditions you live with.
For the most part a new house has the proper trusses and do not need replacing. If you have a home that was built many years ago with different types of trusses, they may become old and rotted causing them to need replacing. If this is the case, your roof will need more help then just shingling. You need to replace the older unsteady trusses with newer trusses.
Always remember to remove the shingles entirely if you have more than three layers already, this is a code in most states, but also necessary for proper roof care. Once you determine your requirements for a roof the rest is easy, even you may want to try it if heights do not brother you. So plan on doing your roof and check out the underlay and the trusses ahead of time to prevent added work and money during the reproofing project.