When it comes time to get a home roof installation because your old roof has deteriorated beyond repair it is time to call a roofing contractor. In many states, like California, roofing is a specialty construction trade and there is a separate license required for it.
And in most instances a roofing contractor will only roof houses and will not perform other construction work for the general public. Of course a lot of people feel that if they have an asphalt shingle roof then it is easy to re-roof it themselves. And in some cases this may be true.
One nice thing about having a reputable roofing contractor re-roof your home is they specialize in home roof installation and will know how to overcome any issues they encounter and will also extend some form of guarantee along with the manufacturers guarantee. This means that if the roofing material fails within the manufacturers warranty period the roofing contractor will handle the communication with the manufacturer or their agent.
This in itself may be a good reason to hire a roofing specialist for your home roof installation.
Another reason to hire a reputable roofing contractor is they can pull the building permit required by the municipality that governs building in your area. Since roofing is considered a specialty trade, many cities or counties require a building permit before any work is performed.
In some cases this may be that in a particular city you can only have so many layers of roofing material, generally asphalt shingles, on a roof. When that point is reached then the entire old roofing material has to be removed. Some cities may allow four layers others, another number of layers. Generally if there is one layer of asphalt shingle roofing material on a roof, a second layer may be added on top of it. And there is no reason that this would not be allowed because one layer of roofing material does not weigh too much and the second layer will cover the old material evenly.
Another reason to limit layers of asphalt shingle roofing material is that if too many layers have been added your basic roof structure or the decking may be rotted. To find this out properly, the shingles may have to come off the roof.