The truth is that anyone can replace their own roof, with the help of the many DIY websites. For residential homes, roof replacement usually takes two days: the first day to remove the old roof and the second day to install the new one. An additional day or two may be needed, depending on the size of the roof. More expensive due to labor and additional materials, your only option is to remove the old roof down to the coating if it already has two layers.
Old House's general contractor, Tom Silva, prefers this route even when the roof is single-layer in order to find the source of any water damage, improve the covers and install shingles to make them flatter. Tearing off the old roof is just muscle and sweat. Applying the new roof requires a little more skill and must be done quickly so that the roof is not vulnerable to the elements. This, combined with the danger of being on a roof, makes it a difficult task for those with little experience with roofing.
No, it's really not that difficult, especially with a roofing nailer and a pancake compressor. If you're going to tear off the old roof, that will be the hardest part. But definitely be sure to handle the covers, the ventilation grilles, the drip bar, etc. And if any of the roof coverings look or feel bad, don't try to escape without changing it.
Depending on where you live, a composite roof should last 15 to 30 years, but it won't if the coating is bad. The packages of shingles weigh approximately 80 pounds each and roofing is hard and hard work, it would be a shame to go through all that trouble just to save your insurance company money.