Removing Old Shingles
It is okay to roof over a single layer of shingles. Shingling over more than one layer can cause problems with the roof's waterproof integrity. It also adversely affects the weight load supported by the structure. If you are doing a shingling job on an older home, it is highly likely that you will need to tear off old shingles.
Shingles are taken off in the reverse order of which they were put on. Which means that you begin at the top, pulling off the ridge shingles first. Be sure to remove old nails, or at least pound them in as you go. Once you get the peak off, you can get behind and above the shingles and wedge your shovel or ripper beneath them, sliding down until you catch a nail and then levering the handle down to raise the front of the shovel up, popping out the nail and freeing the old shingle. Make sure no one is below you, the shingle is likely to slide off the edge of the house if you have any kind of slope at all. If possible position the trash container so that you can simply dump the old shingles into it from the roof. To the best of my knowledge there is no use for old shingles. They pretty much must be hauled to a landfill. Do not think of burying them in your yard or cavalierly disposing of them down some country road or on a vacant lot. Asphalt shingles are a petroleum product and will ultimately create an environmental mess that will need to be cleaned up. Landfills are equipped to deal with them.
In the course of shingle removal, you need not remove old flashing, roof vents, pipe-flashing, sky-lights or stacks unless they are damaged in a way that makes them a problem. However, this is a good time to gauge whether you wish to put new vents, sky-lights or even build a dormer on the roof. Often, the requirements for ventilation have changed since the last time the house was shingled. Remember, vents help to keep moisture (from condensation) and heat out of the attic.
When all the shingles are removed make quick survey of the roof to make sure that you actually got all the old nails.
If your slope is fairly steep, you will want to be sure to use the proper safety equipment and scaffolding for this phase of the project. Carpenters often refer to various slopes with a number - really a ratio. You may have heard someone say that his roof is a 5-12. The first number is the rise, and the second number is the run. So regarding the 5-12 roof, we know that for every five feet of elevation his roof is twelve feet wide out to one side. Roofs vary in pitch depending on style and the amount of snowfall in a region. Steeper roofs will typically be more difficult to work on because they require so much more scaffolding. However, they tend to wear better.