Before you acquire materials you must calculate the surface area of your roof. You cannot simply measure the length and width of your house and multiply them together. You must take into account the pitch of your roof as well as any added gables or dormers. This means that you should draw a map of your roof, then climbing up there and take measurements of the various surfaces. Make use of your high-school math and recall that Length X Width = Area. For triangular areas it is Length X Height X 1/2 = Area. Add up all your surface areas and you should come up with a close approximation of square footage.
Asphalt shingles come in bundles. Normally 3 or 4 bundles make up a square. A square of shingles is the number of shingles covers 100 square feet of roofing area. A bundle of shingles weighs about 70 pounds. There are about 27 strips to the bundle. For transporting these items to a roof, p use conveyor belts or fork lifts. A layer of felt paper is put down before the shingles are hammered home. Felt paper comes in rolls and generally has painted lines that you can use as a guide for keeping shingles straight as they are laid. Rolls come in various lengths. Roofing nails are fairly short and have a fair sized head. This makes them ideal for holding down the soft asphalt shingles. Your nails should be long enough to penetrate through any layers of shingles as well as the plywood sheathing. (1 1/4 inch nails should be sufficient for a new roof. Use 1 1/2 if going over a layer of old shingles.) A large quantity will be necessary. Each shingle will take a minimum of six nails to secure it to the roof. Drip-edge is used help protect the trim boards, facia boards and gable-ends. This is usually made of light aluminum sheet metal, easily cut by tin snips. It also helps prevent moisture coming back up underneath the shingles at the edge of the roof. Flashing is basically a role of sheet metal. Flashing may be cut with tin snips. It is placed around vertical surfaces such as chimneys and dormers as well as in valleys in order to form an impermeable barrier at these vulnerable points. There is specially designed pipe-flashing with a rubber gasket that helps seal vent pipes that protrude through the roof. 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. 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. When all the shingles are removed a quick survey of the roof is taken to make sure that you actually got all the old nails. 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. .