The basic steps to install a roof are:
Removing the old roof first, such as shingles, nails, and any debris that is on the roof.
Clean up the roof's surface so the new material will adhere properly and firmly.
Install the new roof. The contractor will lay down a layer of tar paper or synthetic underlayment and then the roofing material you have selected.
The last steps include trimming the edges of the roof, installing flashing around the gutters, vents, and chimneys, and applying sealant around butt joints to add a waterproof element to your roof to withstand California weather.