Shingle roofs are generally installed over plywood decking. Most residential roof decks are 1/2″ plywood or Oriented Strand Board(OSB). Both types are acceptable for standard roofing applications in a non-coastal area. The picture here shows an OSB deck where the old shingles have been removed. When re-roofing a house it is important to make sure that the decking is clean and without damage.
The deck needs to be fastened down properly all of the old roofing nails removed. This step is a critical part of installing a new roof. Once the inspection is complete then a layer of felt should be installed. Architectural shingle are installed over the felt for a great looking roof that will last for decades.