Sheathing commonly refers to 4x8 wood panels (OSB or plywood) that are used on top of joist, studs and rafters to provide structural support and as a base material for finishing products such as shingles for roofs, siding and hardwood for flooring.

To support sheathing, framing usually employs one of two methods.  The first is blocking that provides a solid outline under the sheathing.  The second employs clips that go between framing members and keep adjoining sheathing boards from warping.

Blocking or Clips

The number of nails that will be needed are determined by the pattern of nails.  There are two different patterns for nails based on whether there is blocking that allows continuous edges or if clips (see picture) are used.  The diagram shows the two patterns.