Source : Webster's Revised Unabridged Dictionary (1913)

Overlayer \O"ver*lay"er\, n.
   One who overlays; that with which anything is overlaid.

Source : WordNet®

overlayer
     n : protective covering consisting, for example, of a layer of
         boards applied to the studs and joists of a building to
         strengthen it and serve as a foundation for a
         weatherproof exterior [syn: {sheathing}, {overlay}]