A lath is a thin, narrow strip of straight-grained wood used under roof shingles or tiles, on lath and plaster walls and ceilings to hold plaster, and in lattice and trellis work.Lath has expanded to mean any type of backing material for plaster including a metal wire mesh or expanded metal that is applied to a wood or metal framework as matrix over which stucco or plaster is applied and wallboard products called gypsum or rock lath. Also reed mat was historically a lath material.