Wire mesh is always needed when installing a scratch coat. A mesh/lath system is integral in creating a structurally sound floor or wall. Wire mesh is metal weaving that usually comes in rolls and is cut to fit the wall or floor. It is nailed or stapled with some room between the mesh and surface to allow the scratch coat to go through the mesh, and fill in the space. The mesh acts as a support system for the scratch coat. It is a strong, interlocking material. Without wire mesh, the scratch coat will not have anything to keep it from cracking and slipping.
Alternatives to Wire Mesh
Some kind of mesh support is always needed when applying a scratch coat. Since wire mesh can be difficult to use because of thinks like unrolling difficulties, heavy weight, and cost, an alternative is available. A fiberglass mesh/lath system can be used in place of wire mesh. The fiberglass mesh serves the same purpose as wire but it is corrosion free and can be installed much easier than metal mesh.