Modern Stucco

Modern Stucco usually consists of 1 layer of wire lath and 3 layers of lime-based plaster. Portland cement is also often used instead of Lime. However, great care must be taken if Portland cement is used because Portland cement does not breathe. It can - and does - trap moisture because of its low moisture permeability. Its crystalline structure cannot accommodate minute movements in the building structure (as Lime does) and is thus very prone to cracking. While Portland cement plaster has high impact resistance and sheds water, its impervious nature can promote mold growth by surrounding the building in the equivalent of zip-lock bag trapping humidity in wall structures and cooler cavities as well as rainwater leaking into the surface through the cracks in the Portland cement surface.

First a Wire Mesh is attached to vapor permeable, water-resistant "tar" paper if stucco is being applied to a timber or light-gauge steel frame structure. The paper protects the sheathing and interior of the wall from outside moisture intrusion without trapping moisture vapor in the wall. Wire lath is used to give the plaster something to attach to and may be expanded-metal lath, woven-wire lath, or welded-wire lath. In Europe, wire lathing with fired clay an integral "brick-mesh" is used in many applications instead of expanded metal lathing.

The first layer of plaster is called a scratch coat. A "comb" is used to scratch the surface horizontally or in a criss cross pattern to provide a key for the second layer.

The next layer is called the Brown coat. It also consists of Lime and sand. It is trowelled on to provide the surface with texture.

The final layer is called the finish coat and is smoothed and leveled with a steel float. Expert plasterers can give the finish coat an almost glossy finish ready for paint or wallpaper.