Stucco on a rigid backing incorporates a rigid backing of sheet material (plywood, fibre-cement or extruded polystyrene) fixed to the cavity battens. A wall underlay is then fixed to the rigid backing, and the sand-cement plaster mix is applied over metal lath reinforcing onto the wall underlay. While the drained cavity also incorporates a wall underlay, the underlay over the backing sheets acts as a slip layer that allows the rigid cured plaster exterior to move independently of the timber frame and rigid backing, which minimises the potential for cracking.

Stucco on a non-rigid backing incorporates a heavyweight absorbent wall underlay that is fixed directly to the cavity battens. While the cavity also incorporates a wall underlay, the underlay on the face of the battens acts as the flexible substrate and slip layer that the metal reinforced sand-cement plaster is applied to. The fixings of the underlay to the battens allows for the plaster to move independently of the timber frame, which minimises the potential for cracking.

Stucco with rigid backing.

Stucco with non-rigid backing.

There is always debate over which of the two systems is the best – the designer will specify the preferred system.

Stucco claddings are used to create a monolithic finish on a building. The weathertight exterior coating creates a weatherskin and a face seal to the cladding that is very airtight. Uncoated plaster edges are very absorbent, and they will also wick water from adjacent surfaces such as waterproof decks and roofs. All exposed edges must be well coated.

Uncoated stucco is very water absorbent, and its weathertight performance is totally reliant upon the exterior coating system creating a face seal weatherskin that is impervious to moisture.

Stucco is also brittle and does not handle movement well, so it is vulnerable to cracking. Movement control joints must always be incorporated into the finished cladding. They may not eliminate cracking entirely but will minimise and manage it.

The application of stucco is a very specialist task, so always employ highly skilled plasterers to carry out this work. The plaster mix must incorporate the right mix and type of materials. The coats must also be adequately cured prior to the next coat being applied.

While generic details can be used as a basis for detailing things such as window and door penetrations and movement control joints, exterior detail design needs to be carried out by a competent and experienced designer. Stucco is not a proprietary system, and there are a range of details that can be used for the same situation. Stucco cladding must be detailed comprehensively and accurately to ensure a weathertight performance.

Stucco claddings must always be installed on a drained and vented cavity, as this ensures that there is a good level of secondary protection in all building types as the cavity provides good drainage and air circulation for drying. Direct-fixed stucco is not covered by E2/AS1, and if proposed, it must be submitted for consent as an alternative method.

Owners should also be made aware that these systems are very high maintenance and that the face seal coating system must be well maintained. Any faults in the cladding such as cracks in the plaster must be repaired immediately as stucco is intolerant of moisture penetration.

Stucco is vulnerable to cracking (followed by moisture absorption) as a result of:

• poor mixing of the plaster
• poor installation
• incorrect curing
• thermal movement
• ground movement
• building movement
• joint cracking
• shrinkage cracking
• poor surface coating application
• lack of maintenance.