Undercoat

Clay undercoat plaster is a blend of clay, fine aggregate and plant fibres that is supplied as a dry powder. It has an indefinite shelf life if stored dry and requires only mixing with clean water to be ready for use. It hardens by drying - no chemical reaction takes place - and so remains workable for a long time and can be re-worked by the addition of water at any time.

Quality of the product is monitored by tests carried out by the manufacturer:

• Tests of physical properties to ensure suitability of clay and aggregates.
• Tests to ensure uniformity and performance of the batched product.

Clay undercoat plaster is suitable for use on internal surfaces of walls and ceilings. It attaches by mechanical bonding to all stable and dry substrates, including masonry, clayboard, reedboard, timber lathing and lime undercoat plasters. Undercoat plasters are not recommended for use on plasterboards because during drying, water will be absorbed by the board which will then become unstable.

It is a practical alternative to using gypsum or lime plasters and is compatible with breathing construction in historic and new buildings.

Clay undercoat plaster can withstand minor movements of the background material without cracking. However the plaster should be reinforced when there is a possibility of structural movement or if different backgrounds are being covered.

Physical properties of plaster:

• Density 1500 kg/m³
• Thermal conductivity (k) 0.66 W/mK
• Vapour diffusion resistance factor (ref air) u8