Thomas Armstrong - Information Zone

Renders & Plasters

Renders & Plasters

Preparation and Good Practice:

• Mortar joints should be raked back 10mm to provide additional key for accepting the render or plaster.
• The wall surface should be clean and free from dust, debris and residues such as moss like growths
• For lightweight and dense concrete block backgrounds, the blocks should not be wet.
• For Airtec aerated blocks, dry blocks should be lightly brushed with water immediately prior to applying the render to overcome the blocks' natural suction and avoid the render/plaster mix drying out too quickly and losing adhesion.
• At least 2 coats should be applied, the first coat being stronger than the following one.
• The first coat should be applied to an approximate thickness of 8-12mm and scratched on its surface. The final coat should be weaker and applied thinner at approxiamtely 6-10mm.
• External renders - apply down to DPC but do not bridge DPC.
  

Render and Plaster Mixes (Lightweight, dense concrete and Airtec aerated blocks):

• The use of cement/lime/sand mixes are recommended because they are less susceptible to crazing and cracking than cement/sand mixes.
• It is important that the undercoat in the rendering is stronger than the finishing coat.

External Renders:

Mix Proportions (by volume)	Base Coat	Finishing Coat	Notes
Cement:Lime:Sand	1 : 1 : 5	1 : 1 : 6	Type iv mixes ( 1:2:9) are more suited to sheltered exposure conditions. Type ii mixes (1:1:6) are more suited to moderate and sever exposure conditions
Cement:Sand (with plasticizer)	1 : 5	1 : 6

 

Internal Plasters:

Plaster Type	Base Coat	Finish Coat	Notes
Dense	1 : 2 : 9 or 1 : 1 : 6 Cement:Lime:Plastering Sand	A skim coat of gypsum plaster approximately 3mm thick	Avoid rapid drying out of the base coat
Lighweight	Proprietary gypsum plasters requiring only the addition of water for both base and finishing coats are available. Always follow the manufacturer's instructions.		Never mix Portland cement and gypsum plaster