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Question on rendering colour variation

New rendering to the gable house wall has colour variation and signs of cracking. The substrate is masonry. What has gone wrong?

Answer

Colour variation can be caused by several factors including variations in substrate suction and variations in mix proportions. Cracking maybe due to overly strong mixes or reflective cracking mirroring cracks in the substrate.

Identification of the reasons for colour variations and/or cracking will determine the recommended course of action. Colour variations can only be rectified by over-decorating. If the rendering has only minor cracking and little hollowness, the cracks can be filled by the decorating material or taped over with scrim before over-decorating. If cracking can be identified in the substrate, the rendering may have to be removed and replaced with the required jointing provided in the substrate.Overly strong coat mixes with cracking and extensive hollowness will have to be removed and replaced.

 
Acknowledgement: The Concrete Society
 
Other references:BRE Good Building Guide 23 Assessing external rendering for replacement or repair
BRE Good Building Guide 24 Repairing external rendering
BS 5262 Code of practice for external rendering
 

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