External finishes

Rendering
Render can be applied to any type of aggregate block walling in order to provide a specific appearance, enhance weather protection in exposed situations, and increase the performance of a wall with respect to sound transmission.

Choice of mix
The choice of render mix will depend on the appearance desired, exposure conditions, nature of the background and the functional requirements. Experience has shown that a porous render not stronger than that required for adequate durability and with an open or rough textured finish is likely to give the best results in the majority of circumstances. Problems can occur with mixes that are too strong. Successive render coats should be specified as being no stronger than the previous coat or background, and no thicker than the previous coat, except in the case of a single coat (refer to specialist manufacturers for guidance and specifications).

Movement joints
If a movement joint exists in the background material a coincident straight joint should be formed in the render. This may be formed with proprietary metal stop beads, with the resulting gap sealed with a suitable joint sealant. Where render is to be continuous across dissimilar background material, any differential movement at the junction should be accommodated by forming a straight joint in the render, which is subsequently sealed, or placing metal lathing not less than 300mm wide, with an isolating membrane across the junction and embedded in the undercoat of the render.

Undercoat
Prior to the application of the render, the background should be sound and assessed for its suitability to receive direct application of the undercoat. A render normally comprises of at least two layers, namely, an undercoat and a final coat. Metal lathing, sometimes used in severe exposure conditions or on weak backgrounds, should have two undercoats. Where improved resistance to rain penetration is desired, two undercoats should be used. Normally undercoats are between 8-12mm thick with the final coat approximately 6mm thick. Overall, the rendering thickness should not be less than 20mm for three coats work and 16mm for two coat work.

Suction
The suction and key of the background are important factors in determining the mixes to be used. Standard lightweight blocks i.e. Fenlite, Superlite, Ultralite and Evalite are medium suction and generally provide adequate surface key to allow direct application of the undercoat. An assessment should be made in case a pre-treatment is necessary for unusually smooth blocks. Facing blocks and Evalast blocks have low suctions and relatively poor keying properties. It is therefore recommended that the joints are raked backed squarely 10mm to12mm deep and a spatterdash, stipple or adhesive slurry is then applied to the surface. In the case of the adhesive slurry it is essential that the rendering undercoat is applied straightaway whilst the slurry is wet, otherwise it will form an effective debonding layer.

Further information on external rendering can be obtained by reference to BS EN 13914: Code of Practice for External Rendering, together with BS 8000: Part 10 Workmanship on Building Sites, Code of Practice for Plastering and Rendering.