Backgrounds shall be appropriate for their intended purpose and suitably prepared to receive render. Issues to be taken into account include:

  1. preparation of masonry backgrounds
  2. preparation of clay brick backgrounds
  3. ribbed metal lath.

Preparation of masonry backgrounds

Masonry backgrounds should be constructed in accordance with Chapter 6.1 ‘External masonry walls’ and include DPCs and cavity trays. The thickness of single-leaf masonry walls should be in accordance with PD 6697.

The surface to be rendered should be free from dust, loose particles, efflorescence and organic growth, and, where applicable, be prepared in accordance with the render manufacturer’s recommendations.

Masonry backgrounds with a smooth surface or close texture should be treated to provide an adequate key by either applying:

  • lath, or
  • a spatterdash or stipple coat.

The suction of the block should be appropriate for rendering. High or low suction will generally require a preparatory coat. The likely suction of the block can be gauged by applying a small quantity of water to the surface and observing the effects:

  • Water being absorbed instantly is an indication of high suction.
  • Water running from the surface with little absorption suggests the background has low suction.

A spatterdash coat typically comprises cement and sand at a ratio of 1:3 mixed with water and often a bonding agent, such as styrene butadiene rubber (SBR) or ethylene vinyl acetate (EVA). The mix should be applied by dashing onto the background to give a rough texture approximately 3-7mm thick.

Generally, raking out mortar joints to blockwork will not sufficiently improve the key, and may extend the curing time of the base coat.

Preparation of clay brick backgrounds

The brick manufacturer’s recommendations for rendering should be followed.

Where S1 bricks are used, the render mix should resist sulfate.

To provide an appropriate bond, clay brick backgrounds with a water absorption rate of between 9% and 15% should generally have sufficient suction to provide a mechanical key. Alternatively, when rendering onto bricks, one or more of the following methods of improving the key can be adopted:

  • Keyed bricks used.
  • A spatterdash coat applied.
  • Mortar joints raked out to a depth of 10-12mm (although this may increase curing time).

Render on an external leaf of clay bricks (F2,S1 or F1,S1 designation bricks to BS EN 771) in severe or very severe exposures is not permitted where the cavity is to be fully filled with insulation.

Ribbed metal lath

Ribbed metal lath should be:

  • fixed in accordance with the manufacturer’s recommendations
  • supported at 350mm and up to 600mm centres for stiffer metal profiles
  • fixed with the correct side to be rendered facing out
  • fixed with a 25mm drained and vented cavity when applied to framed structures
  • austenitic stainless steel to BS EN 10088-1.

Render onto ribbed metal lath can be vulnerable to damage where impact is likely to occur, such as beside communal paths. Appropriate reinforcement may be used to help improve the render’s impact resistance.