6.11.8Render onto board backgrounds

Render onto board backgrounds shall be suitable for the intended use and detailed to provide satisfactory performance. Issues to be taken into account include:

  1. provision of a system manual
  2. compatibility between the render and background
  3. fixing back to the structure
  4. weather resistance
  5. movement joints
  6. board backgrounds.

Provision of a system manual

Where render is applied to a board background, the render manufacturer should clearly define the system in a manual, including:

  • materials and components
  • common details
  • design guidance
  • installation guidance.

The system should be used in full accordance with the manufacturer’s guidance and recommendations.

Compatibility between the render and background

The background should be appropriate for its intended use. Issues to be taken into account include:

  • compatibility between the board and render
  • durability classification of the board and its suitability for use in exterior conditions, including resistance to weather prior to the render being applied.

Render onto board backgrounds should:

  • not be applied where the surface has contamination, dust or loose particles
  • consider the effects of solar radiation (colour, orientation and shading)
  • be mixed to ensure colour consistency where coloured pigments are specified
  • be specified and used with the appropriate ancillary items, such as trims to form corners and returns.

Boards should not be left exposed prior to rendering for longer than is necessary.

Fixing back to the structure

Board backgrounds should be fixed back to the structure in accordance with the manufacturer’s recommendations.

The fixing design should consider:

  • wind load
  • pull-out strength
  • pull-through resistance
  • anticipated movement.

Fixing battens and rails should be installed vertically and not block drainage paths. Timber battens should be suitably treated.

To reduce the risk of damage from impact, especially at low level, where people have access around balconies and where cradle systems etc. can come into contact with the façade, appropriate precautions such as closer supports should be considered.

Cavity barriers should be appropriately detailed to ensure satisfactory performance and:

  • be provided in accordance with building regulations
  • not block ventilation or drainage paths
  • account for movement in the frame
  • be used in accordance with the manufacturer’s recommendations.

Weather resistance

Timber and steel framed backing walls should have a minimum 25mm cavity.

Cavities to timber framed walls should be drained and vented, and cavities to steel framed walls should be drained.

Movement joints

Movement joints should be provided to accommodate movement in timber frame structures. Where board backgrounds are used, movement joints should be:

  • formed in accordance with the system manufacturer’s recommendations
  • continued through the background board
  • positioned to accommodate calculated deflection or movement
  • provided at floor zones.

Board backgrounds

Board backgrounds to be rendered should be external grade and recommended for use in the render manufacturer’s system manual.

Boards should be set out in accordance with the system manufacturer’s recommendations, taking account of possible compression, deflection and alignment of joints in relation to openings in the external wall, such as windows and doors.

The render should have alkali-resistant mesh embedded into the base coat across the whole surface.

Edges of boards should be suitably treated to provide protection from weather during construction and to maintain durability after the render is completed.