Render

6.11
Also See:
PD 6697

6.11.5Accommodation of movement

Rendered walls shall be detailed to reduce the risk of damage due to movement in the background. Issues to be taken into account include:

  1. movement in masonry background
  2. dissimilar materials
  3. movement in ribbed metal lath render.

The construction should include appropriate measures to reduce the risk of damage to the render caused by movement in the background, such as shrinkage, thermal or differential movement. The designer should follow the guidance in this chapter, together with the render/background manufacturer’s recommendations. Alternatively, provision for movement should be designed by an engineer in accordance with Technical Requirement R5.

Areas of the building to be rendered should be identified prior to construction, and movement control considered as part of the design.

Movement in masonry background

Render and masonry backgrounds should be detailed to reduce the likelihood of cracking and crazing in the render. Issues to be taken into account include:

  • the potential for movement in the background and render
  • size, quantity and positioning of openings
  • compatibility with the background
  • density of the masonry
  • the size and geometry of rendered panels
  • the orientation of the building
  • thermal shock
  • moisture content of the materials
  • exposure conditions.

Where length/height ratios are greater than 3:1, consideration should be given to providing suitably designed:

  • movement joints, or
  • bed joint reinforcement.

Where movement joints are provided, they should:

  • be continued through the background and render (including any horizontal beads)
  • be made weathertight with an appropriate sealant
  • not align with openings such as windows, doors or meter boxes.

Bed joint reinforcement should be provided in the first two courses of the external masonry leaf above and below any opening. Where possible, the reinforcement should project 600mm beyond the opening.

Table 2: Concrete block categorisation

CategoryCompressive strength of the blockworkDry density
Low density aircrete2.9-3.6N/mm2<500kg/m3
Normal density aircrete3.6-9.0N/mm2500kg/m3+
Ultra lightweight aggregate3.6-7.3N/mm2<950kg/m3
Lightweight aggregate3.6-7.3N/mm2950-1500 kg/m3
Dense aggregate7.3N/mm2+1,500kg/m3+

Table 3: Preparation of blockwork backgrounds(1)

CategoryNormal movement joint spacingMaximum distance of joint from restrained end, i.e. cornersSuction control
Low density aircreteSpecialist advice required(2)
Normal density aircrete6m3m (half normal spacing)Yes
Ultra lightweight aggregate6m3m (half normal spacing)Not generally required
Lightweight aggregate7.5 - 9mHalf normal spacingNot generally required
Dense aggregate7.5 - 9m Half normal spacingNot generally required

Notes

1 The guidance in this table is generally acceptable for render coats in accordance with Table 5 and factory-made one-coat render based on 1:1:6 mix = 3.5N/mm².
2 Specialist advice from the block and render manufacturer should be sought.
3 Specialist advice should be sought where clay brick backgrounds are used.

Dissimilar materials

Backgrounds should not be constructed from materials of different densities. Where possible, render should not be continuous across dissimilar materials. Where this cannot be avoided, the render should:

  • be stopped at appropriately formed movement joints, or
  • have austenitic stainless steel lath reinforcement carried across the joint with a separation strip, such as building paper, behind.

Where significant differential movement is likely to occur, such as the junction between masonry and board backgrounds, render should be stopped either side of an appropriately formed joint.

Movement in ribbed metal lath render

To avoid cracking, ribbed metal lath backgrounds should be divided with movement joints into bays no more than 5m wide and:

  • site-made render should be applied in three coats
  • factory-made render should be applied in accordance with the manufacturer’s recommendations.