6.11 Render

Also see:

PD 6697

6.11.5 Accommodation of movement

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

movement in masonry background
dissimilar materials
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 considered 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.

Austenitic stainless steel bed joint reinforcement conforming to BS EN 845-3 should be provided in the first two courses of the external masonry leaf above and below any opening to help distribute tensile stress and avoid localised cracking.

Table 2: Concrete block categorisation

Category	Compressive strength of the blockwork	Dry density
Low density aircrete	2.9-3.6N/mm²	<500kg/m³
Normal density aircrete	3.6-9.0N/mm²	500kg/m³+
Ultra lightweight aggregate	3.6-7.3N/mm²	<950kg/m³
Lightweight aggregate	3.6-7.3N/mm²	950-1500 kg/m³
Dense aggregate	7.3N/mm²+	1,500kg/m³+

Table 3: Preparation of blockwork backgrounds⁽¹⁾

Category	Normal movement joint spacing	Maximum distance of joint from restrained end, ie, corners	Suction control
Low density aircrete	Specialist advice required⁽²⁾
Normal density aircrete	6m	3m (half normal spacing)	Yes
Ultra lightweight aggregate	6m	3m (half normal spacing)	Not generally required
Lightweight aggregate	6m	Half normal spacing	Not generally required
Dense aggregate	7.5 - 9m ⁽³⁾	Half normal spacing	Not 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.

Last updated: 2nd January 2024