Render movement
Render movement joint layout along an elevation
- where
- exact number of joints required, before rounding up
- length of rendered wall between corners (m)
- maximum movement joint spacing (m)
- maximum distance from a corner to the first joint (m)
- spacing between joints (m)
- number of joints, n rounded up (0 if L is within both limits)
- distance from each corner to the first joint (m)
Explain this equation
Sets out how many vertical movement joints a rendered wall needs between two corners, and where they go. Use it when setting out beads on a new render job.
L is the wall length, s the maximum joint spacing, and ed the maximum distance from a corner to the first joint, typically half the spacing. n is the exact number of joints required; N is n rounded up. b is the spacing between joints and a the distance from each corner to the nearest joint.
N equal bays of length L/N, with a half-bay at each end, keep every bay within the spacing and every end within the corner limit. Rounding up ensures neither limit is exceeded. If the wall is shorter than both limits, no intermediate joint is needed.
Sense check: a 14 m aircrete wall at 6 m spacing and 3 m from corners needs 3 joints at about 4.7 m centres, with the end ones 2.3 m from each corner.
Watch out: joints must follow background movement joints and should not line up with window or door openings.
Sets out the number of vertical movement joints for a rendered elevation, keeping joints within the maximum spacing and the end bays within the maximum distance from corners.
Enter your values to see the result.
Good to know
NHBC Standards 6.11.5 Table 3 gives normal movement joint spacing on blockwork of 6 m for aircrete and ultra-lightweight aggregate blocks, 7.5–9 m for lightweight and dense aggregate blocks, with the first joint no more than half that spacing from a corner; Weber says up to every 6 m for unreinforced blockwork and to keep panels within a 3:1 length-to-height ratio. Render on metal lath is divided into bays no more than 5 m wide. Joints must line up with, and continue through, background movement joints. The layout here puts N joints at equal spacing L/N with half-bays at each corner; the joints needed is n rounded up.
Common mistake
Positioning joints to line up with window and door reveals. NHBC says joints should not align with openings; place them clear of openings and use bed joint reinforcement or mesh at the opening corners.
Limits
One straight run between two corners, equal spacing. Does not design for openings, horizontal joints, storey heights or differential movement between materials. Follow the render manufacturer's system specification and the background designer's joint layout (BS EN 13914-1).
Sources
- NHBC Standards 6.11.5 Render – accommodation of movement (Table 3: movement joint spacing by blockwork type, half spacing from corners; metal lath bays ≤ 5 m)
- Saint-Gobain Weber: weber.pral M monocouche render application guide (5 °C and rising; movement joints up to 6 m in unreinforced blockwork; 3:1 panel ratio; ±2 mm thickness tolerance)
- BS EN 13914-1:2016 Design, preparation and application of external rendering and internal plastering – Part 1: External rendering
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.