Movement Joint Calculator — Spacing, Mastic & Filler Board

Movement joint spacing depends on the masonry material: clay bricks maximum 12m centres, concrete blocks maximum 6m centres, calcium silicate blocks 7.5m centres. The first joint must be within half the spacing of any corner or return (e.g. within 6m for 12m clay centres), and all spacings halve for freestanding, parapet or unrestrained walls (PD 6697:2019). Joints should also be placed at changes in wall height, thickness or direction. Closer spacing may be needed for very exposed walls. Use our Brick Calculator or Block Calculator for wall material quantities.

Clay bricks expand slightly after manufacture (irreversible moisture expansion), so joints accommodate growth. Concrete blocks shrink as they cure (drying shrinkage), which means more joints are needed. Clay: 12m max centres for expansion. Concrete block: 6m max centres for shrinkage. This is specified in BS EN 1996-2 with the UK guidance in PD 6697:2019 (clause 6.2.6).

Movement joints in external masonry are typically 10-16mm wide. As a guide, joint width (mm) ≈ spacing (m) + 30%, so ~16mm at 12m clay centres and ~8mm at 6m concrete centres (PD 6697). The mastic sealant should be shallow and wide — its depth should be about half the joint width (e.g. 12mm wide = ~6mm deep), and in no case deeper than the joint width, per BS EN ISO 11600 and sealant manufacturer guidance. A deep, narrow bead tears under movement. A compressible filler board or backer rod with a bond-breaker fills the joint behind the sealant; the filler should be slightly wider than the joint so it stays in compression.

Use a low-modulus polyurethane or polysulphide sealant rated for +/- 25% movement. Silicone is not suitable for masonry movement joints. The sealant must adhere to both faces of the joint. Apply a bond-breaker tape at the back of the sealant to prevent three-sided adhesion, which reduces the sealant's ability to stretch.

Internal blockwork walls also need movement joints, typically at the same 6m centres for concrete blocks. Internal walls are less exposed to temperature changes, but drying shrinkage still occurs. Movement joints in internal walls are often simply filled with mastic and decorated over, as visual appearance is less critical.