Expansion Joints in Brickwork: NHBC Guidance, Spacing, Fillers and Sealants

Summary

Clay bricks undergo irreversible moisture expansion after firing — they absorb atmospheric moisture and expand permanently by up to 0.02% of their length over the first few years. On top of this permanent expansion, bricks expand and contract reversibly with temperature changes. Without movement joints to accommodate this dimensional change, walls develop characteristic diagonal cracking at re-entrant corners, over openings, and at changes in wall thickness.

Movement joints are not a sign of poor workmanship — they are a designed-in feature that prevents uncontrolled cracking. The failure to include adequate movement joints (or the filling of existing movement joints with mortar during pointing) is one of the most common causes of diagonal cracking in clay brick walls. Understanding the correct spacing, detailing, and sealant specification for movement joints is essential for any bricklayer or general contractor.

This article covers the physics of clay brick expansion, NHBC guidance on joint spacing, joint filler and sealant selection, and the specific considerations for long runs, parapets, and cavity walls.

Key Facts

• Clay brick thermal expansion — reversible; coefficient approximately 6 × 10⁻⁶ per °C; a 10m wall expands 3.6mm for a 60°C temperature range
• Irreversible moisture expansion — bricks absorb moisture and expand permanently after manufacture; typically 0.02–0.03% of length (2–3mm per 10m)
• Total movement — combined thermal + moisture movement in clay brickwork: approximately 1mm per metre (NHBC guideline)
• NHBC Chapter 6.1 — guidance on movement joint spacing for clay brickwork: 4.5–9m depending on exposure and brick type
• Joint width — 10–20mm for most domestic and low-rise commercial brickwork; wider joints needed for longer runs
• 2-point contact rule — sealant must only contact two surfaces (the two faces of the joint); a backer rod prevents the sealant bonding to the back of the joint (which would create a 3-point failure)
• Closed-cell polyethylene foam — Fillcrete, Kooltherm, or similar; compressible backer rod; fills joint to allow sealant applied at correct depth-to-width ratio (1:2 max)
• Sealant depth — typically 6–10mm of sealant over the backer rod; sealant that is too deep is too stiff and will tear
• Non-hardening sealant — paintable silicone, polysulphide, or polyurethane sealant; must remain flexible at all temperatures; BS EN ISO 11600 Class F/EX
• Calcium silicate and concrete bricks — undergo moisture shrinkage (opposite to clay); movement joints at shorter spacing, typically 7.5–9m, as contraction governs
• Parapet walls — very high thermal and moisture exposure; joints recommended every 3m
• Re-entrant corners — always place a movement joint at internal corners of L-shaped plans, where diagonal cracking would otherwise develop

Quick Reference Table

Detailed Guidance

Why Clay Bricks Expand

Freshly fired clay bricks absorb atmospheric moisture (oxygen, water vapour, and carbon dioxide) and expand permanently. This is the opposite of the behaviour of concrete, which shrinks after casting. The expansion has two components:

Irreversible moisture expansion:

• Occurs immediately after firing as the brick absorbs atmospheric moisture
• Magnitude: typically 0.05–0.1mm per 100mm brick length over the first year
• 90% of the expansion occurs in the first 6 months after firing
• This is why bricks should not be used immediately from the kiln ('green' bricks expand more than seasoned stock)
• NHBC requires a minimum storage period before use; kiln-fresh delivery can cause excessive joint movement

Reversible thermal expansion:

• Daily and seasonal temperature cycling causes the brick to expand and contract
• Coefficient: approximately 6 × 10⁻⁶ per °C (0.006mm per metre per °C)
• UK thermal range for an exposed wall: approximately -15°C to +75°C surface temperature = 90°C range
• Maximum reversible movement: 10m × 0.006 × 90 = 5.4mm

Combined movement in a 9m wall:

• Irreversible expansion: 9m × 0.2mm/m = 1.8mm
• Reversible thermal: 5.4mm (at full range)
• Total: up to 7.2mm — which is why a 10mm minimum joint width is specified

NHBC Chapter 6.1 Spacing Guidance

NHBC Standards Chapter 6.1 (External Masonry Walls) provides the primary guidance used in UK housebuilding for movement joint spacing:

For facing clay brickwork:

• Internal (sheltered) leaf: 9m maximum spacing
• Outer leaf: varies by exposure, typically 6–9m
• High-exposure positions (parapets, fully exposed gables): 4.5–6m
• Next to openings (windows, doors): always position joints to align with the edge of openings where possible

The 'rule of thumb': NHBC guidance is sometimes simplified as "one joint per approximately 6m run." This is a workable rule for standard UK housing in moderate exposure. For high-exposure positions, reduce to 4.5m.

Re-entrant corners: Any internal corner of a wall plan (L-shaped, T-shaped, or U-shaped building) is a stress concentration. Movement joints must be positioned adjacent to all re-entrant corners, typically within 300mm of the corner:

     |
 ----+---- (re-entrant corner)
  ↑
Joint here

Failure to joint at re-entrant corners almost invariably results in staircase cracking from the corner up through the brickwork.

BS EN 1996-2 and Previous Standards

Building Regulations masonry design is now governed by BS EN 1996 (Eurocode 6). BS 5628-3 (Code of Practice for Masonry — Materials and Components, Design and Workmanship) was withdrawn and replaced by BS EN 1996-2 for execution of masonry structures:

• BS EN 1996-2:2006 covers workmanship requirements including movement joint provision
• The specific spacing tables from BS 5628-3 are no longer in a live standard; NHBC guidance and manufacturer's recommendations fill this gap in practice
• Structural engineers designing masonry buildings to Eurocode 6 calculate movement joint requirements from first principles using the expansion coefficient of the specific bricks specified

Joint Construction Details

Standard vertical movement joint in facework:

|--- Brick ---| |--- Brick ---|
              | |
              | |  ← 10-20mm gap
              | |
    [Backer rod fills most of joint]
    [Sealant 6-10mm deep at face]

Construction sequence:

1. Establish joint location from structural drawings or spacing calculation
2. Build both sides of the joint using a straight template or plywood sheet to maintain joint width
3. Do not fill the joint with mortar — leave it open throughout construction
4. After brickwork has settled (ideally 6 months on new build): clean joint faces of all mortar contamination
5. Insert closed-cell backer rod (polyethylene foam rope) to the correct depth; rod diameter should be 25% larger than the joint width for correct compression
6. Apply backing-tape (to prevent adhesion) if not using backer rod
7. Apply sealant with a gun in a continuous bead; tool to concave profile; remove masking tape while sealant is still wet

Backer Rod and Filler Selection

Closed-cell polyethylene backer rod (preferred):

• Available in diameters 6mm to 50mm
• Insert to achieve 6–10mm depth for sealant application above
• Provides consistent backing surface; prevents 3-point sealant adhesion
• Brands: Fillcrete, Fosroc Expandafoam backer rod

Impregnated foam (alternative for horizontal joints):

• Pre-compressed EPDM foam strip with self-adhesive backing
• Expands to fill the joint on contact with air
• Used in horizontal joints in precast and some masonry work
• Less commonly used in vertical brickwork movement joints

Denso tape:

• Petrolatum-impregnated fibreglass tape
• Used as an anti-adhesion backing in some details
• Provides consistent non-adhering surface for sealant

Sealant Selection

The sealant is the functional component — it must remain flexible at all temperatures and accommodate the joint movement without tearing:

BS EN ISO 11600 Class F/EX:

• Class F = façade (external use)
• Class EX = high movement accommodation (≥25% of joint width)
• This is the correct classification for external masonry movement joints

Sealant types:

For most external brickwork movement joints: use a neutral-cure silicone or modified silicone that is specifically marketed as paintable and BS EN ISO 11600 F/EX classified. Silyl-modified polymer (SMP) sealants are increasingly popular — they combine the elasticity of silicone with the paintability and adhesion of polyurethane.

Colour matching: Sealants are available in masonry-matched colours (buff, red, brown, grey). For high-visibility joints, colour-matching the sealant to the brick or mortar colour significantly improves the aesthetic.

Cavity Wall Movement Joints

In a cavity wall, movement joints must be provided in both the inner and outer leaves, but the joint positions do not need to be coincident:

• Outer leaf (clay facework): joints at 6–9m centres (NHBC guidance)
• Inner leaf (blockwork): joints at 6m centres for medium-weight aggregate blocks; cavity blocks are less expansion-prone than clay
• The cavity insulation must be cut cleanly at the joint position; do not bridge the joint with insulation
• Cavity wall ties within 300mm of a movement joint should be slip ties (allow differential movement between leaves)

Parapet Wall Movement Joints

Parapet walls are the most exposed masonry element on any building — exposed on all faces to rain, frost, and temperature extremes. Movement joint spacing must be reduced:

• Clay brick parapets: 3m maximum spacing
• Every movement joint in a parapet must be matched by a movement joint in the parapet coping or cap
• Parapet movement joints must be maintained through the DPC and flashing layers

Failure to provide parapet movement joints is one of the leading causes of parapet cracking and associated water ingress into flat roofs and ceilings.

Frequently Asked Questions

What happens if a movement joint is filled with mortar during pointing?

The mortar restrains the natural expansion of the brickwork. As the bricks try to expand, the restrained movement is transferred to the brickwork as compressive stress. When the compressive stress exceeds the tensile strength of the mortar or brick, diagonal cracking occurs — typically at 45° from re-entrant corners, over openings, and at changes in wall thickness. The mortar in the movement joint must be raked out and replaced with backer rod and flexible sealant.

How do I know where to position movement joints on a new extension?

Start from the NHBC spacing guidance (6–9m for most clay brick outer leaf). Then identify mandatory joint positions: at all re-entrant corners, adjacent to all door and window openings where possible, at junctions between old and new work, and wherever the wall changes thickness or height. If the extension plan is L-shaped or T-shaped, joints at the internal corners are mandatory. If the extension is longer than 6m in any direction, at least one joint mid-run is required.

Can a movement joint be at the corner of a building?

Yes — a corner movement joint (quoin joint) is sometimes used on large buildings to isolate a corner section. The joint runs through the corner detail, separating the two wall leaves. This requires careful detailing to remain weathertight while remaining unrestrained. In domestic construction, quoin joints are unusual; most corners are tied masonry without movement joints.

Do cavity wall ties affect movement joint position?

Within 300mm of a movement joint, use slip ties (ties that allow differential horizontal movement between inner and outer leaves). Standard twisted wire ties or double-triangle ties are not slip ties — they restrain differential movement and can cause cracking if placed too close to a movement joint. Helical wall ties (Teplo-tie, Ancon Swing-Tie) are inherently flexible and are often used throughout the cavity wall to reduce the need for specific slip ties at joints.

Regulations & Standards

• BS EN 1996-2:2006 — Eurocode 6; execution of masonry structures; workmanship requirements including movement provision

• NHBC Standards Chapter 6.1 — external masonry walls; movement joint guidance for UK housebuilding

• BS EN ISO 11600:2003 — building construction; jointing products; classification and requirements for sealants; Class F/EX for external masonry

• BS EN 845-1 — masonry accessories; wall ties (relevant to slip tie selection adjacent to movement joints)

• NHBC Standards Chapter 6.1 — National House Building Council

• Brick Development Association: Movement in Brickwork — BDA technical guidance note

• Sika: Movement Joint Sealants — Sika sealant selection guide for masonry

• BSI BS EN 1996-2 — British Standards Institution

• Ibstock: Movement Joint Guide — leading UK brick manufacturer

• brick types — brick selection and frost resistance classification

• efflorescence — salt migration and moisture movement in brickwork

• pointing repointing — mortar joint repair and repointing