Brick Slips: Adhesive Installation, Grouting and BS EN 771-1 Requirements

Summary

Brick slips (sometimes called brick tiles or brick cladding) are thin sections of fired clay — typically 20–25mm thick — that replicate the face of a standard brick. They are used to clad steel-framed structures (SFS), concrete, timber frame, and render substrates with a brick appearance without the self-weight of full brickwork. Their use has grown substantially in the UK with the rise of modern methods of construction (MMC) and timber frame building.

However, brick slips are also one of the most frequently poorly installed cladding products in the UK. The consequences of incorrect installation — detachment of individual slips or entire panels — range from aesthetic failure to serious injury if slips fall from height. The Grenfell Tower fire and subsequent regulatory changes have fundamentally changed the regulatory landscape for external wall systems, and brick slips are directly affected by the requirements for non-combustible cladding above 18m.

This article covers substrate requirements, adhesive selection, joint detailing, movement joints, the fire safety requirements, and the planning implications for brick slip cladding in conservation areas.

Key Facts

• Brick slip dimensions — typically 20–25mm thick × 215mm × 65mm (full brick face); also available as half-brick and custom sizes
• BS EN 771-1 — brick slips are classified as fired clay masonry units under this standard; same classification system as full bricks
• Substrate requirements — must be mechanically keyed (roughened render, SFS sheathing board, direct concrete, or masonry); cannot be applied to smooth painted surfaces
• Full-bed adhesive — all slips must be fully bedded in adhesive; spot-fixing creates hollow spots that trap moisture and lead to adhesive failure
• BS EN 12004 — classification for tile adhesives; C2S2 = cement-based, enhanced (C2), with improved slip resistance and deformability class S2; required for external brick slips
• C2S2 adhesive — enhanced cement-based adhesive with S2 (high deformability); resists differential thermal and moisture movement between slip and substrate
• Joint width — 10mm nominal to match standard brickwork coursing; minimum 6mm required by BS EN 12002
• Grout/pointing mortar — pigmented mortar (BS EN 13888) or polymer-modified pointing compound matched to the brick slip colour
• Movement joints — 3m maximum on external applications; 5m maximum internally; coincide with structural movement joints
• Height limit for standard systems — above 18m, non-combustible backing required (A1 or A2 rated); standard OSB or plywood sheathing is not acceptable
• PAS 9980 — code of practice for the assessment of external wall systems on existing blocks of flats; includes assessment of brick slip systems
• Mechanical fixings — for slips applied above 18m, mechanical fixings through each slip are often required as a secondary safety retention system
• Planning — brick slips in conservation areas must match the appearance of traditional brick; full planning applications may be required for changes to external appearance

Quick Reference Table

Detailed Guidance

Substrate Preparation

The substrate is the foundation of a brick slip system. Failure here causes panel failure regardless of adhesive quality:

Acceptable substrates:

• Rendered masonry (scratch coat): float coat or key coat render, minimum 10mm thick, with a cross-hatched key scratched into the surface while wet
• SFS (Steel Frame System) sheathing: cement particle board (Powerboard, Supalux) or glass reinforced cement (GRC) board mechanically fixed to the SFS; DO NOT use standard OSB or plywood as the direct substrate for external brick slips above 18m
• Concrete (in-situ or precast): bush-hammer or scabble the surface to remove laitance; apply bonding slurry if smooth
• Blockwork — coursed blockwork with keyed or roughened joints; remove any existing paint or coating

Substrate checks before fixing:

1. Suction: brick slip adhesive relies on the substrate having sufficient suction to form an initial bond; pre-wet dusty or high-suction surfaces; allow to reach surface-dry condition before adhesive application
2. Flatness: substrate must be within ±5mm in any 2m length; high spots or hollow sections telegraph through to the slip face
3. Strength: substrates weaker than C20 concrete may need reinforcing or priming before slip application
4. Cleanliness: oil, dust, loose material, efflorescence, and biological growth must be removed

Adhesive Selection: BS EN 12004 Classification

BS EN 12004 classifies tile and brick adhesives by:

• C (cementitious) or D (dispersion) or R (reactive)
• 1 (normal) or 2 (enhanced — improved bond strength)
• T (slip-resistant for vertical surfaces)
• S1 (deformability up to 5mm) or S2 (deformability over 5mm)

For external brick slips: C2S2 is the minimum acceptable classification. This ensures:

• Enhanced bond strength for the adhesive weight
• S2 deformability to accommodate differential thermal movement between the slip and substrate
• Slip resistance (T) to prevent slips sliding during installation

Full-bed application (mandatory):

• Apply adhesive to the substrate by notched trowel (6mm V-notch for slips up to 15mm thick; 8mm or 10mm V-notch for slips 15–25mm thick)
• Back-butter each slip with a thin skim of adhesive (2–3mm)
• Press slip firmly onto the substrate with a slight twisting motion to collapse the notch ridges and achieve full contact
• Adhesive coverage: minimum 75% for internal; minimum 90% for external; 100% for external above 6m or frost-exposed positions
• Check coverage by lifting a slip after pressing — adhesive should cover the full back face with no dry areas or hollow spots

Spot-fixing is not acceptable. Spot-fixing leaves hollow areas behind slips that trap moisture, create freeze-thaw expansion risk, and reduce the bond area by 50–75%. Spot-fixed slips are a falls-from-height risk.

Joint Tooling and Grouting

Joint widths:

• 10mm nominal to match standard brickwork coursing; this maintains the visual authenticity of the brickwork appearance
• Minimum 6mm (BS EN 12002); joints below 6mm are too narrow to fill and tool effectively
• Maximum 15mm; wider joints look disproportionate and require high-shrinkage grout

Joint filling methods:

Mortar gun/pointing mortar:

1. Mix pointing mortar (cement : sand 1:4 with plasticiser, plus pigment to match brick colour)
2. Apply with a pointing gun or pointing trowel; fill joint completely with no voids
3. Tool with a jointing iron when mortar has reached firm but not hard consistency (thumb-print hard)
4. Match the joint profile to the existing brickwork: bucket-handle, flush, or weathered as specified

Pre-mixed sanded grout:

1. Polymer-modified sanded grout (BS EN 13888 class CG2WA); available in brick-matched colours
2. Applied by rubber float or pointing tool; joints packed fully
3. Wipe excess from slip face with a damp sponge before grout hardens

Pigmented polymer pointing compound:

1. Single-component pre-mixed product (e.g. Parex, Remmers Fugenmortel); coloured aggregate gives natural brick mortar appearance
2. Applied by mortar gun; tooled to match specification
3. Slower cure than cementitious mortar; better stain resistance

Movement Joints

Movement joints in brick slip systems must accommodate:

1. Thermal movement of the slip (clay brick: 6 × 10⁻⁶ per °C)
2. Differential movement between slip and substrate (particularly on SFS where the steel frame moves differently to the brick)
3. Structural movement joints in the backing structure

Movement joint spacing:

• External: 3m maximum horizontal and vertical
• Internal: 5m maximum
• Must coincide with any structural movement joints in the backing

Movement joint detailing:

• 10–15mm wide open joint
• Closed-cell polyethylene backer rod
• Flexible sealant (C3 or C4 classification under BS EN ISO 11600, matching brick colour)
• At corners: move joints as close to the corner as the bond pattern allows

Fire Safety and Height Requirements

The Grenfell Tower fire in 2017 led to a fundamental review of external wall systems and combustibility. The key changes affecting brick slip systems:

Buildings above 18m (approximately 6 storeys) — residential:

• Under the Building Safety Act 2022 and amendments to Building Regulations Approved Document B, external wall systems on residential buildings above 18m must use A1 or A2-s1,d0 rated materials throughout
• Standard OSB, plywood, and many PIR insulation boards are Class C or D — not acceptable
• Acceptable backing boards: calcium silicate board, magnesium oxide board (where fire-tested), cement particle board (A1/A2 rated grades)
• The clay brick slip itself is A1 (non-combustible) — the issue is the substrate and fixings

PAS 9980 (External Wall Assessment):

• PAS 9980:2022 provides the methodology for assessing and reporting on external wall systems on existing residential buildings
• Any brick slip system on a residential building over 11m should be assessed under PAS 9980 by a qualified EWS assessor
• A positive EWS1 certificate (B1 rating or above) is required for mortgage lending purposes on such properties

Mechanical fixings above 18m:

• Adhesive alone is not considered sufficient secondary retention for brick slips above 18m
• Stainless steel mechanical fixings (proprietary slip retention systems from Eurobrick, SH Structures) are required as a secondary safety measure
• Fixings are typically 2 per slip, driven into the substrate through the mortar joint and hidden by pointing

Conservation Area and Listed Building Considerations

Brick slips on buildings in conservation areas or on listed buildings require careful consideration:

• The appearance must closely match the character and material of the original building
• Full brickwork is almost always preferred over slip cladding for listed building repairs
• Many local planning authorities explicitly prohibit brick slip systems on listed buildings as a like-for-like repair material
• On new extensions in conservation areas, brick slips may be accepted where the visual result is indistinguishable from full brickwork
• The planning application must include material samples and a statement of justification

Frequently Asked Questions

Are brick slips as durable as real bricks?

A correctly installed and jointed brick slip on a properly prepared substrate can achieve a service life equivalent to full brickwork — 50+ years. The failure modes are different: full brickwork fails by frost damage, mortar decay, and movement cracking; brick slips fail primarily through adhesive bond failure (detachment) if incorrectly installed. With correct installation — full-bed C2S2 adhesive, movement joints at 3m, mechanical fixings above 6m — brick slips are a durable cladding system.

Can brick slips be installed on a timber-framed house?

Yes, with a suitable substrate. A cement particle board or glass reinforced cement (GRC) board mechanically fixed to the timber frame provides a stable, non-combustible backing. The timber frame must be clad in a breathable breather membrane with a minimum 20mm drained cavity between the membrane and the backing board. This ventilated cavity manages any moisture that penetrates the brick slip layer. Direct application of brick slips to OSB or plywood is not recommended for external use.

What is the difference between C2S2 and C2TE adhesive?

C2TE is cement-based, enhanced (C2), with the T designation (tile adhesive — reduced slip for vertical surfaces), and E designation (extended open time — allows repositioning the slip before the adhesive skins). C2TE does not include the S2 deformability classification. For external brick slips where differential movement between slip and substrate is a concern, C2S2 is the correct choice. C2TE is more appropriate for dry internal tile work where open time is more important than deformability.

How do I handle the corners of brick slip panels?

Corner detailing is one of the most challenging aspects of brick slip installation. Options:

1. Return slips: purpose-made corner slips with a 90° return (L-shaped cross section) that replicate a brick corner; most authentic appearance
2. Mitre: two slips mitre-cut at 45° to form the corner; requires precision cutting and careful grouting
3. Corner bead: metal or PVC corner bead defines the arris; slips butt against it on each face; less authentic but simpler

Return (L-shaped) corner slips are the professional standard for conservation area work. Mitre joints tend to open with thermal movement, allowing water ingress at the corner.

Regulations & Standards

• BS EN 771-1:2011+A1:2015 — fired clay masonry units; specification applicable to brick slips as well as full bricks

• BS EN 12004:2017 — adhesives for ceramic tiles; classification system including C2S2 for external use

• PAS 9980:2022 — code of practice for the assessment and remediation of external wall systems; relevant to brick slip systems on residential buildings above 11m

• Building Regulations Approved Document B (2022 edition) — fire safety; combustibility requirements for external wall systems above 18m

• BS EN ISO 11600:2003 — jointing products; classification for movement joint sealants

• Eurobrick: Technical Installation Guide — leading UK brick slip manufacturer

• RIBA: External Wall Systems Guidance — Royal Institute of British Architects

• BSI BS EN 12004 — British Standards Institution

• PAS 9980:2022 — British Standards Institution

• MHCLG: External Wall Systems — Ministry of Housing, Communities and Local Government

• brick types — brick classification, frost resistance, and salt content

• expansion joints — movement joint spacing and sealant selection

• efflorescence — salt migration and treatment