Summary

Brick is the most consequential single material specification on most domestic masonry projects. The wrong brick in the wrong position produces visible failure within years — spalling on exposed copings, efflorescence streaking down a freshly built wall, or a glaring mismatch on an extension to a Victorian terrace. Yet many projects specify on price and colour alone, ignoring the technical classification that determines whether the brick is fit for the position.

Selection is governed by BS EN 771 (the product standard, in three parts), by Approved Document A for structural use, by Approved Document C for moisture resistance, and by the manufacturer's technical data for the specific brick. This article covers the UK classification system, the principal brick types by manufacturing process and use, the dimensional standards (modern metric vs imperial), and the regulation environment for specification. It is written for the bricklayer, builder, or specifier choosing brick for a real project — not the materials scientist.

Key Facts

Quick Reference Table

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Brick Type Compressive Strength Water Absorption Frost Class Typical Use
Engineering Class A (e.g. Staffs Blue) ≥125 N/mm² ≤4.5% F2 DPC course, manholes, retaining walls, paving, ground works
Engineering Class B ≥75 N/mm² ≤7% F2 Below DPC, foundations, hard-wearing exposed positions
Facing brick (premium) 25–55 N/mm² 6–12% F1 or F2 External walls, decorative
Facing brick (standard) 15–35 N/mm² 10–18% F1 External walls, sheltered positions
Common brick (fletton) 15–25 N/mm² Variable F0–F1 Internal walls, behind render, hidden cavities
Stock brick (London Yellow, Cambridge White) 15–25 N/mm² 12–20% F1 Traditional stock brick facing
Handmade facing 20–45 N/mm² 8–18% F1–F2 Heritage / premium new build
Reclaimed brick Variable Variable Variable (test) Heritage extension matching
Calcium silicate (BS EN 771-2) 15–60 N/mm² n/a Frost-resistant equivalent Uniform colour facing; internal
Concrete brick (BS EN 771-3) 7–35 N/mm² Variable Variable Facing or common; structural blockwork backup
Engineering paviour ≥75 N/mm² ≤7% F2 Paving (specific paviour spec)
Dimension Metric (post-1965) Imperial (pre-1965)
Length 215mm ~225mm (varies)
Width 102.5mm ~112.5mm
Height 65mm ~75mm
Course height (with 10mm joint) 75mm ~85mm
Stretcher count per m² (single skin) ~60 ~48

Detailed Guidance

Engineering bricks — when strength and moisture resistance matter

Engineering bricks are the workhorse for any position that must resist water or carry high loads. The two classes differ on water absorption and strength:

Class A (e.g. Staffordshire Blue):

Class B (Engineering Red, Engineering Blue):

Common error: using a "engineering-look" facing brick (red, dense appearance) as a structural engineering substitute. Verify the BS EN 771-1 declaration — water absorption is the key test.

Facing bricks — appearance is the spec

Facing bricks are selected primarily for appearance — colour, texture, blend. The technical performance must still meet the position's requirements (F2 for parapets/copings; S2 for all external).

Manufacturing categories:

Wirecut: Clay extruded as a column, wire-cut to brick lengths. Most common modern process. Uniform shape, often with sand-coated face for texture. Mid-range price.

Waterstruck: Wet clay pressed into moulds with water as release agent. Slightly distressed face. Premium price.

Soft-mud / handmade: Clay thrown by hand or machine into moulds dusted with sand. Highly characterful face with creases and folds. Premium / heritage price.

Pressed: Dry-pressed in a die. Crisp arrises, flat faces. Used for engineering bricks and some facing bricks.

Selection drivers:

Common bricks (flettons) — out of sight only

Common bricks are not weather-rated and are used only where they will not be exposed to the weather:

The classic UK common brick is the "fletton" — a low-density Lower Oxford Clay brick (made from clay containing organic matter that fuels its own firing). Cheap, light, pink/red colour, soft enough to score with a coin. Acceptable for hidden positions only.

A common error is using flettons as the outer skin of a wall that's "going to be rendered later" — if the render delaminates or never gets applied, the wall fails within winters. Specify a properly weather-rated facing brick or budget for protection.

Calcium silicate (sandlime) bricks

Calcium silicate bricks (BS EN 771-2) are made from sand and lime, autoclaved under steam. Properties:

Used for facing where colour uniformity is required, and as a structural facing in regions where they're traditional (more common in some parts of Northern Europe than the UK, but still produced).

Concrete bricks (BS EN 771-3)

Concrete bricks are dense aggregate concrete blocks formed at brick size. Properties:

Concrete "facing brick" products are not clay brick; despite some catalogue presentations they perform differently in service, and the technical data must be checked against the position requirements.

Imperial brick matching (pre-1965 stock)

Buildings constructed before 1965 (when metrication of UK brick took effect) used imperial-sized brick — approximately 9 × 4½ × 3 inches (225 × 112.5 × 75mm work size). The exact dimensions varied by region (London 8⅝ × 4⅛ × 2⅝; northern industrial 9 × 4½ × 3; some regions used distinctive proportions).

For an extension, repair, or matching project on an imperial-era property:

  1. Measure the existing brick over 10 courses and divide by 10 to get the course height (this averages out variation).
  2. Measure 10 stretchers along a wall to get the brick length plus joint.
  3. Note the bonding pattern (Flemish, English Garden Wall, Stretcher) — see brick bonds.
  4. Note colour, texture, frog/perforation pattern.
  5. Source matching brick from a specialist (Bulmer, York Handmade, Furness, Imperial Brick Co., reclaimed yards).
  6. Mock up a sample panel for approval before laying significant brickwork.

Modern metric brick (65mm high, 75mm course) does not match imperial brick (75mm high, 85mm course) — laying metric brick into an imperial wall produces a course misalignment that grows with height and is immediately visible. Always match the actual brick dimensions.

Reclaimed brick

Reclaimed brick is used for heritage matching, character new builds, and conservation work. Key considerations: origin (source local stock for local character match — Victorian London terraces want London-yard stocks); cleaning (old lime mortar must be off the bedding faces — cleaned brick at premium, uncleaned at discount); frost performance (historic brick may have been made for sheltered positions and may fail in exposed positions today — test or assume F1); salt content (old brick may have absorbed salts in prior use; consider F1/S1 equivalence at best); cost (reclaimed stock brick £600–£1,200 per 1,000 cleaned; reclaimed handmade £1,500–£3,500+ per 1,000). For Listed Buildings, reclaimed match is often the only consent-acceptable option and a sample panel approved by the conservation officer is standard.

Specifying brick — checklist for any external work

Before placing the order: BS EN 771-1/-2/-3 declaration with class; compressive strength meets structural design; water absorption appropriate (Engineering Class A/B if below DPC; ≤12% for normal facing); frost resistance F2 for copings/parapets/paving and F1 for sheltered facing; soluble salt S2 for external work; colour and texture match approved sample panel; brick dimensions correct (especially on heritage/extension match); bonding pattern compatible with brick proportions; mortar specification compatible (NHL2/3.5/5 for heritage lime; 1:1:6 cement:lime:sand for modern; never 1:3 OPC straight on traditional brick).

Brick selection on Listed / Conservation work

For a Listed Building (Grade I, II*, II) or for a property in a Conservation Area with an Article 4 Direction, brick selection requires:

  1. Match to existing — colour, texture, dimension, bonding pattern.
  2. Often a mock-up sample panel (typically 1m²) on site, photographed and approved by the conservation officer.
  3. Listed Building Consent in place before any work that affects character.
  4. Lime mortar (NHL2 / NHL3.5 for typical work) — NEVER cement mortar on traditional brick; cement mortar is harder than the brick and causes spalling.

See repointing lime vs cement and brick matching for the mortar and matching detail.

Frequently Asked Questions

What's the difference between Engineering Class A and Class B?

Class A is the higher specification — minimum 125 N/mm² compressive strength and maximum 4.5% water absorption. Class B is minimum 75 N/mm² and maximum 7% water absorption. Both are F2 frost-resistant. Class A is for the most demanding applications (sewer brickwork, severe-exposure ground works, paviours). Class B covers most domestic engineering positions including DPC course brickwork and foundation walls.

Can I use a facing brick below ground?

Generally no. A facing brick is rated for above-ground external use (F1 or F2) but is not necessarily rated for the chemical attack of soil and groundwater that a true engineering brick is. Use engineering brick (Class B minimum) below DPC, especially in clay soils or where sulfates are present. Where a facing brick is taken to ground for aesthetic reasons, specify F2 with low water absorption and consult the brick manufacturer for ground-contact suitability.

Why does my Victorian terrace have different brick sizes to a modern brick?

Brick dimensions were metricated in 1965. Pre-1965 (imperial) brick is approximately 225 × 112.5 × 75mm; post-1965 (metric) brick is 215 × 102.5 × 65mm. The 10mm difference in height is the most visible — over a single storey, that's 30+ mm of course difference between imperial and metric brick. For matching, source dimensionally-correct imperial brick from a specialist or reclaim yard.

Are perforated bricks weaker than solid bricks?

Per unit area, no — the perforations reduce the brick's weight without significantly reducing compressive strength, because the loaded area is the bearing face (top and bottom, not the perforated face). Perforated bricks are commonly used in load-bearing walls and are fully structural. The perforations also aid mortar bond — mortar squeezes into the perforations and creates mechanical key.

What's a frog and does it go up or down?

The frog is the recess pressed into one (sometimes both) bed faces of a traditional brick. The traditional rule is "frog up" — laying the brick with the frog facing upward and filled with mortar — which improves bond, increases compressive bearing area, and produces a stronger joint. Some specifications (especially for fast-laid commercial walls) accept "frog down" for speed, but frog up is the technically correct method and the default for any heritage or conservation work.

Why does my new brickwork have white streaks down it?

Efflorescence — soluble salts drawn to the surface by water as the wall dries. Caused by high-soluble-salt brick (S0/S1 instead of S2), excessive rainfall during construction, salt-rich mortar, or contaminated sand. Mostly cosmetic — brushes off with a stiff dry brush; do not wash off (water re-dissolves salts and recycles the problem). Specify S2 brick for external work to minimise. See efflorescence.

Regulations & Standards