What is an engineering brick?

An engineering brick is a dense, hard, low-water-absorption clay brick fired at higher temperatures than standard facing or common bricks. Its low porosity makes it highly resistant to water penetration, chemical attack, frost damage, and abrasion. Engineering bricks are classified into Class A (water absorption below 4.5%, compressive strength minimum 125 N/mm2) and Class B (water absorption below 7%, compressive strength minimum 75 N/mm2). Class A bricks are used in the most demanding applications; Class B bricks are more common in general below-DPC and drainage work.

Where are engineering bricks used?

Engineering bricks are used wherever standard bricks would deteriorate from moisture, chemical exposure, or heavy loading. Common applications include: below DPC level in external walls (where ground moisture is present); copings and sills exposed to driving rain; manhole chambers and inspection chambers (where sewage creates a chemically aggressive environment); retaining walls below ground; bridge piers, culverts, and sewer linings; damp-proof courses (a two-course slip of engineering bricks acts as an effective DPC in older buildings); and factory floors and heavily trafficked industrial surfaces.

What is the difference between a Class A and Class B engineering brick?

Class A engineering bricks have a water absorption of no more than 4.5% and a minimum compressive strength of 125 N/mm2. Class B bricks have a water absorption of no more than 7% and a minimum compressive strength of 75 N/mm2. Class A is the more demanding specification - it is used in the most exposed or chemically aggressive conditions, such as sewer linings, bridge work, and coastal retaining walls. Class B is used for general below-ground brickwork, manholes, and damp-proof courses in domestic and commercial construction.

Why are engineering bricks blue or red?

The colour of engineering bricks depends on the clay type and firing conditions. Blue engineering bricks (the traditional Staffordshire Blue) get their distinctive blue-grey colour from the reduction firing process (oxygen-starved kiln atmosphere) used with specific Staffordshire marl clays. Red engineering bricks are fired in an oxidising atmosphere with different clay types, giving a deep red or red-brown colour. Both can achieve Class A or Class B specification - colour alone does not determine the class. Blue engineering bricks are widely used in Victorian railway and canal infrastructure and remain the traditional choice for manhole rings and copings.

Masonry noun

Engineering Brick

/ ˌɛndʒɪˈnɪərɪŋ brɪk /

Also known as: Class A brick, Class B brick, blue engineering brick, Staffordshire Blue

Definition

An engineering brick is a dense, hard, low-water-absorption clay brick fired at high temperatures, classified as Class A (max 4.5% water absorption, min 125 N/mm2 compressive strength) or Class B (max 7% water absorption, min 75 N/mm2). Its very low porosity makes it resistant to moisture, frost, and chemical attack. Engineering bricks are used wherever standard bricks would fail - below DPC level, in manholes and sewers, in retaining walls, at copings and sills, and in heavily trafficked industrial floors.

In practice

In domestic construction, engineering bricks appear most commonly below the DPC in external walls (Class B is standard here), at oversite level on slab edges, and as manhole rings and inspection chamber walls. The two-course engineering brick slip DPC - two courses of engineering bricks laid in strong cement mortar (1:3) - was the traditional method of providing a damp-proof course in Victorian and Edwardian buildings before bituminous felt DPCs became standard, and still functions well where intact.

Engineering bricks require a stronger mortar than facing bricks - 1:3 or 1:4 OPC:sand rather than the 1:5 or 1:6 used for most facing brickwork. Their low absorption means they do not bond as readily with mortar, and joints must be well filled and struck carefully. On Victorian infrastructure (railway bridges, retaining walls, sewer linings), engineering brickwork in lime mortar has proved extraordinarily durable - many structures built in the 1840s and 1850s remain in service with minimal maintenance.

Building Regulations

StandardBS EN 771-1 - Specification for clay masonry units

Class AMax 4.5% water absorption, min 125 N/mm2 strength

Class BMax 7% water absorption, min 75 N/mm2 strength

Below DPC mortarMin 1:4 OPC:sand - stronger than above-DPC designations

Approved Document C requires external walls to resist moisture rising from the ground - engineering bricks below DPC level satisfy this requirement by their inherent low water absorption. Approved Document A requires masonry to be specified to meet the structural demands of its application. Engineering brick classification is defined in BS EN 771-1 and the mortar specification for below-ground masonry is covered in BS EN 1996-1-1 (Eurocode 6).

Full Building Regulations guidance

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Engineering Brick

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