Engineering Bricks: Class A vs Class B, Where They Go, and How Many You Need
UK homeowner guide to engineering bricks: Class A vs B specs, mortar requirements, quantity calculations, and what building control will check below DPC.
Your building control inspector will check that engineering bricks are in place below the damp proof course before signing off the foundations. Use ordinary facing bricks or common blocks in that zone and you will be digging them out again. Engineering bricks are not interchangeable with other masonry units below ground level. The water absorption rate is the reason, and understanding that single number will stop you making the most expensive beginner mistake in extension groundwork.
What they are and what they're for
Engineering bricks are fired clay bricks manufactured to a much higher density than standard facing bricks or common bricks. The manufacturing process vitrifies (fuses into glass-like density) the clay body, which dramatically reduces porosity. The result is a brick that barely absorbs water, resists freeze-thaw damage, sheds ground salts, and carries compressive loads well beyond what any standard brick is required to do.
The formal definition comes from BS 6100 (Glossary of building and civil engineering terms): "brick sized fired clay units having a dense, strong body conforming to defined limits for water absorption and compressive strength." The current manufacturing standard is BS EN 771-1:2011+A1:2015, the European harmonised specification for clay masonry units. Engineering brick classes are defined in the UK National Annex to that standard, having previously been specified under the now-withdrawn BS 3921.
They go where moisture is constant or severe: below the damp proof course in all masonry walls, in inspection chambers and manholes, in retaining walls with earth pressure on one face, and in any location where standard bricks would eventually saturate, spall (crack from frost damage), or fail structurally. Above DPC, in most wall locations, ordinary facing bricks are fine. Below DPC, they are not.
The distinctive deep blue colour of classic engineering bricks (the "blues" or "Staffordshire Blues") comes directly from the firing process. Clay fired at high temperature in a low-oxygen atmosphere turns blue and simultaneously becomes almost impervious. The colour is not applied or accidental. It is the physical result of the same process that creates the low porosity. Red engineering bricks are fired with access to oxygen, which produces iron oxide and the red colour, but with the same high density achieved through the vitrification process.
Class A vs Class B: which one do you need
Two engineering brick classes exist under the UK National Annex to BS EN 771-1. They are not interchangeable, though for most domestic extension work you will only ever need Class B.
| Property | Class A | Class B |
|---|---|---|
| Minimum compressive strength | ≥125 N/mm² | ≥75 N/mm² |
| Maximum water absorption | ≤4.5% | ≤7% |
| Freeze-thaw durability | F2 (frost resistant) | F2 (frost resistant) |
| Active soluble salts | S2 (low) | S2 (low) |
| Standard dimensions | 215 × 102.5 × 65mm | 215 × 102.5 × 65mm |
| Colours available | Blue only | Red or Blue |
| Configuration | Solid | Solid or Perforated (15–28% voids) |
| Typical weight (perforated) | - | ~2.5 kg |
| Typical weight (solid) | ~3.0–3.5 kg | ~3.0 kg |
| Fire classification | A1 (non-combustible) | A1 (non-combustible) |
The compressive strength figures above are from BS EN 771-1 using the current test method. You may encounter older references (or product literature predating the transition from BS 3921) citing Class A at ≥70 N/mm² and Class B at ≥50 N/mm². Those numbers came from the superseded BS 3921 standard and a different specimen preparation method. The actual brick hasn't changed; the measured number is higher under the new test. Use the BS EN 771-1 figures (≥125/≥75) for any specification work.
Class A is stronger and has lower water absorption than Class B. It is used in applications with severe chemical or hydraulic exposure: sewers, culverts, canal infrastructure, industrial floor bases, and below-ground construction where the water table is high and aggressive. For a domestic rear extension in standard ground conditions, Class A is not required and not worth the price premium.
Class B is the standard specification for domestic extension groundwork. Building control acceptance of Class B for below-DPC courses in domestic construction is the norm across England, Wales, Scotland, and Northern Ireland. Your structural engineer's drawings will almost certainly say "Class B engineering bricks" or equivalent. If they say "Class A," follow the drawings without substitution.
The practical rule: unless your SE has specified Class A, or you are building in an area with known aggressive ground chemistry (your SE will flag this), Class B is the right choice. Specifying Class A when Class B is sufficient is money wasted.
Sizes and configurations
The standard UK engineering brick is 215 × 102.5 × 65mm. Some retailers list these as 215 × 100 × 65mm, which is rounding to the nearest 5mm for display purposes. The nominal dimension is 102.5mm.
A 75mm nominal brick (215 × 102.5 × 75mm) is also available, primarily from Forterra. These are less common in domestic groundwork but occasionally specified for thicker wall bases or retaining wall applications. Pack sizes differ: 424 per pack vs 504 per pack for the 65mm version.
Perforated bricks have 15-28% voids running vertically through the body. Solid bricks are fully dense throughout. The structural performance of both is similar (perforations are vertical, so they don't compromise compressive strength in the direction of loading). Perforated bricks are slightly lighter and marginally cheaper. The instruction on perforated bricks is to lay them with the perforations running vertically, not horizontally. Horizontal perforations would allow water to collect inside the brick. This matters below DPC.
Solid bricks are used where the full cross-section needs to be impervious, for example in manhole construction where bricks are in constant contact with standing water.
How to work with engineering bricks
Weight and handling
A standard perforated Class B brick weighs approximately 2.5 kg. A solid brick runs to 3.0-3.5 kg. These are not heavy compared to a 19 kg dense concrete block, but bricklaying involves repetitive picking, placing, and adjusting. A bricklayer laying 300-400 bricks a day is handling 750-1,000 kg of material through their hands, wrists, and lower back. That's why bricklayers care about brick weight and why lighter perforated bricks are preferred where both options are available.
On site, engineering bricks arrive on pallets of 380-504 depending on manufacturer and brick type. Forterra's 65mm perforated comes in packs of 504. Ibstock's blue engineering bricks come in packs of 380. Wienerberger packs 400. Know your pack size before ordering.
Mortar specification
This is the detail most homeowner guides skip entirely. Engineering bricks below DPC must be laid in M12 mortar. That is designation (i) in the mortar classification system: a cement:lime:sand mix of 1:0–¼:3. M12 is the strongest prescribed mortar, designed to reach 12 N/mm² compressive strength at 28 days.
M12 mortar is appropriate for all work below DPC and for the DPC course itself. Above DPC, in the main body of a wall, M6 mortar (1:½:4–4½ cement:lime:sand) is standard. The Wienerberger and Brick Development Association guidance is explicit on this: M12 below DPC, M6 above.
Using M6 mortar below DPC is a defect. Building control inspectors check mortar mixes. If the mix is too weak for the exposure condition, they will ask for it to be made good. A bricklayer asking to use standard mix "because it's easier to work with" should be directed to the drawings and specification.
Do not render over engineering bricks below DPC. Render traps moisture against the wall surface, risks bridging the DPC, and defeats the purpose of using impervious bricks in the first place.
Never lay engineering bricks with the perforations horizontal. The voids run vertically for a reason: so water drains out and doesn't pool inside the brick. Horizontal perforations below ground level mean water sits trapped in the brick body. The brick will eventually spall from freeze-thaw cycles or salt crystallisation pressure. Check your bricklayer's bond pattern before the course is set.
Cutting engineering bricks
Engineering bricks are harder and denser than standard bricks. You cannot split them cleanly with a bolster chisel and lump hammer as you would a facing brick. The material is too hard and the body is too uniform for a clean break.
Use a disc cutter (angle grinder with a masonry diamond blade, or a dedicated cut-off saw). Score the cut line all around the brick first, then cut through. The dust generated from cutting fired clay is fine silica dust. P3 rated dust mask, safety glasses, and hearing protection are not optional here. Silica dust accumulation causes silicosis (progressive and irreversible lung scarring) and is a recognised occupational hazard. Cut outside or in a ventilated area.
A brick splitter (guillotine hire tool, available from HSS Hire or Speedy Hire) will cut engineering bricks in most cases, though the dense body puts more stress on the blade than a softer facing brick. For large numbers of straight cuts, a splitter is faster than a disc cutter. For angled or precision cuts, use the disc cutter.
How many do you need
The standard formula is 60 bricks per linear metre of wall per course. This assumes the standard 215mm brick on a 225mm modular course (brick height 65mm + 10mm mortar joint = 75mm per course) in a single-leaf wall. The wall length determines bricks per course; multiply by the number of courses to get your total.
Worked example for a typical extension:
A single-storey rear extension might have a perimeter of 14 linear metres of external wall (3 new walls). Below DPC, you typically need 2-3 courses of engineering bricks above finished ground level (minimum 150mm clearance means two standard courses, but 3 courses is common practice to allow for ground level variation during landscaping).
- 14m perimeter × 60 bricks per metre = 840 bricks per course
- 3 courses = 2,520 bricks for the outer leaf
- Inner leaf below DPC: concrete blocks are sometimes used here instead of engineering bricks. Check your drawings. If engineering bricks are specified for both leaves, double the figure.
- Add 10% for cuts and wastage: 2,520 × 1.1 = 2,772 bricks
- Order in whole packs (Forterra 65mm perforated = 504 per pack): 6 packs = 3,024 bricks
In practice, most domestic extensions sit well under 1,000 engineering bricks total for the outer leaf. The inner leaf below DPC is often dense concrete blocks, not engineering bricks, and the number of engineering brick courses is typically 2-3 on the outer skin. Confirm both with your SE's drawings and your building control officer before ordering.
Cost and where to buy
Engineering bricks range from £0.45 – £1.90 depending on class, configuration (perforated vs solid), colour (red vs blue), and where you buy.
The price spread is wide and worth understanding before you order:
Class B red perforated is the cheapest option. Brick Wholesale lists these at around £0.47-0.56 per brick inc VAT; BuilderDepot at £0.49. These are utility bricks - colour variation between packs is common and expected.
Class B blue perforated sits at £0.80-1.35 per brick inc VAT depending on supplier and brand. Wickes lists the Marshalls blue perforated at around £1.03 per brick (pack of 440 at £455). Brick Wholesale quotes Wienerberger K209 blue perforated at £0.81-0.97 per brick.
Class B blue solid runs £1.20-1.90 per brick inc VAT. Wienerberger solid blue is around £1.21-1.45 per brick from Brick Wholesale, higher from retail. Ibstock blue solid sits at £1.57-1.88 per brick.
Class A is rarely stocked at retail. Where available, expect £1.50-2.00+ per brick. Source through a specialist brick merchant or directly from Travis Perkins trade account.
Buying in full packs is always cheaper per brick than single purchasing. Pack sizes are 380-504 depending on manufacturer, so work out your quantity requirement first, then round up to the nearest pack. Most builders' merchants (Jewson, Travis Perkins, Marshalls, MKM) require an account or branch selection to see pricing; their online prices are indicative. Call the branch for a firm trade quote.
For domestic quantities (typically 500-1,500 bricks), delivery from a builders' merchant rather than a specialist brick supplier is standard. Marshalls engineering bricks are stocked at Wickes branches. Wienerberger and Forterra products are available through Jewson, Travis Perkins, and Buildbase.
Key manufacturers
Engineering brick manufacturing in the UK is dominated by a small number of producers. The differences within a class (Class B from Forterra vs Class B from Wienerberger) are minor in structural terms. However, colour and surface texture vary meaningfully between manufacturers, and mixing manufacturers mid-project produces visible inconsistency, even within the same class.
Forterra (Butterley/Accrington) is the largest UK engineering brick manufacturer. The Accrington factory has produced engineering bricks for over 150 years. Class B 65mm perforated in packs of 504. Also produces Class B 65mm solid and Class B 75mm perforated (424 per pack). Class A is available to trade order. Gross dry density around 2150 kg/m³.
Wienerberger offers the widest range of styles under the engineering brick classification: K209 blue perforated, K109 solid blue, Sandown red, Kingsbury red, Denton red, and others. Packs of 368-400. Most commonly stocked at Jewson and Travis Perkins. The Wienerberger range also includes some products that sit at the boundary between engineering and facing bricks.
Ibstock (Lodge Lane) manufactures the Staffordshire Slate Blue Smooth and Lodge Lane Blue Mix. Both are Class B with perforated configuration and are smooth-finish enough to be used as facing bricks in some applications. Pack of 380. Weight around 2.5 kg per brick.
Marshalls produces both clay and concrete engineering bricks. Their clay Class B perforated range (red and blue) is stocked at Wickes in packs of 440. Their concrete engineering brick (50 N/mm² equivalent) performs comparably for DPC applications and is harder with age.
Do not mix manufacturers mid-project for visible brickwork. Even Class B bricks from different manufacturers vary in hue, texture, and size tolerance. The difference is subtle on a quick inspection but visible when you stand back from a 10-metre wall. The r/Bricklaying community has documented disputes over mid-project manufacturer switching. Order all bricks for a run from the same batch.
Managing the aesthetic problem
This is the issue no technical guide addresses directly, but it dominates homeowner discussion online. Engineering bricks below DPC are mandatory, but they often look wrong against the facing bricks above. The dense, smooth blue or red engineering brick against a textured buff or sandy facing brick creates a visible colour break at ground level.
There are a few practical approaches, each with trade-offs:
Choose your engineering brick colour to reduce contrast. Red engineering bricks will clash less with warm-toned facing bricks. Blue engineering bricks look appropriate with grey, slate, or cooler coloured facing bricks. Ask your bricklayer (or merchant) to show you the two bricks side by side before committing.
Use a render stop bead at DPC level. If the extension walls are to be rendered above DPC, the render terminates at the DPC line using a stop bead. The engineering bricks below the bead are not rendered over (rendering over them bridges the DPC) but are often barely visible once planting or landscaping goes in. This is the approach most commonly recommended in the forums.
Accept it. In many extensions the engineering brick zone ends up below finished ground level once paths, patios, or planting is complete. Building control requires 150mm of clearance between the DPC and external ground level (paving, soil, or path). That means at least two courses of engineering bricks are always visible above the final ground surface. But in practice, once a path goes in at 150mm below DPC, the engineering brick zone is only two courses high and visually minor.
Do not raise ground levels to hide engineering bricks. Landscaping, paving, or soil that reaches or exceeds the DPC level bridges the DPC, allows moisture to travel up into the wall, and creates a rising damp problem. Building regs require a minimum 150mm clearance between the DPC and external ground level. Your building control officer will check this clearance at the foundation inspection stage and again at the final inspection if the landscaping is complete.
Alternatives
Dense concrete blocks (100mm) are sometimes specified for below-DPC inner leaf construction in cavity walls, rather than engineering bricks on both leaves. Dense blocks have lower water absorption than standard aggregate blocks, and the inner leaf of a cavity wall below DPC is less exposed to direct water ingress than the outer leaf. Whether engineering bricks or dense blocks are required for the inner leaf below DPC depends on your structural engineer's specification. Read the drawings.
Concrete engineering bricks (Marshalls produce one at 50 N/mm²) are available and accepted for DPC applications. They perform comparably to clay Class B for moisture resistance and harden with age. Less commonly specified for domestic extensions but a legitimate alternative where clay engineering bricks are unavailable or significantly more expensive locally.
Facing engineering bricks are a hybrid product: smooth-finish bricks that meet Class B structural and water absorption requirements but have a more decorative finish than utility engineering bricks. Ibstock's Lodge Lane Blue Smooth is an example. The appearance bridge between structural and decorative can help with the aesthetic problem above. They are more expensive than utility engineering bricks.
Where you'll need this
Engineering bricks appear in the groundwork phase of any masonry extension or renovation involving a damp proof course:
- Damp proof course - 2-3 courses of engineering bricks below the DPC line, approximately 60 bricks per linear metre of wall per course, in M12 mortar
Engineering bricks appear across the groundwork phase of any project involving masonry cavity wall construction at or below ground level.
Common mistakes
Ordering Class A when Class B is specified. Class A costs 50-100% more than Class B and is not required for domestic groundwork in standard conditions. If your SE's drawings say Class B, order Class B. If the drawings say "engineering bricks" without qualification, confirm the class with your SE or building control before ordering.
Using the wrong mortar. Laying engineering bricks in M4 or M6 mortar below DPC because it's easier to work with is a defect. M12 mortar is the required specification for below-DPC and in-DPC coursework. The mortar mix must match the exposure class and structural zone. No shortcuts here.
Mixing manufacturers or batches mid-project. Engineering bricks are specified for structural and moisture resistance performance, not appearance. But mixing Forterra Accrington bricks with Wienerberger or Ibstock products, or even mixing batches from the same manufacturer, produces visible colour variation. Order all bricks for a continuous run of wall from the same manufacturer and, ideally, from the same production batch. Confirm availability before committing to a supplier.
Rendering over engineering bricks below DPC. Render over engineering bricks traps moisture against the wall face, bridges the DPC where the render crosses it, and creates conditions for rising damp. The engineering bricks below DPC should be left exposed to allow the wall to breathe. If aesthetics are a concern, use a render stop bead at DPC level.
Laying perforated bricks with holes horizontal. Standard perforated engineering bricks must be laid with the perforations vertical. Horizontal voids collect and retain water. This is a basic site instruction that is sometimes missed by inexperienced bricklayers.