Blinding Concrete: The 50mm Layer That Halves Your Reinforcement Cover

The structural engineer's drawing for a reinforced ground slab calls for 40mm of concrete cover over the mesh. The builder skips the blinding layer to save a day and pours the slab straight onto the hardcore. At inspection, the building control officer measures actual cover with a probe and finds 25mm in places where the mesh sat on a high stone. The slab fails. Either the floor comes out and gets re-poured, or the engineer issues a remedial design adding a topping screed and proves equivalent cover. Both options cost more than the modest cost of blinding concrete that would have prevented the problem.

That's what blinding does. It's not glamorous, it's not structural, and most homeowners have never heard of it. But on a reinforced extension slab, blinding is the difference between passing first-time and re-pouring a floor.

What blinding concrete is

Blinding is a thin layer of weak concrete, typically 50-75mm thick, poured over compacted hardcore (or directly onto excavated ground) to create a clean, level surface for whatever goes on top. It's not load-bearing in any structural sense. It's a working platform.

The standard grade is C8/10 GEN 1 under BS 8500 (the British Standard for specifying concrete). The "C8/10" tells you the minimum compressive strength: 8 N/mm² when tested as a cylinder, 10 N/mm² as a cube. That's roughly a third of the strength of a foundation concrete, which is exactly the point. Blinding is meant to be cheap, easy to place, and just rigid enough to walk on the next day.

You'll hear it called by several names depending on who's talking. The merchant calls it "C10". The structural engineer writes "GEN 1" on the drawing. Old-school builders call it "lean mix". Some sites just call the layer "the blinding". They all mean the same thing.

Sand blinding vs concrete blinding

Two materials carry the name "blinding". They do related but distinct jobs.

Sand blinding is a 50mm layer of sharp sand (also called grit sand or concreting sand, never building sand) laid over compacted hardcore. Its job is to fill the gaps between stones and provide a smooth bedding surface for a polythene damp proof membrane that goes on top. Without it, sharp edges on the hardcore puncture the membrane.

Concrete blinding is a 50-75mm layer of weak C8/10 concrete poured over the hardcore. It does the membrane-protection job too, but more importantly it gives you a rigid, flat working surface for placing reinforcement chairs, formwork, or insulation boards on top.

The decision between them matters. For a domestic extension slab the rule is straightforward:

The community forums get this wrong constantly. BuildHub threads cycle through the same debate every few months: do I need concrete blinding or is sand fine? The answer comes down to one question. Is the slab reinforced? If yes, you need concrete blinding. The reinforcement chairs (the small plastic or steel supports that hold the mesh at the correct height) sink into sand under the weight of a worker placing the mesh. By the time the structural concrete arrives, the chairs have settled, the mesh is too low, and the cover is wrong.

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Why the cover-reduction matters

This is the technical argument for using concrete blinding rather than sand on any reinforced slab, and it's almost entirely missing from homeowner-facing content.

BS 8500:2023 Table A.10 (note E) sets the rules for concrete cover: the depth of concrete between the outside face of the slab and the steel reinforcement inside. Cover protects the steel from corrosion (which would split the concrete from inside), gives it bond to develop strength, and provides fire resistance. Get cover wrong and the slab has reduced design life, possibly catastrophically reduced.

For concrete cast directly against soil, the standard requires nominal cover of 25mm plus a deviation allowance of 50mm, totalling 75 mm of cover. That's a thick chunk of concrete protecting the rebar.

For concrete cast against a blinding layer, the deviation allowance drops from 50mm to 15mm because the blinding gives a flat, predictable surface. Total nominal cover becomes 25mm + 15mm = 40 mm.

That 35mm difference matters in two ways. It saves concrete on every cubic metre of slab. More importantly, it makes the cover achievable on site. Holding 75mm of cover over a lumpy hardcore surface is genuinely difficult. Holding 40mm over a flat blinding layer with castle spacers is straightforward. Eurocode 2 (BS EN 1992-1-1) and the UK National Annex confirm the same numbers.

The structural engineer's calculation assumes 40mm cover. That assumption only holds if the blinding is in place. Skip the blinding and the engineer's design no longer matches what you've built.

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How thick should it be

Anywhere from 50mm to 75mm. Thinner than 50mm and the layer breaks up under foot traffic before the structural slab goes on. Thicker than 75mm wastes concrete and money. The NHBC Standards 2024 (clause 5.1.16) are explicit on this: blinding should be the "minimum thickness necessary" to provide a suitable surface.

The standard answer for a reinforced extension slab is 50mm. Step up to 75mm only if the hardcore is uneven enough that 50mm wouldn't fully cover the high spots, or if the blinding is doing a secondary job as an additional protective measure (APM) under BRE Special Digest 1 for sulfate-rich ground. APM blinding must be at least 50mm thick and of equivalent quality to the foundation concrete itself, which is a separate specification call.

For volume calculation, multiply your slab footprint by the layer thickness. A 30m² rear extension at 75mm of blinding needs 30 x 0.075 = 2.25m³. Round up to 2.5m³ to allow for the hardcore eating some of the depth on the high spots.

Where blinding fits in the build sequence

There's a fixed order to the work between excavation and the structural slab pour, and it matters because building control inspects parts of it and the rest cascades from those inspections.

1. Excavate to the reduced level specified on the drawings, accounting for slab thickness, insulation, hardcore, and blinding.
2. Compact the formation (the natural ground at the bottom of the excavation) using a wacker plate. Soft spots get dug deeper and re-filled.
3. Lay and compact hardcore in lifts no thicker than 225mm before each pass of the compactor. Deeper lifts cannot compact fully and leave voids.
4. Building control inspection of the formation, hardcore, and oversite preparation. The BCO has 24 hours' notice from your builder and they verify subbase suitability before anything goes on top.
5. Pour blinding (concrete or sand depending on what the slab requires).
6. Lay DPM with 300mm laps and 150mm turn-up at perimeter walls onto the wall DPC.
7. Lay insulation (typically 100-150mm of PIR for Part L compliance) directly on the DPM.
8. Position reinforcement on castle spacers achieving the engineer's specified cover.
9. Pour structural slab in a continuous operation.

The blinding sits between two hold-points: the BCO inspection of the oversite is the hard stop before it goes down, and the DPM lays directly on top of it. Skip the BCO inspection and pour the blinding too soon and you're at risk of being told to dig out and start again. Pour the blinding when it's raining and you'll have standing water under your DPM.

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How to order it

Blinding concrete is ordered the same way as any other ready-mix grade, with one wrinkle: the volumes are usually small enough to trigger a short-load surcharge.

The ordering specification, written out in full, is: GEN 1 designated concrete to BS 8500-1, strength class C8/10, S3 consistence, 20mm maximum aggregate. You don't need to recite all of that on the phone. "C8/10 GEN 1, 20mm agg, S3" gets you there. S3 consistence is the slump class, meaning a wet, easily workable mix. For blinding you want it wet enough to flow and self-level rather than needing aggressive screeding.

Most ready-mix plants run a 6m³ standard barrel truck. A typical blinding pour for a 30m² extension is 2-3m³, well below the standard threshold. You have three options.

Option 1: Standard ready-mix with short-load surcharge

Order from a national supplier (Tarmac, Hanson, Cemex, Breedon) or a regional plant via your builders' merchant. The truck arrives with your 2-3m³, you discharge in 20 minutes, and you pay a short-load surcharge on top.

For a 2.5m³ pour at the national average rate shown above, expect to pay roughly 2.5 times the per-m³ figure for the concrete itself, plus the short-load surcharge listed alongside. London and the South East run noticeably higher per m³ across the board. Some suppliers (Mister Concrete is a known case) impose a flat minimum charge regardless of volume, which makes them poor value for blinding pours specifically.

Option 2: Volumetric mixer

A volumetric truck carries cement, aggregate, and water separately and mixes on site as the chute runs. You pay only for what's dispensed. There's no minimum order, no short-load surcharge, and no 90-minute clock running from batching to discharge.

The per-m³ price is typically 20-35% higher than the standard ready-mix rate shown above. For a 2.5m³ blinding pour, the maths often comes out roughly even with a short-load standard delivery, with the volumetric option giving you no waste risk and unlimited time on site. Bell Concrete, Base Concrete, and a long list of regional volumetric specialists run these trucks across the UK.

For any blinding pour under 3m³, get a quote from both. Volumetric usually wins.

Option 3: Site-mixed from bags

Possible for very small pours (under 0.5m³) but rarely worth it. Site-mixing 1m³ of C8/10 needs around 7-8 bags of cement at £6.50–8.50 each (C8/10 GEN 1 uses only 180kg of cement per m³, which is why it's called a lean mix), plus 1.8 tonnes of all-in ballast, plus a hire mixer at £20–40/day inc. VAT. The labour-hours and quality risk push it out of contention against ready-mix delivered. Reserve it for a tiny isolated pour (a 0.2m³ mass-fill behind a manhole, say) where ordering a truck doesn't make sense.

How to place it properly

Blinding is the easiest concrete pour you'll ever attend. It's also the one that gets done sloppily because nobody treats it as important. Three things matter on the day.

The base must be clean and damp, not wet. Sweep the hardcore free of loose stones, dust, and debris. Lightly damp it down with a hose if the weather has been hot and dry, because bone-dry hardcore sucks moisture out of the mix and makes it crumbly at the bottom. Standing water is the opposite problem. Do not pour blinding onto a puddled surface; the water dilutes the concrete locally and creates weak spots. If it has rained the night before, mop out any standing water before the truck arrives.

The level matters more than the strength. Blinding's whole purpose is providing a flat surface for the next layer. Spread the concrete with a rake, then strike it off with a screed board (a straight piece of timber dragged across two pre-set rails). For an extension footprint with no specific gradient, set up two parallel timber rails at the desired blinding height, fill between them, and screed off. Do not power-float blinding; a basic tamped finish is correct because you actually want some surface texture for the next layer to bond against.

Don't bother curing it. Blinding doesn't need 28-day strength to do its job. By the next morning it'll be hard enough to walk on, place chairs on, and lay DPM over. If the weather is hot and windy, drape it with polythene overnight to slow surface drying, but otherwise just leave it alone.

You don't need a power float, a vibrating poker, or any of the kit that goes with a structural slab pour. Two people with rakes, a screed board, and a bull float (or just a piece of straight timber) can place 2-3m³ of blinding in 30-40 minutes.

Cold weather

Don't pour blinding when air temperature is below 5°C or when frost is forecast within 24 hours. The concrete won't reach the strength it needs to support DPM laying within the next day or two, and the surface can spall. Above 8°C is fine. Between 5 and 8°C, cover with insulating blankets after pouring and accept a slower programme.

What about not using blinding at all

This comes up on every BuildHub thread and the answer is "depends on what's going on top".

Plain ground-bearing slab with no reinforcement, on well-compacted hardcore, with sand blinding under the DPM. That works. You don't need concrete blinding. NHBC 5.1.16 is satisfied as long as the sand blinding is doing its membrane-protection job and the hardcore is voids-free.

Reinforced slab with A142, A193, or A252 mesh. You need concrete blinding. There is no safe shortcut here. Castle spacers on sand will sink. Mesh laid directly on hardcore will rest on high stones and miss its design height. The cover requirement set by your engineer cannot reliably be met without a blinding layer.

Slab over contaminated ground, sulfate-rich ground, or made-up ground with potential for fines migration. Concrete blinding is required as part of an APM under BRE Special Digest 1, at minimum 50mm and of foundation-equivalent strength.

Trench-fill or strip foundation. Blinding to the bottom of the trench is sometimes specified by the engineer to provide a clean platform for the rebar cage. This is a different blinding application and the rules are the same: GEN 1, 50mm, flat surface for placing steel.

What to check on delivery and afterwards

The driver hands you a delivery ticket. For blinding, three things on it matter.

The grade should read C8/10 GEN 1 or some clear equivalent (some suppliers print "C10/12" which is the same designated mix under older notation). If the ticket says GEN 0 and your engineer specified GEN 1, refuse the load. The cost of refusing wrong concrete is much smaller than the cost of pouring it.

The batch time should be within 90 minutes of arrival on site, the standard window for ready-mix to retain workability. Volumetric deliveries don't have this issue, because the mix is fresh by definition.

The plant should hold QSRMC or BSI Kitemark certification, indicated on the ticket. This is the third-party quality verification that building control inspectors look for. Your delivery ticket is your evidence the concrete met spec.

Keep the ticket. Building control don't always ask, but when they do, having no record is a problem you don't need.

After the pour, the surface should be flat to within 5-10mm under a 2m straightedge. Anywhere that exceeds this needs to be levelled before the next layer goes on, either by skimming additional concrete, sanding back high spots, or accepting a thicker DPM packing layer.

Common mistakes

Pouring before the BCO has inspected the hardcore. Building control inspect the oversite preparation specifically. Concrete poured before they've seen the hardcore can be ordered out, exposing whatever is underneath. The inspection takes 15-30 minutes and costs nothing extra. Do not let your builder skip it to save half a day.

Using GEN 0 instead of GEN 1. Some suppliers will offer GEN 0 (C6/8) as a cheaper alternative for "non-structural blinding". Most structural engineers won't accept it for a slab they've designed. Check the engineer's drawings and specify what's there.

Ordering too thick a layer. Two pours per project go thicker than they need to: foundations and blinding. Both waste money. NHBC says minimum thickness necessary. 50mm is fine for a flat hardcore base. 75mm is the upper end. 100mm is excess that costs an extra cubic metre on a typical extension.

Building sand instead of sharp sand for sand blinding. Building sand is too fine, clumps when damp, and washes out under groundwater. Every BuildHub thread on the topic carries this warning. If you're doing sand blinding, ask for sharp sand (also sold as grit sand or 0-4mm washed concreting sand). Building sand is for mortar above DPC, not for blinding.

Pouring blinding onto frozen hardcore. Frozen ground reflects pour temperature, the concrete sets uneven, and the surface flakes off the next day. If frost is forecast, postpone.

Letting the truck dump and leaving the screed for later. Blinding wants screeding while it's still wet. Once it starts to stiffen, levelling becomes harder and the surface ends up rough. Two people on the screed board, working from one end to the other immediately as the concrete is placed, gives you a clean platform.

Where you'll need this

• Foundations and footings: blinding to the bottom of trenches before placing rebar cages, where specified by the structural engineer.
• Building control inspection: drainage and oversite: the BCO inspects the hardcore and oversite preparation before any blinding is poured. The inspection is a hard stop in the build sequence.
• Damp proof course: DPM lays directly on the blinding layer, with a 150mm turn-up onto the wall DPC at the perimeter.
• Walls and blockwork: first course of blocks bears on the foundation strip; blinding to the trench base is the platform that sets the rebar cage at correct cover.

These tasks appear in the groundwork stage of any extension or renovation project that involves a reinforced floor slab or trench foundations, not just kitchen extensions. The cover-reduction story applies wherever Eurocode 2 design rules are used: a side return, a garden room with a structural raft, a garage conversion with a new floor, or a basement build will all run the same calculation.