100mm Dense Concrete Blocks: The Below-DPC Workhorse Explained
Everything a UK homeowner needs to know about 100mm dense concrete blocks: specs, weight, blocks per m², where to use them, and the most common beginner mistake.
Order the wrong block and your building control inspector will ask you to expose completed work. Order the right one, in the wrong quantity, and you're waiting two days for a re-delivery of 40 kg of blocks when your bricklayer is on site and on the clock. Dense concrete blocks are not complicated, but they have one critical rule (dense below the DPC, aerated above it), and the consequences of getting that wrong are expensive.
What they are and what they're for
A 100mm dense concrete block is a solid aggregate concrete masonry unit, 440 mm long, 215 mm tall, and 100 mm wide (the "100mm" refers to the width, which is the wall thickness). They're manufactured from sand, gravel, and Portland cement, compacted and cured. The result is a dense, heavy block with excellent compressive strength, good sound insulation, high thermal mass, and near-zero water absorption.
The standard specification for domestic construction is 7.3 N/mm2 compressive strength. That number (called the normalised mean compressive strength) tells you how much load per square millimetre the block can take before crushing. For context, your structural engineer will have specified a minimum strength on the drawings. If the spec says "7N," a 7.3N block is acceptable because it exceeds the requirement. If the spec says "10.4N," a 7.3N block is not acceptable. Read the drawings.
All UK-supplied blocks must conform to BS EN 771-3, the harmonised European standard governing aggregate concrete masonry units. It sets requirements for dimensions, compressive strength, density, and durability. Any block sold through a reputable UK merchant will carry CE marking and a Declaration of Performance showing the tested values for your specific block.
Gross dry density is approximately 2000 kg/m3 (range 1800-2100 kg/m3 depending on aggregate mix and manufacturer). That density is what gives dense blocks their strength, their acoustic mass, and their moisture resistance. It is also what makes them so heavy to handle, a point we'll come back to.
Types, sizes, and specifications
The 440 x 215 x 100mm size is the UK standard. One block covers a face area of 440 x 215mm, which with a 10mm mortar joint becomes a 450 x 225mm module. That's the number you use when calculating how many blocks you need per square metre of wall.
Compressive strength variants are available for specific applications:
| Strength | Use case | Common specification |
|---|---|---|
| 7.3 N/mm² | Standard external walls, below-DPC courses, typical domestic load-bearing | Default for most domestic extension work |
| 10.4 N/mm² | Heavily loaded walls, shallow foundations, some retaining walls | Specified by SE where 7.3N insufficient |
| 17.5 N/mm² | Retaining walls, high-load columns, multi-storey at lowest level | Less common in domestic projects |
| 21–30 N/mm² | Commercial and industrial, specialist structural applications | Rarely specified for domestic extensions |
For most domestic extensions, 7.3N/mm2 is what will be on your structural engineer's drawings. If you're ordering blocks and the spec simply says "dense concrete block" without a strength, confirm with your SE or building control before ordering. Getting 7.3N when 10.4N was required means your inspector will require demolition and rebuild.
Higher-strength blocks are heavier and often harder to source locally. Forterra's Conbloc range covers 7.3, 10.4, 17.5, 22.5, and 30 N/mm2. Lignacite's Lignacrete goes up to 30 N/mm2. For standard domestic work, you won't go above 10.4N.
How to work with dense blocks
Weight and handling
Each 100mm dense block weighs approximately 19-20 kg. Manufacturers vary slightly: Tarmac Topcrete comes in at 19 kg, Cemex ReadyBlock at 19.8 kg, WDL at 20 kg. Moisture content adds to this: blocks stored outdoors in wet weather can absorb several additional kilograms. A freshly delivered, rain-soaked pallet will always feel heavier than the spec sheet suggests.
HSE/CONIAC guidance (Construction Sheet No. 37) flags high risk of injury in the single-handed, repetitive manual handling of blocks heavier than 20 kg. The Concrete Block Association recommends a sustained laying rate of no more than 20 blocks per hour for blocks in this weight range. That's one block every three minutes. Your bricklayer knows this. If they're motoring through at 40 blocks an hour for hours at a time, their back will know about it by day three.
Plan your delivery and storage carefully. A pack of 72 blocks weighs approximately 1.4 tonnes. A pack of 88 weighs around 1.7 tonnes. Most builders' merchant deliveries come on a flatbed lorry with a crane arm (hiab). You need a firm, level standing area for the pallet close to where the blocks will be used, ideally within 20 metres. Moving 1.4 tonnes of blocks by hand across a muddy site is a real labour cost that doesn't appear in most quotes.
Store different block grades on clearly marked, separate pallets. Dense 7.3N and dense 10.4N blocks look almost identical. On a busy site where multiple deliveries have arrived, mix-ups happen. A block used in the wrong location may not be caught until building control inspection, at which point you're digging out completed work. Mark pallets clearly when they arrive and keep grades segregated.
Cutting dense blocks
You cannot cut a dense concrete block with a hand saw. The aggregate is too hard. Options are:
A disc cutter (also called a cut-off saw or angle grinder with a masonry disc) is the fastest method. Score the cut line on all four faces first, then cut through. Always cut outside or in a well-ventilated area; cutting concrete generates fine silica dust, and you must wear a P3 dust mask (not a basic disposable), safety glasses, and hearing protection. Silica dust is a serious health hazard with cumulative effects.
A cold chisel and lump hammer (bolster chisel) is the traditional method for making rough cuts where precision isn't critical. Score around all four faces with the chisel, then strike along the scored line. The block will split along the line if the cut is clean. This works for closing cuts and cut-and-bond courses but doesn't give you the accuracy of a disc cutter. Not suitable for cuts that will be visible or that need tight tolerance.
A block splitter (guillotine-style tool) is fast and clean for straight cuts. Trade hire from HSS or similar. Worth it for large volumes.
Laying dense blocks
Dense blocks are laid with standard mortar. Do not use thin-joint mortar systems (Celfix or similar) designed for aerated blocks. Those systems rely on the dimensional precision and low water absorption of aerated blocks. Dense blocks need standard 10mm mortar joints.
A 1:6 cement:sand mortar mix (or 1:1:6 cement:lime:sand for a more workable mix) is appropriate for above-ground blockwork in most exposures. Below DPC and in exposed locations, a stronger 1:4 or 1:3 mix is used for improved durability. Your structural engineer's specification will state the mortar designation required.
Keep mortar joints consistent at 10mm. Inconsistent joints affect both the structural performance of the wall and the aesthetics if any of the blockwork will be visible. Building control inspectors check joint consistency. NHBC inspectors flag uneven joints as a defect.
How many blocks do you need
The calculation is straightforward once you know the module size.
Each block covers a face area of 440 x 215mm. Add 10mm mortar joints: effective face area is 450 x 225mm = 101,250 mm2 = 0.1013 m2. Divide 1 m2 by 0.1013 = 9.87 blocks per m2. Round up to 10 blocks per m2. That's the standard UK figure.
Worked example: a cavity wall with two skins, 4 metres long and 2.4 metres high.
- Face area per skin: 4 x 2.4 = 9.6 m2
- Both skins: 9.6 x 2 = 19.2 m2
- Blocks needed: 19.2 x 10 = 192 blocks
- Add 10% for cuts and wastage: 192 x 1.1 = 211 blocks
- Order in whole packs (72 per pack): 3 packs = 216 blocks
Always add 10% for wastage. Cuts at corners, around openings, and for closer blocks at the end of each course eat into your count faster than expected. Ordering a single extra pack is far cheaper than a separate delivery charge for a small re-order.
For padstones beneath steel beams, the quantity is specific to your structural drawings. Dense blocks are used as padstones to spread the concentrated point load from a steel beam into the wall below. Typically 2-4 blocks per beam bearing point, laid in a specific configuration shown on the SE's drawings. Do not substitute aerated blocks for padstones; they lack the compressive strength.
Cost and where to buy
Dense 100mm blocks are priced at £1.40 – £2.65 per block (ex VAT) across UK builders' merchants. The range reflects the difference between retail (Travis Perkins quoted online at the high end) and independent merchant pricing.
The major national merchants (Travis Perkins, Jewson, Wickes, and Buildbase) all carry dense blocks from the main manufacturers (Tarmac Topcrete, Forterra Conbloc, Cemex ReadyBlock). Jewson and Wickes typically require a postcode or account login to see pricing; their online listed prices may not reflect what trade customers actually pay. National chain list prices tend to sit at the top of the range, but trade account holders pay considerably less.
Independent builders' merchants consistently undercut national chains. If you are buying more than two or three packs and you do not hold a trade account, call three local independents before ordering from a national chain. The savings add up quickly across a full extension's worth of blockwork.
Pack sizes vary by manufacturer: 66, 72, or 88 blocks per pack. Buying by the pack is always cheaper per block than buying individually. Delivery costs add up fast for heavy materials. A two-pack delivery weighs over a tonne. Local merchants often have lower or free delivery thresholds for bulk orders. Factor delivery into every quote comparison.
Key manufacturers
You won't always get to choose which brand arrives on your pallet (merchants stock what they have), but knowing the main manufacturers helps you cross-check product data sheets and verify what you've been sent is what was specified.
Tarmac Topcrete is the most widely distributed dense block in the UK. Density 1950 kg/m3, thermal conductivity 0.95 W/mK. Available through Jewson, Wickes, and Travis Perkins.
Forterra Conbloc and Evalast cover the full strength range from 7.3 to 30 N/mm2. Density 1990 kg/m3, thermal conductivity 1.12 W/mK. Forterra is a major market supplier.
Cemex ReadyBlock 2000 Solid Dense runs at 1960 kg/m3 density, 19.8 kg weight, thermal conductivity 1.29 W/mK.
Newlay Concrete (Newcon) and WDL Concrete are regionally strong manufacturers (Yorkshire and Wales respectively), both at around 2000 kg/m3 and available through Jewson. WDL's Declaration of Performance is publicly available and shows all tested properties for their 7.3N blocks.
All compliant manufacturers produce blocks to BS EN 771-3 and carry CE marking. The differences between manufacturers at the same strength grade are marginal for domestic use. The structural performance is governed by the declared compressive strength, not the brand.
Alternatives
Aerated blocks (100mm) are the standard alternative for above-DPC inner leaf construction. The difference in thermal conductivity is stark: dense blocks conduct heat at 0.95-1.33 W/mK; aerated blocks at 0.11-0.15 W/mK. For meeting Part L U-value requirements for extension walls, aerated blocks for the inner leaf are the only practical choice without adding substantial extra insulation board. The strength trade-off (3.6 N/mm2 vs 7.3 N/mm2) is acceptable for inner leaf above-DPC construction in domestic buildings.
Aerated blocks also weigh 8-10 kg each (versus 19-20 kg for dense), can be cut with a hand saw, and cause far less physical strain during laying. The switch from dense below DPC to aerated above DPC is welcomed on every site.
Engineering bricks are sometimes specified below DPC where blockwork is below the water table or in particularly aggressive ground conditions. Higher compressive strength than dense blocks, lower water absorption, more expensive. Your structural engineer will specify these if required.
140mm dense blocks are available for walls requiring additional compressive strength or where the wall needs to carry greater loads. Weight increases to around 26-27 kg per block. Specified for foundation walls deeper than 1 metre, certain party wall applications, and at the lowest level of multi-storey structures.
Where you'll need this
Dense concrete blocks appear at the foundations and structural phases of any extension or renovation involving masonry walls:
- Walls and blockwork - below-DPC courses from foundation to ground level, using approximately 10 blocks per m2 of wall face
- Steels and lintels - padstones positioned beneath steel beam bearing points to distribute concentrated loads, typically 2-4 blocks per bearing point per structural engineer's drawings
These materials appear across groundwork and structure phases of any project involving masonry cavity wall construction.
Common mistakes
Using dense blocks above DPC for the inner leaf. This is the most common beginner error, and it's documented as such in competitor content and forum threads. Dense blocks above DPC on the inner leaf creates walls with thermal conductivity around ten times worse than an aerated inner leaf. Meeting Part L U-value requirements becomes very difficult, and even then the calculation may not pass. Your structural engineer's drawings will show which block type is specified in which location. Read them.
Ordering blocks without confirming the strength. "Dense block" at a merchants' desk can mean 7.3N, 10.4N, or 7N (which is a merchant shorthand for 7.3N and is fine). If your drawings say 10.4N and you order 7.3N, building control will require you to remove and replace the incorrect blocks. Check the strength before the delivery is placed.
Underestimating delivery logistics. A pack of 72 dense blocks weighs 1.4 tonnes. Two packs is 2.8 tonnes. You need vehicle access for a crane-arm lorry, a firm standing area for the pallet, and a realistic assessment of how far the blocks need to be moved on site. If access is tight, discuss it with your merchant before ordering. Redelivery and crane hire charges for aborted deliveries are charged at cost.
Skipping the 10% wastage allowance. Cuts at corners, closing cuts at window reveals, and occasional broken blocks during handling mean your net usage is always higher than the theoretical calculation. Order 10% extra. The cost of an extra half-pack is negligible compared to a separate re-delivery.
Dense blocks and aerated blocks can look similar when stacked on a pallet in poor light, especially if packs have become mixed during site deliveries. Pick up a block. A dense block at 19-20 kg is unmistakably heavy. An aerated block at 8-10 kg can be lifted with two fingers. If you're uncertain which is which, weigh one before using it in a load-bearing application.