With over 30 years in the builders’ merchant trade, Andrew brings deep knowledge of everything from civils to timber. Now part of the Gilmore team, he helps customers make the right choices with advice built on decades of hands-on experience.
The key difference between 7N and 10N blocks is simply a matter of compressive strength, the higher the number, the greater the structural integrity. A 10N blockwork wall will be able to support higher loadings, and will also be denser, making it better at sound deadening in partition walls.
It’s not unusual to have both 7N and 10N specified on the same project, however, they will be used for different parts of the build. Make sure they are stored separately as they can sometimes be difficult to tell apart.
Here’s what you need to know about the differences between 7N and 10N blocks.
The N stands for Newtons, which is an international measure of force (an SI). The N should actually be followed by mm2, so 7N blocks will actually normally be 7.3N/mm2. 10N blocks are most commonly 10.4N/mm2.
In relation to block strength it determines the point at which a block will fail, so a 7N block will stand up to a direct loading of 7 Newtons per millimeter (which is actually 67 tonnes before the safety factor is considered*!) before it is crushed.
It follows that a 10N block will take a greater loading and have a higher fail point. This is why you will see 10N blockwork specified for foundations and lower storeys, corner sections, and high loading areas such as window and door openings.
Block strengths (e.g., 7.3 N/mm², 10.4 N/mm²) are laboratory test values for the unit, not the strength you design to on site. In real projects, masonry design follows Eurocode 6 / BS EN 1996 (with the UK National Annex), which converts unit strengths into a characteristic masonry strength and then applies partial safety factors (γ) for materials, workmanship, load effects, eccentricity, slenderness, and bearing conditions.
You’ll often hear builders or designers talk about a “three-to-one” safety factor when it comes to blockwork — and in everyday terms, that’s not far off. Once you account for the formal safety and partial factors set out in Eurocode 6 / BS EN 1996, along with reductions for workmanship, wall geometry, mortar strength, and load eccentricity, the usable design stress in a wall usually ends up being around a third (or less) of the block’s laboratory-tested value.
In other words, the familiar “3 to 1” rule of thumb loosely reflects the effect of all those code-based adjustments, even though it isn’t a fixed standard.
Bottom line: the allowable design stress is significantly lower than the lab value, and it depends on the entire wall build-up — unit strength, mortar class, geometry, loading, and detailing. Always follow the project specification or consult your structural engineer for the correct design values.
The typical use for 7N blocks is as a general purpose unit for walls and floors, for upper stories and for inner leafs of brick-skinned cavity walls. 7N blocks are perfect for your rear single-storey extension, a garage conversion, or for the gable end of a two storey extension.
Use 7N blockwork for internal partition walls, built off a concrete slab or beam and block floor. The resulting wall will be fire resistant and have good sound and thermal insulation properties.
Either an aggregate concrete block, or an aerated equivalent 7.3N block can be specified for use when building a standard low-rise traditional construction project.
I’d expect to see 10N blocks used below the DPM on top of the footings, to give support to the rest of the structure, but they’re not essential in most low-rise construction.
Part A of the Building Regulations covers structural specifications. It sets out a height requirement whereby stronger blocks will be required to manage additional loadings for greater storey heights.
In a three storey construction of a townhouse, for example, internal walls must be built from blocks that are a minimum of 140mm thick. Despite their size, these blocks still have a perfectly acceptable 7.3N rating.
You may see this term when it refers to the party wall between dwellings. In this case it usually refers to the sound deadening required. A denser block, such as a 10N aggregate or dense block will help to fulfill the requirements for Building Regulations.
Note; the regulations in Scotland require more sound deadening than the rest of the UK. Consult your local surveyor for advice.
The price differences between 7N and 10N blocks will have more to do with whether the blocks are facing (for show) or regular blocks. There is little difference with regard to cost, so it would not be wise to economise and use 7N, when 10N are specified.
If you mix and match, using up partial pallet loads of blocks, the wall will have the compressive strength of the lower rated blocks.
They refer to compressive strength in N/mm² (megapascals). In UK blockwork, common grades are 3.6, 7.3 and 10.4 N/mm². The higher the value, the stronger (and typically denser) the block. It is not the total weight a single block can carry.
Use 7.3N for many internal partitions and upper storeys in low-rise work where loads are modest. Use 10.4N where higher loads are expected—below DPM, around openings, bearing points (joist ends/rafter feet), and in taller or more heavily loaded walls—always follow the project spec or an engineer’s design.
Generally, yes. Denser aggregate blocks tend to improve sound reduction, which helps with Approved Document E targets and Robust Details party-wall constructions. Always check the full wall build-up and the manufacturer’s data.
Yes, but don’t mix strengths within the same structural element unless the design allows it. Keep pallets clearly separated and build to the lowest strength present if mixed inadvertently. When in doubt, ask the designer/engineer.
They set performance requirements rather than telling you a single strength. Approved Document A points to design standards (e.g., Eurocode 6 / BS EN 1996). The required strength depends on loads, wall height/thickness, support conditions, and exposure. Follow the issued specification or obtain structural advice.
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Please note: The information provided on this website is for general guidance only and should not be relied upon as professional advice.
Building methods, material specifications, and regulations can vary depending on location, project design, and site conditions.
Always refer to the latest Building Regulations, manufacturer data sheets, and consult with a qualified structural engineer, surveyor, or building control officer before starting any construction work or making design decisions.
Gilmore Building Supplies accepts no responsibility for loss, damage, or injury resulting from reliance on the information provided.
