Dense vs Hollow Concrete Blocks: Which Type Should You Use?

Dense vs Hollow Concrete Blocks: Which Type Should You Use?

Dense and hollow concrete blocks can both form strong masonry walls, but their internal construction is different. Dense blocks contain a high proportion of solid concrete, giving them substantial mass, while hollow blocks use large formed voids to reduce weight and create a different structural geometry. The correct choice depends on wall thickness, loading, acoustic requirements, fixings and the complete design.

JJ Building Materials stocks dense BLOC blocks in 100mm and 140mm widths and a 215mm hollow BLOC block, all with a 7.3N designation. This guide compares the practical differences and explains why one format should not be substituted for another without checking the specification.

What Is a Dense Concrete Block?

A dense concrete block is made from relatively high-density concrete and has a largely solid cross-section. Its mass gives it robust handling characteristics and can contribute to acoustic performance. Dense blocks are widely used in internal and external masonry, foundations and load-bearing walls where the product is suitable.

The live JJ Building Materials range includes a 100mm BLOC dense 7.3N block and a 140mm BLOC dense 7.3N block.

What Is a Hollow Concrete Block?

A hollow concrete block contains large internal voids formed during manufacture. The shells and webs around the voids carry load while reducing the amount of concrete and unit weight compared with a fully solid block of the same overall size.

JJ Building Materials currently stocks a 215mm BLOC hollow 7.3N block. This is much wider than the dense products, so differences in unit weight cannot be judged from configuration alone.

Dense vs Hollow: Strength

Do not assume dense automatically means stronger. All three current BLOC products carry a 7.3N designation, showing that hollow geometry can still achieve the specified compressive-strength level. The actual wall capacity depends on width, mortar, height, restraint and loading.

Where the structural design specifies a particular block type as well as a strength, match both. A hollow 7.3N unit and dense 7.3N unit are not necessarily interchangeable because their geometry and fixing behaviour differ.

Dense vs Hollow: Weight

Hollowing reduces the mass of a block compared with a solid unit of identical external dimensions and concrete density. However, a 215mm hollow block can still weigh more than a 100mm dense block because it is more than twice as thick.

Plan manual handling using actual product weight where available, not the words dense or hollow. Larger blocks can also be awkward even if their kg weight is manageable.

Dense vs Hollow: Wall Thickness

In the current JJ Building Materials range, dense products are 100mm and 140mm, while the hollow product is 215mm. That means selection often begins with wall thickness rather than void configuration. If drawings specify a 215mm hollow wall, changing to a thinner dense block alters the structure.

Our concrete block size guide explains the dimensional implications.

Dense vs Hollow: Fixings

Dense concrete generally provides a continuous material zone for expansion anchors, screws and resin systems. Hollow blocks require an anchor that can work with a shell or bridge a void. Specialist hollow-block sleeves, mesh resin systems and other fixings may be used depending on load.

For heavy fixtures, obtain load data from the fixing manufacturer for the exact masonry substrate. Do not use a dense-block load table for a hollow unit.

Dense vs Hollow: Cutting

Dense blocks require powerful masonry cutting equipment for clean cuts. Hollow blocks can be easier to cut through some zones but the remaining shells can be fragile near the edge of a cut. Plan module lengths to minimise small pieces.

Use dust controls, guards and suitable PPE for all concrete cutting.

Dense vs Hollow: Acoustic Performance

Mass is valuable in many sound-insulation systems, so dense blocks are commonly used where acoustic separation is important. Hollow blocks can also form successful acoustic walls depending on thickness, finishes, cavities and whether voids are filled in a designed system.

Use tested or calculated wall-system performance rather than judging by unit type alone. Flanking paths at floors, roofs and junctions often determine real-world results.

Dense vs Hollow: Thermal Performance

Neither conventional dense nor hollow concrete should be assumed to offer the same low thermal conductivity as aircrete. Hollow voids can change heat flow, but full wall U-value depends on the exact block data, insulation and build-up.

For lightweight thermally efficient masonry options, compare Thermalite aerated blocks.

Dense Blocks for Internal Partitions

100mm dense blocks can be useful in robust internal partitions, plant rooms and walls that need good fixing strength or mass. However, floor loading should be considered because dense masonry is heavy. Check whether the supporting slab or floor is designed for the wall.

Dense Blocks for External Walls

Dense blocks can form the inner leaf of insulated cavity walls or other external systems. Because the block itself is relatively conductive, insulation design is important. Check wall ties, cavities, DPCs, thermal bridges and finishes.

Hollow Blocks for Structural Walls

Thick hollow blocks can be used in structural masonry systems where their geometry and strength are designed for the loads. Some systems may use reinforcement or grout, but only where specifically detailed. Do not fill voids or add reinforcement casually because that changes the designed system.

Hollow Blocks in Retaining or Below-Ground Work

Only use hollow blocks in retaining or below-ground applications where the wall has been designed for soil pressure, moisture and drainage. Retaining structures require more than block strength; foundations, reinforcement where applicable, waterproofing and drainage are critical.

7.3N Dense and Hollow Blocks

A shared 7.3N designation means the products meet the stated unit compressive-strength level, not that their walls perform identically. See our 7.3N concrete block guide for a detailed explanation.

Mortar Bedding Differences

Dense solid blocks can usually be fully bedded across their width. Hollow blocks should be laid in accordance with product and masonry guidance so shells and webs are correctly supported. Consistent joint thickness and filled perpend joints where required are essential.

Quantity per Square Metre

If dense and hollow products share approximately 440 x 215mm face dimensions, they use a similar number of blocks per square metre of wall face: around 10 units per m² as an early estimate with typical joints. Width affects wall volume, but not face coverage.

See our concrete block quantity guide for calculations and pack planning.

Pack Quantities

Current JJ Building Materials packs differ by size: 100mm dense blocks are supplied in packs of 66, 140mm dense blocks in packs of 44 and 215mm hollow blocks in packs of 38. Wider or heavier blocks normally result in fewer units per pack.

Which Is Better for DIY?

Ease of use depends on the task. A 100mm dense block is smaller but heavy for its size; a 215mm hollow block is larger and needs careful handling. If the work is structural, DIY convenience should never override the designed masonry specification.

Dense vs Hollow Selection Checklist

• Check the specified wall thickness.
• Confirm whether dense or hollow configuration is required.
• Verify compressive strength.
• Review structural loading and wall height.
• Consider acoustic mass and thermal build-up.
• Plan appropriate fixings.
• Check handling weight and pack sizes.
• Calculate quantity by wall area.

Dense vs Hollow FAQs

Are hollow concrete blocks strong?

Yes, hollow blocks can have substantial declared compressive strength. The current JJ Building Materials hollow BLOC product is designated 7.3N.

Are dense blocks stronger than hollow blocks?

Not necessarily. Compare the declared strength of the exact products and the complete wall design.

Are hollow blocks lighter?

They are lighter than a solid block of the same size and density, but a wider hollow block can still weigh more than a thinner dense unit.

Which block is best for heavy fixings?

Dense blocks often provide a straightforward solid substrate, but suitable fixings exist for hollow masonry too. Use tested anchor data.

Which block is best for sound insulation?

Dense block mass can be useful, but acoustic performance should be based on a complete tested or calculated wall system.

Can hollow blocks be filled with concrete?

Only where the wall system is specifically designed to be grouted or filled. Do not modify the block system without design guidance.

Do dense and hollow blocks need different quantities?

If their face dimensions are the same, block count per square metre is broadly similar. Pack quantities may differ.

Where can I compare dense and hollow blocks?

Browse JJ Building Materials' concrete blocks collection for live sizes and pack information.

Final Advice

Choose dense or hollow blocks by the wall design, not by assumptions. Dense blocks provide high mass and a continuous fixing substrate; hollow blocks reduce material and can create thick structural masonry efficiently. Width, strength and configuration all need to line up with the drawings before price or handling convenience is considered.