Question 1

1. What considerations are important when choosing foundation blocks?

Accepted Answer

When selecting foundation or footing blocks, several factors should guide your decision.

Foundation block size - Understanding foundation block sizes is important. Blocks are typically chosen to suit cavity width and wall construction rather than load alone. You should also assess ground conditions early, as these influence the foundation design and may require engineered solutions to prevent movement or settlement over time.
Type of block - Another key consideration is the type of foundation block needed. In the UK, this usually means choosing between aerated concrete foundation blocks and trench blocks, both designed for below-ground use. Aerated blocks offer lighter handling and improved thermal performance, while trench blocks are used to speed up construction from footing to damp-proof course. Note that dense concrete blocks may also be specified for block footing where higher strength and durability are needed, but these are typically standard blocks used in a foundation context rather than a distinct foundation block type.
Block strength ratings - These should also be checked to make sure they meet the structural demands of the project. If you need to, consulting a structural engineer or experienced professional will help confirm the correct specification and ensure compliance with UK building regulations.

Taking these factors seriously at the outset helps prevent costly issues later and ensures your foundation performs reliably over the lifetime of the building.

Question 2

2. Foundation blocks vs footing blocks – what's the difference?

Accepted Answer

In construction, the ‘footing’ is the lowest part of the structure – designed to evenly distribute the weight of the building above. On most modern UK builds, the footing is made from poured concrete, with foundation blocks, or similar masonry units, used to build up from that footing to the damp proof course.

‘Footing blocks’ aren't a formal product type, but in UK trades the term is often used informally to refer to foundation blocks, since they sit on top of the footing.

Question 3

3. How many concrete blocks do I need for a shed foundation?

Accepted Answer

This depends on the shed size and support layout, as well as the area. Small sheds may only need 6 blocks, while medium sheds typically use 9–12, with larger or heavier sheds requiring more support points. Blocks should be placed under all load-bearing points, including corners and mid-spans, rather than evenly covering the whole area.

Question 4

4. How many foundation blocks do I need per m²?

Accepted Answer

In terms of coverage, most systems of foundation block work require around 10 blocks per m² when laid as continuous blockwork with standard mortar joints. However, this is only a guide. When used in foundations, especially trench block systems, the number of blocks needed depends on foundation depth, layout, and number of courses, not just surface area.

Larger-format foundation or trench blocks can reduce the number of courses below ground, helping speed up installation and reduce labour on site. It’s best to base your quantities on the foundation design and trench dimensions, rather than relying on m² estimates alone.

Question 5

5. Can I use concrete blocks as a foundation?

Accepted Answer

Standard concrete blocks can be used in foundations in the UK, but only in the right context. Standard dense concrete blocks are often used to build up foundation walls from a concrete footing to the damp-proof course, as they provide good compressive strength and durability.

However, they‘re not usually used as the footing itself. Most foundations still rely on poured concrete footings or trench fill to spread the load into the ground, with blockwork built on top.

In some cases, standard blocks may be used below ground as part of the concrete block foundation, but they must be suitable for the exposure conditions and have the correct strength rating. For many modern builds, purpose-made options like trench blocks or aerated foundation blocks are preferred for speed, handling, or thermal performance.

Question 6

6. Can Thermalite blocks be used in foundations?

Accepted Answer

Yes, Thermalite blocks can be used for a concrete block foundation, but only where they’re specific foundation-grade blocks. Thermalite units are commonly used below ground in cavity walls and beam and block floors. However, they must be suitable for the exposure conditions and loading and are often used above footing level rather than in direct ground contact.

Foundation blocks

Thermalite 300mm Trenchblock T&G 3.6N

Celcon Foundation Block Standard Grade Block - 440 x 215 x 300mm

Hi-strength Thermalite Trench Blocks 7.3N 300mm