09/05/2003
When embarking on a new paving project, whether it's a charming garden path, a robust driveway, or a large commercial area, one element often sparks debate and confusion: the sub-base. Is it truly necessary? Can you cut corners, perhaps by using old materials lying around? Understanding the role and requirements of a proper sub-base is paramount to the longevity and performance of your paved surface. It’s the unsung hero beneath your beautiful blocks or slabs, providing crucial support and stability. This guide will delve into the essential considerations for sub-bases, helping you lay a foundation that stands the test of time and British weather.
Do You Really Need a Sub-Base for Your Paving Project?
This is perhaps the most common question posed by DIY enthusiasts and even some seasoned contractors. The short answer is unequivocally clear: if your paved area will experience vehicle traffic, then a sub-base is not just recommended, but absolutely essential. For areas designated purely for foot traffic, the necessity is less rigid, though often still highly beneficial. The primary function of a sub-base is to distribute the loads applied to the surface over a wider area of the underlying ground, known as the sub-grade. Think of it as the 'underlay' beneath the 'carpet' of your paving – it prevents localised pressure points, reduces settlement, and provides a stable, uniform platform. Without it, your paving is far more susceptible to cracking, sinking, and ultimately, premature failure.
The type of paving you choose is a significant factor in determining sub-base requirements. Some materials, such as flexible block paving, almost always demand a robust sub-base to maintain their integrity and prevent movement. Others, like pattern imprinted concrete, might technically function without one on a perfectly stable sub-grade, but including a sub-base will invariably enhance durability and reduce the risk of future issues. It's a foundational layer designed to absorb stress and provide a consistent base for whatever lies above it. Unless there's a compelling, expert-backed reason to omit it, incorporating a sub-base is generally the wisest choice for any paving installation.
Table 1: Requirements for Sub-base and Recommended Thicknesses by Paving Type
| Type of Paving | Sub-base Required? | Typical Thickness (mm) | Comments |
|---|---|---|---|
| Flexible Block Paving | Always | 75-150 | 75mm minimum for foot traffic only; 100-150mm for driveways; 150mm+ for commercial applications. Crucial for load distribution and preventing block movement. |
| Rigid Block Paving | Usually | 100-225 | May be used beneath the base layer to provide additional stability and load-bearing capacity for heavier applications. |
| Patio Flags | Occasionally | 75-100 | Not strictly necessary for light foot traffic on stable ground, but highly useful if levels need to be elevated or for added stability. |
| Indian Sandstones | Occasionally | 75-100 | Similar to patio flags; not essential for all situations but beneficial for elevation, stability, and preventing differential settlement. |
| Yorkstone Flags | Occasionally | 75-150 | Not necessary for foot traffic on well-drained, stable ground, but often employed when constructing a driveway or for heavier-duty applications to ensure longevity. |
| BS Flags | Usually | 75-150 | While not always specified for small patios, they are frequently used for driveways and are almost always a requirement for commercial works due to their robust nature. |
| Gravel | Usually | 75-150 | May be omitted in purely decorative, untrafficked areas, but absolutely essential for any areas subject to vehicle or significant foot traffic to prevent rutting and dispersal. |
| Setts/Cubes | Usually | 75-150 | Optional for rigid construction where the bedding material provides sufficient support; however, it is absolutely essential for flexible construction to prevent movement and settlement. |
| Cobbles | Occasionally | 75-150 | Can be used beneath a base layer to enhance structural integrity, particularly in areas expecting moderate to heavy traffic, providing a stable foundation. |
| Tarmac | Always | 100-150 | A minimum of 100mm is typically recommended for footways and light driveways. Tarmac requires a solid, well-compacted sub-base to prevent cracking and rutting under load. |
| Concrete - PIC (Pattern Imprinted Concrete) | Usually | 100-150 | Often, and sometimes misguidedly, omitted beneath PIC on driveways. While concrete itself is rigid, a sub-base provides crucial support, especially on variable sub-grades, preventing cracks. |
| Resin Systems | Usually | 75-150 | The requirement and thickness depend heavily on the existing substrate type and the expected traffic. A stable, well-drained sub-base is key to preventing future issues with the resin surface. |
How Deep Should Your Sub-Base Be?
Determining the appropriate depth for your sub-base is as critical as deciding whether to use one at all. Generally, a flexible sub-base made of unbound material, such as crushed aggregate (like DTp1), needs to be at least 100mm deep. Similarly, a bound sub-base composed of Cement Bound Material (CBM) or Hydraulically Bound Material (HBM) also typically requires a minimum of 100mm. For bitumen-bound materials, a slightly thinner minimum of 50mm might suffice, but this depends heavily on the specific application and expected loads.
A useful 'rule of thumb' often applied to fill layers, including sub-bases, is that their thickness should always be at least twice the size of the largest particle within the material. This principle is fundamental to ensuring the layer functions correctly. It prevents the creation of 'points' where a single large particle could be in direct contact with both the upper and lower surfaces of the layer. Instead, each particle should be 'cushioned' by finer material both above and below it, ensuring that the layer effectively spreads the loads it carries. This also eliminates any potential 'rocking points' within the sub-base, which could lead to instability in the paving above.
For instance, if you are using DTp1 (Department of Transport Type 1), which theoretically has a maximum particle size of 37.5mm, then applying this rule of thumb, the minimum layer thickness would be 37.5mm × 2 = 75mm. If your sub-base material is a 50mm crusher run, then the minimum layer thickness would be 50mm × 2 = 100mm. Conversely, if your project space dictates a maximum fill depth of, say, 50mm, then the largest particle size you should use would be 50mm ÷ 2 = 25mm. This simple calculation helps ensure structural integrity.
So, assuming a sub-base is going to be used, which, as we've established, is often the sensible choice, just how deep should it be for various projects? The following table provides typical recommended depths when using materials like DTp1, which is a widely accepted and robust sub-base material in the UK.
Table 2: Recommended Sub-base Depths Using DTp1 Material
| Project Type | Typical Recommended Depth (mm) |
|---|---|
| Patios, Garden Paths (Foot Traffic Only) | 75-100 |
| Driveways, Public Footpaths (Light Vehicle Traffic) | 100-150 |
| Heavy Uses (e.g., Commercial Parking, Access Roads) | 150-225 |
| Highways (Major Roads) | 150+ |
It's important to remember that these are typical depths. Specific site conditions, soil type, and anticipated loads may necessitate adjustments. Always consult with a professional if you are unsure.
Can I Use Old House Bricks as a Sub-Base? The Pitfalls of a False Economy
This question often arises from a desire to save money or repurpose materials. While admirable in spirit, the short answer is a resounding no. Sub-bases are not included in a pavement design simply for aesthetics; they are often the crucial load-bearing layer that ensures the long-term stability and performance of your entire paved area. Trying to save a few quid by using old house bricks or other random rubbish you have lying around is almost always a false economy that will cost you far more in repairs and heartache down the line.
Consider this analogy: would you be comfortable if a carpet fitter suggested using a few old tea towels under your living room carpet instead of proper underlay, just to save a bit on cost? Of course not! You'd expect a proper, uniform foundation. The same principle applies, perhaps even more so, to your outdoor paving.
The fundamental problem with using old bricks, especially whole ones, is that people tend to simply 'chuck them in as is'. Whole bricks and half-bricks are dumped haphazardly over the sub-grade, creating hundreds of irregular gaps and voids between them. The same issues arise when using other so-called 'hardcore' like old flagstones, broken kerbs, or random lumps of concrete. These materials, when not properly processed, cannot form a stable, uniform layer.
What happens next? After just a few weeks or months, any finer material used as additional fill (or even the bedding material for your paving) begins to trickle down and settle into all those numerous voids. This initial settlement is just the beginning. Following this, the irregular outlines of the bricks start to 'ghost' through to the surface. You'll find that your bitmac surface develops unsightly ups and downs, or your patio flags begin to 'rock' precariously on the high points of the underlying, poorly laid bricks. This is a common failure point that leads to trip hazards, drainage issues, and a generally unsightly and failing surface.
Even breaking bricks in half isn't ideal, though it slightly reduces the severity of the problem. It doesn't eliminate the fundamental issue of irregular shapes and large voids. For any old brick material to be even remotely suitable, you need to break the bricks down significantly – into quarters or even smaller pieces. The smaller and more uniform the pieces, the better. Once broken, this material then needs to be 'blinded' with finer aggregates such as grit sand, rock fines, or crushed stone. The purpose of this 'blinding' is to fill all the voids between the smashed brick pieces, creating a denser, more cohesive layer. Following this, the entire layer must be thoroughly compacted, repeatedly running a plate compactor or roller over the area until all the 'fines' are vibrated and settled into the voids, creating a reasonably stable 'capping layer'.
Even after all this effort, it's highly recommended to then regulate the top of this makeshift layer with a minimum of 75mm (or more, depending on the project) of quality sub-base material. This could be a DTp1, DTp2, or a high-quality crusher-run material. This additional layer provides the essential uniform, load-spreading platform that your paving truly needs.
Ultimately, skimping on the sub-base is one of the surest ways of significantly shortening the life of any driveway or heavily trafficked paved area. For the relatively small additional cost, spending a few extra quid on quality sub-base material is a sound investment that will undoubtedly pay for itself many times over through years of extended service and minimal maintenance. Don't compromise the foundation of your paving; it's the bedrock of its performance.
Frequently Asked Questions (FAQs)
Q: What is DTp1 material?
A: DTp1, or Department of Transport Type 1, is a widely specified granular sub-base material in the UK. It consists of crushed rock, concrete, or brick, graded from 0/40mm down to dust, designed to compact well and provide a stable, load-spreading layer. It's often referred to as MOT Type 1.
Q: Can I use scalpings instead of DTp1?
A: Scalpings are a broader term for crushed aggregate. While some scalpings might be suitable, they don't always meet the strict grading specifications of DTp1. Ensure any scalpings you use are clean, well-graded, and free from contaminants, and ideally, that they can be compacted to achieve a similar density and stability as a specified DTp1. Always check with your supplier about their suitability for sub-base applications.
Q: How do I compact a sub-base effectively?
A: Effective compaction is crucial. For smaller areas like patios, a vibrating plate compactor (often called a 'wacker plate') is sufficient. For larger driveways or commercial areas, a roller might be required. The material should be laid in layers (lifts) of no more than 150-200mm, with each layer thoroughly compacted before the next is added. You should see no further movement or imprint from the compactor once a layer is adequately consolidated. A good indication of proper compaction is when the material feels firm and doesn't 'spring' when walked on.
Q: What happens if I don't use a sub-base where one is needed?
A: Omitting a necessary sub-base will almost certainly lead to premature failure of your paving. Common issues include:
- Sinking and Unevenness: The paving will settle unevenly due to inadequate load distribution, creating dips and hollows.
- Cracking: Rigid paving (like concrete or large flags) will crack under stress if the sub-grade isn't uniformly supported.
- Rocking Slabs/Blocks: Individual paving units will become unstable and 'rock' when stepped or driven on.
- Weed Growth: Poorly prepared bases can allow easier ingress for weeds.
- Drainage Issues: Uneven settlement can create puddles and poor surface drainage.
Ultimately, this leads to costly repairs or even complete replacement of the paved area much sooner than expected.
Q: Is there ever a scenario where a sub-base is truly not needed?
A: For very light foot traffic areas, such as a decorative garden path with stepping stones, on extremely stable and well-draining ground (e.g., solid chalk or very firm, undisturbed sand), a sub-base might technically be omitted. However, even in these cases, a thin layer of compacted grit or sharp sand often serves a similar purpose, providing a uniform bedding layer. For anything more substantial, or where the ground conditions are uncertain, a sub-base is always the safer and more professional approach.
If you want to read more articles similar to Sub-Base: Essential or Optional for Your Paving?, you can visit the Automotive category.