How to Calculate Patio Sub Base Requirements

20th Aug 2024 -

If you want to build a strong and long-lasting patio for your customer then it's important to build a strong patio sub base. For this feature, we guide you through accurately determining how much sub base for patio projects you need.

We also explore depth requirements, pro tips for laying patio sub base and common mistakes to avoid.

• How Deep Should Patio Sub Base Be?
• How Much Sub Base Do You Need for a Patio?
• Pro Tips on How to Lay a Patio Sub Base
• 9 Common Patio Sub Base Building Mistakes to Avoid
• Boost Your Patio Projects with High-Quality Patio Sub Base

How Deep Should Patio Sub Base Be?

Patio sub base depth ensures the finished surface is stable. Below is an overview of the typical sub base depths depending on the material.

Sub Base Material	Typical Depth
Crushed Stone/MOT Type 1	100-150mm
Recycled Aggregate	100-150mm
Sand and Gravel Mix	75-100mm
Cement-Treatment Base	75-100mm
Permeable Sub Base	150-200mm+

While there is no one-size-fits-all answer, several key factors influence the depth of sub base for patio areas.

• Soil type: The nature of the underlying soil greatly impacts sub base depth. Clay soils are prone to movement and typically require a deeper sub base than sandy or well-draining soils. For clay soils, you may need up to 200mm of sub base. Sandy soils may only require 100mm.
• Expected load: Consider the intended use of the patio. Areas that will bear heavy loads require a deeper sub base than those designed for light foot traffic.
• Paving material: The type and weight of the paving material influence sub base depth. Heavier materials like natural stone paving slabs generally need a deeper sub base than lighter options like concrete pavers.
• Climate: In locations with freeze-thaw cycles, a deeper sub base helps prevent frost heave. Areas with milder climates may require less depth.
• Drainage requirements: Locations with poor natural drainage may benefit from a deeper sub base to improve water management.

A general rule of thumb for calculating sub base depth is the following:
Total excavation depth = paving thickness + bedding layer + sub base depth. 

For example, 50mm thick pavers with a 30mm bedding layer and 150mm sub base need an excavation depth of 230mm.

How Much Sub Base Do You Need for a Patio?

Proper calculation ensures you have sufficient material to complete the job while avoiding inconvenient shortages. This section guides you through how much MOT Type 1 or other sub base you need for your patio. We take into account factors such as patio size, required depth and material characteristics. 

Step-by-step calculation - find out how much sub base you need

• Determine the patio area: Area = Length x Width
• Calculate the volume of sub base: Volume = Area x Sub base depth
• Apply the material compression factor: Sub base materials typically compress by about 25-30% when compacted. To account for this, multiply your calculated volume by 1.3.
• Add a wastage allowance: Add an extra 5-10% to account for wastage and ensure you have enough material.
• Convert volume to the appropriate unit: Sub base is often sold by the tonne. The conversion from cubic metres to tonnes varies depending on the material. However, for MOT Type 1, it is approximately 1.6 tonnes per cubic metre.

Formula: Total sub base needed (tonnes) = (Area x Depth x 1.3 x 1.1) x 1.6.

The below outlines how much type 1 per square metre is needed for a patio measuring 4m x 3m. This example also has a sub base depth of 100mm.

• Patio area: 4m x 3m = 12 square metres (sq m)
• Volume of sub base: 12 sq m x 0.1m depth = 1.2 cubic metres
• Apply compression factor: 1.2 x 1.3 = 1.56 cubic metres
• Add 10% for wastage: 1.56 x 1.1 = 1.716 cubic metres
• Convert to tonnes: 1.716 x 1.6 = 2.75 tonnes

For this standard 4m x 3m patio with a 100mm sub base, you need approximately 2.75 tonnes of sub base material.

Pro Tips on How to Lay a Patio Sub Base

Several advanced techniques go beyond standard practices. These methods boost performance, durability and efficiency, especially in challenging conditions. Here are the advanced techniques that set your patio installations apart. 

Geogrid reinforcement

Incorporating geogrid layers within the sub base material can increase its load-bearing capacity. Install a layer of bi-axial geogrid every 150-200mm within the sub base. Use appropriate aperture size (geogrid mesh openings) and tensile strength.

It reduces the overall sub base thickness required and minimises differential settlement. This is particularly useful in areas with poor soil conditions or for patios designed to support heavy loads. 

Hydraulic binding

Mixing a hydraulic binder with the sub base material before compaction improves its strength and stability. Add 2-4% (by weight) of hydraulic binders like blast furnace slag or cement to the sub base. Mix thoroughly before spreading and compacting. 

The process reduces the sub base's susceptibility to moisture and improves long-term performance. A mechanical mixer and moisture meter help achieve the right blend. It is beneficial in high-traffic spaces or areas with challenging weather conditions.

Intelligent compaction systems

Compaction equipment fitted with GPS and accelerometer technology offers real-time data on compaction levels across the sub base area. They ensure uniform compaction, identify weak spots and provide documentation of compaction quality for assurance purposes.

GPS-enabled rollers or plate compactors, along with data analysis software, are required for this technique.

Capillary break layer

Introducing a capillary break layer beneath the main sub base may prevent upward moisture migration. Install a 50-100mm layer of clean gravel (typically 20-40mm in size) beneath the main sub base material. 

Overall, this reduces frost heave risk and improves long-term stability. It is ideal for areas with high water tables (underground level) or frost-susceptible soils. A laser level is essential for the precise grading of this layer.

Pore water pressure relief system

Installing a network of horizontal drainage pipes within the sub base can rapidly use up excess pore water pressure. Place perforated pipes in a herringbone pattern within the lower portion of the sub base, connected to a drainage outlet. 

This system enhances stability and reduces the risk of sub base failure during extreme weather events. It is particularly useful for large patio areas or in regions prone to heavy rainfall. A trenching shovel and laser level are necessary for pipe installation. 

Thermal insulation layer

Adding a layer of extruded polystyrene (XPS) insulation within the sub base structure may reduce frost penetration depth. Install a 50-100mm layer of high-density, moisture-resistant XPS boards above the main sub base but below the bedding layer. 

This technique minimises frost heave risk and improves energy efficiency for heated outdoor spaces. It is beneficial in cold climates or for patios adjacent to heated structures. Insulation-cutting tools and protective gear for handling materials are essential.

Electro-osmosis treatment

Applying electro-osmosis (electric current to reduce moisture) to clay soils beneath the sub base can improve their bearing capacity. Install a series of anodes and cathodes in the soil beneath the sub base area and apply a low DC voltage to induce water movement from anodes to cathodes. 

Electro-osmosis treatment dehydrates and stabilises clay soils, reducing moisture-related volume changes and speeding up consolidation. It is effective for patios with expansive clay soils. 

An electro-osmosis system and monitoring equipment (moisture and voltage sensors) are needed for this technique.

9 Common Patio Sub Base Building Mistakes to Avoid

Various pitfalls can compromise the stability and longevity of a patio. Below, we outline the frequent errors made during sub base preparation and installation, along with solutions to address each issue.

1. Inadequate soil preparation

Failing to properly assess and prepare the existing soil before laying the sub base.

Solution: Conduct a thorough soil assessment before beginning. For clay soils, consider adding a layer of hardcore sub base or MOT Type 1 to improve drainage. Ensure proper compaction for sandy soils.

2. Incorrect sub base depth

Adopting a one-size-fits-all approach to sub base depth without considering specific site conditions.

Solution: Carefully think about how much hardcore you need and calculate the required sub base depth. For heavy-duty applications or poor soil conditions, lay a deeper sub base. Use a string line and measure frequently during installation to ensure consistent depth across the whole area.

3. Poor compaction of the sub base material

Inadequate compaction of the sub base material, leading to settling and instability.

Solution: Compact the sub base in layers no thicker than 75-100mm at a time. Use a plate compactor and make multiple passes in different directions. For larger areas, consider using a roller compactor. Check the compaction of each layer before adding the next. The surface should feel firm and not shift under foot traffic.

4. Ignoring drainage requirements

Not accounting for proper drainage leads to water accumulation and potential frost heave.

Solution: Ensure the sub base has a slight slope (typically 1:60) away from buildings. With poor drainage areas, install a perforated drainage pipe at the lowest point of the slope. Use open-graded materials in the lower sub base layers to facilitate water movement.

5. Neglecting edge restraints

Failing to install proper edge restraints causes spreading and instability of the sub base and paving.

Solution: Install robust edge restraints around the entire perimeter of the patio before laying the sub base. Use concrete haunching (extra concrete to reinforce edges), plastic edging systems or suitable landscaping blocks. 

Ensure the edge restraints are securely anchored and level with the finished paving height.

6. Overwatering during compaction

Adding too much water to the sub base material during compaction leads to a soft, unstable base.

Solution: While some moisture may help compaction, avoid overwatering. The material should be damp but not saturated. If the sub base becomes too wet, allow it to dry before continuing compaction. In wet conditions, consider a sub base with a lower fine content.

7. Rushing the process

Attempting to complete the sub base installation too quickly, leading to poor compaction and uneven settling.

Solution: Allow enough time for each stage of the process. Properly compact each layer, allowing time for any moisture to evaporate if necessary. If possible, let the completed sub base settle for a few days before laying the bedding layer and paving.

8. Not using a geotextile fabric

Failing to use a geotextile fabric in areas with poor soil conditions mixes the sub base with the underlying soil.

Solution: Lay a geotextile fabric before adding the sub base material. This is key in areas with clay soils or spots with a risk of soil mixing with the sub base. It helps maintain the integrity of the sub base and prevents sinking or unevenness over time.

9.  Don’t ignore frost susceptibility

Ignoring frost heave in colder climates will cause cracking and unevenness.

Solution: Use frost-resistant sub base materials in areas prone to freezing and increase the sub base depth. Consider adding a layer of insulation or using a capillary break layer to prevent moisture from rising.