Last Modified: February 27, 2024

Sub-Base Depth for Patio Installation


A paa template

By Ben Parrot - Landscape Gardener & Amateur Geologist


Welcome to our latest part of our "People Also Ask About" series, where we delve into common queries and provide comprehensive answers to help you renovate your outdoor space and save you time searching online. In this edition, we're focusing on aspects of preparing a sub-base for patios, although the principles apply to most landscaping features with a foundation. Our aim is to provide answers as fully but as brief as possible. With a professional landscaper's perspective, you will get practical insights and expert advice so that your search ends with this, the first article you find, saving you time and hassle. Please check for your specific question in the quick links menu. Let's take your patio to the next level!

Garden gnome levelling subbase

Key Takeaways

  • Preparation: Get the steps for effective ground levelling which is the first step towards a solid patio.
  • Depth: Learn about the optimal sub-base depth for a patio's longevity and stability.
  • Compaction: Find out about proper compaction techniques for constructing a firm foundation.
  • Materials: Get tips about choosing the right type of sub-base material.
Gravel path over an excavated area

Understanding Sub-Base for Patios

One of the key elements in the successful construction of a patio is the establishment of a well compacted, level and free draining sub-base. In this section we tackle many of their common questions around sub-base installation for a patio. We also have an article called Patio Gravel Ideas which may also be helpful to you.

How do you level ground for a sub-base?

Properly preparing the ground for a sub-base is a vital part of the process when constructing a stable foundation. Here is an outline of the steps:

  1. 1
    Clear the area by removing vegetation, debris, and large stones.
  2. 2
    Calculate the required thickness of the sub-base and take into account any additional layers.
  3. 3
    Excavate the site to the necessary depth by hand or digger.
  4. 4
    Use a spirit level or laser level to check the level of the area.
  5. 5
    Fill any depressions with sub-base material to achieve a precise level.
  6. 6
    Remove any excess from elevated sections.
  7. 7
    Lightly compact the soil before adding the sub-base material.

This preparation plays a crucial part in the longevity and stability of the project, whether it be for patios, driveways or pathways. If you are planning to build a sub-base, check out our primer: What is Sub Base?

Preparing a garden for patio installtion

How do you compact a sub-base for a patio?

You compact a sub-base using a plate compactor or heavy roller. Compacting the sub-base materials is critical step to ensure its durability and stability. To ensure proper compaction of the subbase material, start from one corner and work your way across in a systematic pattern, overlapping each pass. Compact the material in layers not exceeding a depth of 100mm (4 inches), moistening it each time for best results. This helps prevent settling and shifting of the patio topping over time, by providing a firm and level foundation.

How thick should a paving sub-base be?

Your paving sub-base should be a minimum depth of 100mm if you are building patio, driveway or pathway. This depth ensures a robust foundation capable of withstanding various weather conditions and usage patterns. In regions where the soil freezes below more than 150mm (6 inches), it is necessary to build foundations that are 300mm (12 inches) below the ground level. This may result in the requirement of a thicker layer of subgrade and topping materials. Check out our article What is Sub Base.

A thinner sub-base may suffice for light patios, which are generally less frequented areas; however, installing to the 100mm standard is advisable. We cover some of the issues in Laying Gravel Tips.

Modern patio with wooden furniture. Jpg

Image by Anja from Pixabay

Can you lay slabs on sub-base?

Yes, you can lay paving slabs on the sub-base, although it is necessary to excavate to a depth that accommodates a 100mm sub-base, a 50mm mortar layer and the depths of the slabs. If you are installing a patio next to a building, the surface level of the slabs must be at least 150mm below the property's damp-proof course to prevent moisture ingress.

Is 50mm subbase enough?

No, for most standard sub-base installations using bound or unbound materials, a minimum depth of 100mm is recommended. For areas subjected to traffic, such as public paths or highways, a greater depth is required. A sub-base depth of 50mm may suffice when using bitumen-bound materials, although it is not recommended. 

Poor installation of sub base leading to uneven surface

Foundation Materials

How thick should sand be under pavers?

Your sand should be at a depth of 25mm (1 inch) under the pavers to secure them effectively and give you the flexibility to make incremental adjustments. The sand acts as a cushion and levelling medium, so it's important to account for the sand that naturally settles into the base and joints of the pavers. Therefore, it's advisable to purchase a little more than you think you need, especially for larger paving areas.

Can you use sharp sand as a sub-base?

Yes, you can use sharp sand as a sub-base layer for patios at a recommended thickness of 30 - 50mm for optimal depth. To get the best result from your patio, starting with a compacted 100mm layer of MOT Type 1 sub-base beneath the sharp sand is advisable. This prevents the sand from dispersing over time while improving drainage.

Concrete sharp sand close up

Should I put membrane under sub-base?

No, you don't need to place a membrane between the bedding layer and the sub-base. An industry-standard sub-base material typically possesses a tight or close finish, effectively preventing the upward migration of fines or weeds without the need for a separate membrane. However, a woven heavy duty weed membrane is recommended and usually enough for standard patios, as drainage underneath is not required. The membrane prevents the sub-base from mixing with the ground, avoiding any issues with the patio slabs. Read more about it in Membrane Plastic Under Gravel.

Can I use gravel as a sub-base?

Yes, you can use gravel as a sub-base material, particularly larger sizes ranging from 20mm gravel to 40mm crushed rock. Due to gravel's superior load-bearing qualities, it is highly suitable for use as a patio sub-base to improve the stability and drainage of your foundation.

What is the best sub-base for pavers?

The best sub-base for pavers is an open-graded base built with 6mm - 40mm (1/4" to 10") crushed aggregates or stone. This type of sub-base material is highly permeable, allowing water to drain efficiently. This reduces the risk of hydrostatic pressure build-up, which can lead to significant problems with the pavers.

Need to work out how much gravel your project needs? Try our free gravel calculator.

Landscaped garden with paving stone patio image by jesse bridgewater from

Image by Jesse Bridgewater from Pixabay


In conclusion, understanding and implementing the correct sub-base for your patio is key to a long-lasting and attractive finish. By selecting appropriate materials, ensuring thorough compaction, and correctly preparing the ground, you can build a stable and long-lasting outdoor living space. Remember, a well-prepared sub-base is the cornerstone of any successful foundation.

We have every type of gravel available to choose from in the UK. Check them out!

About the author 

Ben Parrot is a landscape gardener and amateur geologist. He is the founder of Outdoor Aggregates, with the mission to create the first encyclopaedia for decorative aggregates. From natural stone through to decorative bark, these aggregates are critical to garden and landscape design, universally.

The problem: There isn't any place where all the different types of decorative landscaping materials and construction aggregates can be found, nor are the specifications of each type readily available. So, enriched by his extensive experience, and inspired by the timeless elegance and raw beauty of stone and rock, etched by nature's artistry over millennia, Outdoor Aggregates was born.