A few years after moving into our current house, we lifted and relaid the patio that came with it. The previous owners had put it down themselves and it had always had a problem - a noticeable dip near one corner that held water after rain and a couple of slabs that rocked when you stood on them.
When we lifted the slabs, the cause was immediately obvious. There was no meaningful sub-base. Under about 20mm of loose sharp sand was bare soil, and in the problem corner the sand was barely there at all. One frost had been enough to start the movement. The slabs themselves were fine - it was entirely a preparation problem.
Redoing it properly, with 100mm of compacted MOT Type 1 and a proper mortar bed, took a weekend. The patio has not moved since.
The complete build-up
To understand sub-base depth, you need to see where it fits in the whole system:
| Layer | Typical depth | Notes |
|---|---|---|
| Paving slabs | 35-50mm | Depends on slab type |
| Mortar or sand bed | 25-40mm | For levelling and bedding |
| Compacted MOT Type 1 | 75-150mm | The structural sub-base |
| Geotextile membrane | - | Between soil and sub-base |
| Natural ground | - | Compacted subgrade |
Total excavation depth: approximately 150-240mm below finished surface level.
The sub-base is the most important layer in this build-up. The slabs, mortar and membrane are all quite forgiving - they can be adjusted, relaid, replaced. The sub-base, once laid and compacted, defines whether the whole structure moves or holds steady.
Recommended sub-base depth by ground type
| Ground type | Recommended depth |
|---|---|
| Firm sandy or gravelly soil | 75-100mm |
| Average garden soil | 100mm |
| Clay soil | 100-150mm |
| Very soft or made ground | 150-200mm |
| Near large tree roots | 150mm + root barrier |
The clay question is the most important one in most UK gardens. Clay soil holds water, expands when wet and shrinks when dry. This seasonal movement is transmitted to the surface unless you have enough sub-base to absorb and distribute it. On clay, I would always specify 150mm of compacted MOT Type 1. The extra material costs perhaps 15-20% more than 100mm but significantly reduces the risk of slabs rocking or settling over the first few years.
Sub-base vs bedding layer: an important distinction
A common confusion worth clearing up: the sharp sand or mortar layer immediately under the slabs is the bedding layer, not the sub-base.
Bedding layer (25-40mm): Fine-grained material (sharp sand, semi-dry mortar or wet mortar) that allows you to set slabs accurately to level. It is workable and compressible. It is NOT structural.
Sub-base (75-150mm): Compacted coarse aggregate (MOT Type 1) that distributes load. This IS structural.
A patio with only a bedding layer and no sub-base is what the previous owners of our house had put down. It works for a short time and then doesn't. A patio needs both.
How much MOT Type 1 do I need for the sub-base?
Remember the compaction factor: MOT Type 1 reduces in depth by around 20-30% when properly compacted. You need to order 1.3 times the compacted volume.
Formula: Compacted volume (m3) = Area x depth (in metres) Loose volume to order = Compacted volume x 1.3 Weight (tonnes) = Loose volume x 2.1 t/m3 Add 15% wastage
Quick reference at 100mm compacted depth
| Patio area | Tonnes to order (inc. 1.3x factor + 15% wastage) |
|---|---|
| 5m2 | 1.6t |
| 10m2 | 3.2t |
| 15m2 | 4.8t |
| 20m2 | 6.4t |
| 25m2 | 8.0t |
| 30m2 | 9.6t |
At 150mm compacted depth
| Patio area | Tonnes to order |
|---|---|
| 10m2 | 4.8t |
| 20m2 | 9.6t |
| 30m2 | 14.4t |
For precise figures, use the Sub Base Calculator.
The geotextile membrane: worth doing
Below the sub-base, a non-woven geotextile membrane serves one important purpose: it stops the sub-base stone from gradually being pressed down into softer ground by vehicle weight and foot traffic over the years.
Without it, on softer soils, the sub-base slowly migrates downward. You end up with a sub-base that is thinner than you specified, and eventually a patio that starts to show movement. The membrane costs very little and is one of those things you will never regret installing.
Use a proper non-woven geotextile (150-200g/m2 specification or higher). Not a lightweight weed membrane - those are not strong enough to survive sub-base compaction.
Drainage falls: set them at sub-base stage
One of the advantages of spending time on the sub-base is that it gives you a controlled surface to set your drainage falls on before any slabs go down.
Aim for a fall of at least 1:60 (approximately 16mm per metre) away from any buildings. This is barely visible to the eye but essential for keeping water away from house walls and damp-proof courses.
If you try to introduce falls at slab level by adjusting mortar thickness, you are making the job much harder than it needs to be. Get the sub-base level right and the slabs will follow.
My tips for getting the sub-base right
After relaying enough patios to have developed strong opinions about sub-base preparation:
Do not skimp on the plate compactor hire. A plate compactor compacts MOT Type 1 to the density it needs to be. A hand tamper does not, full stop. The difference in finished stability is obvious - you can push a properly compacted sub-base with your foot and it does not move at all. The same material tamped by hand will continue to settle under load.
Compact in layers. 150mm of sub-base should go down in two 75mm layers, each compacted before the next goes on. A single 150mm layer compacted in one pass is dense at the top and loose at the bottom.
Check your levels carefully. Take time to check the sub-base level at multiple points with a long straight edge and spirit level before any mortar goes down. Low spots in the sub-base translate to low spots in the finished patio.
Add the mortar bed the same day you lay the slabs. Pre-laying the mortar bed and coming back the next day means the mortar may be too stiff to work properly. Mix and lay on the same day, keeping unused mortar covered in polythene.
Use the Sub Base Calculator and Patio Slab Calculator together to plan the full project.