How Deep Should a Patio Sub-Base Be?
If you have ever seen a patio with sunken, wobbly or cracked slabs, the problem almost always lay beneath them. The sub-base is the hidden layer that carries the load, and its depth is the first thing to get right. Here is how deep a patio sub-base should be, what changes the answer and how it fits together with the other layers.
The short answer
For a typical domestic patio on firm ground, a compacted sub-base of around 100mm of MOT Type 1 is the usual recommendation. Go deeper in these cases:
- Soft, clay or made-up ground. More stone spreads the load.
- Heavy loads such as a hot tub, a summerhouse or a heavy outdoor kitchen.
- Occasional vehicle use, where 150mm is more common. See our driveway sub-base guide.
Some paving manufacturers specify a depth for their products, especially porcelain. Always follow the manufacturer's laying instructions where they differ from general advice.
All the layers: how deep do you dig?
A patio is built up from several layers, and you need to dig out enough for all of them:
| Layer | Typical depth |
|---|---|
| Paving slab or porcelain | Usually 20 to 50mm, depending on the product |
| Bedding (sand or mortar) | Commonly roughly 30 to 50mm |
| MOT Type 1 sub-base | Around 100mm |
| Total excavation | Often around 150 to 200mm |
Add a little more if you are laying a membrane. Then work backwards from the finished level you want. The finished patio should sit below any damp-proof course on the house, usually at least 150mm, and it should fall gently away from the building so water runs off.
Why layers matter
- Thickness spreads the load. A deeper layer of well-compacted stone spreads weight over a wider area of soil.
- Compaction does the work. A thick layer of loosely placed stone is not a sub-base. Build up in layers and compact each one with a plate compactor, following the machine's guidance on layer thickness. If your total is more than about 100mm, lay it in two layers.
- Drainage. A firm, well-graded base that sheds water protects against heave and frost damage.
Checking the depth as you dig
- Set string lines or pegs at the finished level and slope.
- Measure down from the string at several points as you dig.
- Remove topsoil, roots and any soft spots, and fill them with compacted stone.
- Compact the exposed ground before adding stone.
- Measure again after each layer is compacted, since stone settles.
What if I cannot dig that deep?
Sometimes there is a limit, for example where the patio abuts a house or where there are pipes or tree roots. If you cannot dig the full depth, do not simply skip the sub-base. Talk to your installer about alternatives, such as a thinner base on compacted ground suited to the paving, or a raised patio with a retained edge. Reducing the sub-base depth should not be the first compromise.
How much MOT Type 1 does that mean?
A 100mm depth over a 4m × 3m patio is 1.2m³ finished, or about 2.55 tonnes with a 25% allowance for compaction, which is 4 bulk bags. See how much MOT Type 1 you need for a patio for the full working. We stock MOT Type 1 sub-base bulk bag (800kg) and a hardcore / crusher run bulk bag for heavy-duty lower layers. Our MOT Type 1 vs Type 3 guide explains the options.
Frequently asked questions
Is 100mm of MOT Type 1 enough for a patio?
For normal foot traffic on firm ground it is a typical depth. For soft ground, heavy loads or vehicle use you will want more.
Do I need a sub-base for porcelain paving?
Usually yes, and porcelain has specific laying requirements. Check the manufacturer's instructions, since many specify the sub-base and bedding.
Can I lay slabs straight onto sand?
Not on a bare soil base. Sand alone does not stop settlement. A compacted sub-base under the bedding keeps the patio stable.
How much should a patio slope?
A gentle fall away from the house is usual, commonly quoted at around 1 in 80. Check with your installer or paving manufacturer for your project.
Should I compact in one go or layers?
In layers. Build up the sub-base, then compact each layer before adding the next.
Ordering and delivery
You can browse the full range in our cement and aggregates collection. Bulk bags are sold at approximately 800kg each (the packed weight can be up to 850kg depending on moisture), and most sands and gravels are also available in 20 to 25kg trade packs for small jobs. Prices on our website are shown excluding VAT, and VAT is added at checkout.
We deliver to addresses in mainland England only; we do not currently deliver to Wales, Scotland, Northern Ireland or the islands. Timber, aggregates and cement have a minimum order of £250 ex VAT, and delivery is £25 ex VAT on orders under £500 ex VAT and free over £500 ex VAT. Bulk bags are delivered by Moffett lorry (truck-mounted forklift) and offloaded at your site, so someone must be there to receive them and you will need a firm, level area with suitable access. We do not offer next-day delivery. Our shipping policy has the full details, or call us on 03330 908479 or email sales@insulationwarehousedirect.co.uk if you would like a hand working out your order.
The quantities in this guide are estimates. Ground levels, how well the material is compacted and how wet it is all change what you need, and densities vary between quarries, so check the amount with your installer or the manufacturer's guidance before you order. Prices and pack sizes are correct at the time of writing.
Working out quantities? See our MOT Type 1 sub-base guide: how much do I need? or go straight to our aggregates calculator.