Common Mistakes Laying a Patio (UK): Slabs, Base, Joints
Patios go wrong from the bottom up. A 4 × 3 m patio of 600 × 600 mm slabs with 10 mm joints takes 36 slabs with 10% extra, and 2.8 tonnes of MOT Type 1 sub-base at 100 mm. The paving calculator sizes your patio, and the patio materials calculator the whole job.
Mistake 1: not allowing for waste
Cut slabs at the edges and breakages need extra:
| Extra | Slabs |
|---|---|
| 5% | 34 |
| 10% | 36 |
| 15% | 38 |
Mistake 2: forgetting that slab size and joint change the count
The same patio by slab size:
| Slab | Slabs needed |
|---|---|
| 600 × 600 mm | 36 |
| 450 × 450 mm | 63 |
| 400 × 400 mm | 79 |
| 300 × 300 mm | 138 |
| 600 × 300 mm | 70 |
And by joint width, with 600 × 600 mm slabs:
| Joint | Slabs needed |
|---|---|
| 0 mm | 37 |
| 5 mm | 37 |
| 10 mm | 36 |
Mistake 3: a thin or uncompacted base
Slabs rock and sink on a poor base. MOT Type 1 sub-base for the example by depth:
| Sub-base depth | MOT Type 1 |
|---|---|
| 100 mm | 2.8 tonnes |
| 150 mm | 4.2 tonnes |
| 200 mm | 5.5 tonnes |
Mistake 4: no fall and no drainage
Set a fall away from the house, and keep the finished level below the damp proof course. Do not let water run back toward a wall.
Mistake 5: poor bedding and pointing
- Bed each slab on mortar spots or a full bed as the slab maker says, not on dabs of sand alone.
- Tap each slab to level, and check with a long level every few slabs.
- Point with a brush-in or a slurry mix once the slabs are set, and keep traffic off until it hardens.
Estimates only: check the base depth for your ground and use.
Frequently asked questions
How many slabs for a 4 × 3 m patio?
With 600 × 600 mm slabs and 10 mm joints and 10% extra, 36 slabs.
How much fall should a patio have?
A gentle fall away from the house, often about 1 in 80, so water runs off and does not pool or bridge the damp proof course.
How deep should the base be?
Often 100 mm of compacted sub-base for a patio, and more for a drive. The depth depends on the ground and the load.
Do I need planning permission?
A patio usually does not, but one over a certain height, or a front garden hard surface, may need drainage or permission. Check locally.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.