How to lay a patio
A patio is four layers, and only one of them is the bit you can see. Get the excavation, sub-base and bedding right and the paving is the easy day. The single most important thing to know before you start: slabs and block paving are bedded completely differently — slabs on a full mortar bed, blocks on unbound sand — and mixing the two methods up is why so many patios end up rocking.
The build-up, bottom to top
- Subgrade — the natural ground, stripped of topsoil and compacted. Soft or waterlogged? Dig deeper and add more sub-base.
- Sub-base — at least 100 mm of compacted MOT Type 1. This is the structural layer that spreads load.
- Bedding — mortar for slabs, screeded sharp sand for blocks. Details below; they are not interchangeable.
- Paving — slabs, blocks or setts, laid to a fall.
Dig to the total of all four, not to the depth of the slab. A 40 mm slab on 50 mm of bedding over 100 mm of sub-base means excavating 190 mmbelow the finished level — and the finished level must sit at least 150 mm below the damp-proof course of the house.
Slabs: a full mortar bed
Flags and slabs need continuous support. Bed them on20–50 mm of mortar, mixed by volume asPavingexpert publishes it:
| Grit sand : cement | Known as | Use for |
|---|---|---|
| 10 : 1 | DIYer's mix | light-use patios and paths |
| 6 : 1 | Contractor's mix | medium duty, or wet ground where the bed could migrate |
| 3 : 1 | Heavy duty | vehicular loading |
Use a full bed, not dabs. Spot bedding — five dollops of mortar under each corner and the middle, also called dot-and-dab — leaves voids that fill with water and crack the unsupported slab under load.Pavingexpert reports that BS 7533-4 requires flags to be laid on a full bedding layer, and that this applies to all classes of pavement — patios and driveways included, not just commercial schemes. Once the mortar is mixed it is no extra work. (We cite Pavingexpert's summary rather than the standard itself: BS 7533 is a paid document and we have not read it first-hand.)
Block paving: screeded sand, no cement
Block paving works the opposite way. The laying course is25–40 mm of screeded sharp (grit) sand, left completely unbound — no cement, ever. The blocks are laid on the loose screed, then vibrated in with a plate compactor, which beds them and locks them together. Cement in the laying course stops the pavement flexing and re-seating, which is the whole point of the design. Joints are then filled by sweeping in kiln-dried sand and vibrating again. Ourblock paving calculator counts the blocks, bedding sand and jointing sand together.
Fall and drainage
Set a fall of about 1:80 — roughly 12 mm per metre — running away from the building, so rain sheds instead of pooling or tracking back toward the damp course. Where drainage is poor, or the patio sits under a downpipe, go to 1:60. Establish the fall with a level and a shim on your straight edge before laying, and check it as you go; trying to introduce a fall halfway through is how patios end up with a dip in the middle.
Worked example: a 5 m × 4 m patio
- Area: 20 m²
- Sub-base at 100 mm compacted: 20 × 0.1 = 2 m³ → about 4.2 tonnes of MOT Type 1 at2.1 t/m³
- Bedding at 50 mm: 20 × 0.05 = 1 m³ → about 1.8 tonnes of sharp sand at1.8 t/m³ (plus cement at your chosen ratio for slabs)
- Excavation: roughly 190 mm deep — about3.8 m³ of spoil to move, which is more than most people expect and usually the real cost of a skip
Run your own numbers with theMOT Type 1 calculator for the sub-base and the aggregate calculator for any other layer — both take metres and millimetres and return tonnes and bulk bags.
The mistakes that cost a weekend
- No sub-base, or too thin. The paving is not structural. 100 mm compacted, minimum.
- Compacting in one thick lift. Sub-base consolidates in layers; a single deep spread stays loose underneath.
- Spot bedding slabs. Covered above — it cracks them.
- Cement in a block-paving laying course. Turns a flexible pavement into a rigid one that cannot re-seat.
- Forgetting the DPC. Finished level at least 150 mm below the damp-proof course, or you channel water into the wall.
- No fall. Flat patios puddle, and in winter those puddles freeze.
Frequently asked questions
How do you lay a patio, step by step?
Mark out and excavate to the full build-up depth, compact the subgrade, lay and compact a sub-base of at least 100 mm of MOT Type 1, then bed the paving. Slabs go on a full mortar bed of 20–50 mm; block paving goes on a screeded, unbound sharp sand bed of 25–40 mm. Set a fall of about 1:80 away from the house, joint the paving, and let mortar cure before loading it.
What goes under a patio?
From the bottom: firm compacted subgrade, then a compacted sub-base of MOT Type 1 — a minimum of 100 mm for a patio on good ground — then the bedding layer, then the paving. The sub-base is the structural part; a beautiful slab on a poor base still rocks and sinks. Skipping it is the single most common reason patios fail.
Can you lay a patio without cement?
For block paving, yes — that is the correct method: blocks are laid on a screeded, unbound sharp sand bed and vibrated in, with no cement anywhere in the laying course. For flags and slabs, no: they need a full mortar bed to spread load and stop them rocking. Laying slabs loose on sand is a common shortcut and it is why so many DIY patios end up uneven within a season.
What mortar mix should I use for patio slabs?
Pavingexpert publishes three by volume: 10 parts grit sand to 1 part cement as the DIYer’s mix for light-use patios, 6:1 as the contractor’s mix for medium duty or wet ground where the bed might migrate, and 3:1 where vehicles will drive on it. Use a full bed rather than dabs of mortar under the corners.
Should I use spot bedding or a full mortar bed?
A full bed. Spot bedding — five dollops of mortar under each slab, also called dot-and-dab — leaves voids that collect water, and the unsupported middle of the slab cracks under load. Pavingexpert reports that BS 7533-4 requires flags to be laid on a full bedding layer, and that this applies to all classes of pavement including patios and driveways, not just commercial work. It is no more work once the mortar is mixed.
How much fall does a patio need?
About 1:80 — roughly 12 mm of drop per metre — running away from the building, so surface water sheds rather than pooling or reaching the damp-proof course. Use 1:60 where drainage is poor or the patio sits below a downpipe. Set the fall with a level and a shim on the straight edge before you start laying, not afterwards.
How thick should the sub-base be under a patio?
At least 100 mm of compacted MOT Type 1 for a patio on good ground, and Pavingexpert notes this can be reduced to 75 mm only for paths or patios in exceptional circumstances on genuinely firm ground. At the UK trade density of 2.1 tonnes per cubic metre, a 20 m² patio at 100 mm needs about 4.2 tonnes.
About these numbers
The bedding mixes, bed depths and the full-bedding requirement come from Pavingexpert's flag-laying and block-paving guidance; the MOT Type 1 and sharp sand densities are the same cited figures our calculators compute with, quoted verbatim on thematerial density reference and rendered onto this page from that data so the two cannot disagree — readhow CubicMath works. Depths and falls are standard UK practice rather than a single specification; your project drawing or installer outranks them. Laying turf alongside the patio? Theturf calculator sizes that.