Patio bedding layer: how much sand and cement?
Last reviewed 1 July 2026
Short answer
A full mortar bed for slabs is typically 30–50 mm. At 40 mm over 20 m² that is 0.8 m³ of mortar, which needs about 1.28 tonnes of sharp sand and 256 kg of cement at a 1:5 mix — roughly 11 bags of 25 kg. Block paving uses a 30–40 mm sharp sand screed instead, with no cement.
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Full mortar bed or sand screed?
| Full mortar bed | Sharp sand screed | |
|---|---|---|
| Used for | Slabs, natural stone, porcelain | Block paving, some permeable systems |
| Typical depth | 30–50 mm | 30–40 mm |
| Materials | Sharp sand and cement, usually 1:5 | Sharp sand only |
| Edge restraint | Recommended | Essential |
| Working time | Limited — mix what you can lay | Unlimited |
| Tolerance to movement | Rigid; cracks if the base moves | Flexible; re-levellable |
Converting a mortar bed into materials
Start with the volume: area × bedding depth. A 20 m² patio at 40 mm is 0.8 m³ of mortar.
For a 1:5 cement to sharp sand mix, allow 1.6 tonnes of sand and 320 kg of cement per cubic metre of mortar. The sand figure includes an allowance for bulking, since damp sand occupies more volume than its dry weight suggests.
So 0.8 m³ needs 1.28 tonnes of sand — two UK bulk bags — and 256 kg of cement, which is 11 bags of 25 kg.
sand (t) = mortar volume × 1.6 cement (kg) = mortar volume × 320
Bedding quantities for common patios
| Patio area | Mortar volume | Sharp sand | Cement (25 kg bags) |
|---|---|---|---|
| 10 m² | 0.40 m³ | 0.64 t | 6 |
| 20 m² | 0.80 m³ | 1.28 t | 11 |
| 30 m² | 1.20 m³ | 1.92 t | 16 |
| 50 m² | 2.00 m³ | 3.20 t | 26 |
Getting the bed right
- Lay on a full bed, not on dabs. Five spots of mortar under each corner and the middle leaves voids that fill with water, freeze and crack the slab. It is the most common cause of a rocking, cracked patio.
- Keep the mix fairly dry. A wet, sloppy bed will not support the slab as you tap it down. It should hold its shape when squeezed.
- Mix only what you can lay. Mortar that has started to go off will not bond, and re-tempering it with more water ruins the strength.
- Prime the underside of porous natural stone with a slurry primer. Without it the bond fails and you get picture-framing where moisture migrates through.
- Screed sand beds to a consistent depth and do not compact them before laying — the units are vibrated in afterwards.
Important: Falls before finish
Safety notice: Material planning only