Can I lay paving slabs straight onto sand?

The question is really about unit size. A block 200mm long sits on sand and is held by its neighbours; there is nowhere for it to rock to. A slab 600mm square laid on sand has enough area to bridge any small hollow, and every step on it pumps sand out from underneath. That is the whole distinction, and it explains every rule you will hear about this.

Step by step

1. Look at what is under the sand, first.Sand is a bedding course, never a foundation. Whatever the unit, it sits on a properly excavated and compacted sub-base of MOT type 1. Slabs laid on sand laid on soil will fail whatever their size.
2. Judge by unit size and load.Small units — blocks, setts, small pavers — work on a screeded sharp sand course, vibrated in, with restrained edges. Large slabs on a foot path with occasional use can work on a well-compacted sand or grit bed. Large slabs on a patio with furniture, or anything a vehicle touches, want a full mortar bed.
3. Use the right sand.Sharp sand, which is angular and drains, not soft building sand, which holds water and slumps. For paths and stepping stones, grit or a sand-and-grit mix is better still on heavy ground because it does not pump when saturated.
4. Screed the bed rather than spreading it.Rails and a straightedge to a consistent loose depth of 30 to 40mm. An unevenly spread sand bed produces slabs at different heights and voids under some of them.
5. Restrain the edges however you lay.Sand-laid paving spreads sideways under load. An edging bedded and haunched in concrete is what holds the field together, and it matters more on sand than on mortar.
6. Accept the trade-off.Sand-laid paving is quicker, cheaper, permeable, and individual units can be lifted and re-laid easily. Mortar-bedded paving is rigid, will not pump or rock, and takes furniture and heavy use. Choose which set of properties you want.
7. For a patio, bed the slabs fully.A continuous mortar bed under the whole slab, not five spots. This is the single change that separates a patio that stays flat from one that rocks — and it is worth doing even though it is more work.

Common mistakes

• Laying large slabs on sand over soil — Two problems compounding. The soil compresses unevenly and the sand pumps out under the slab. This is the classic rocking patio and it appears within one winter.
• Using building sand — Soft sand holds water, does not drain and slumps under load. It is a mortar ingredient, not a bedding material.
• Spot-bedding on five blobs of mortar — It is faster and it is the direct cause of most cracked and rocking slabs. The voids between the spots fill with water, freeze, and the unsupported middle of the slab cracks under load.
• Skipping the edge restraint — Sand-laid paving spreads. Without a haunched edge the outer units creep, joints open, sand escapes and the failure works inwards.

Alternatives

• Screeded sharp sand: The correct method for block paving and small unit paving on a compacted sub-base with restrained edges. Permeable, quick, easily lifted for repairs. Not suitable for large slabs.
• Full wet mortar bed: The standard for slab patios. A continuous bed of sharp sand and cement supporting the entire underside. Rigid, no rocking, no pumping, and the slab cannot be lifted casually afterwards.
• Grit or sand-and-grit bed: A good middle ground for informal paths, stepping stones and slabs in a lawn. Drains better than sand, does not pump on heavy ground, and stays easy to adjust.
• Dry sand-cement bed: A semi-dry mix of sharp sand and cement laid as a screed and watered after the slab is set. Often used for large-format paving. It sets to give full support, and it is more forgiving to work with than a wet bed, but it wants a primer on non-porous slabs.

Questions people ask