Slabs should be bedded on a full mortar bed 30 to 40mm thick, laid at a 1:4 cement to sharp sand ratio, over 100mm of compacted sub-base. The alternative — five dabs of mortar under each corner and the middle — is how most patios that fail were laid.
The extra mortar costs perhaps £60 on a 20m² patio. The difference in service life is measured in decades.
Planning: levels and falls
Start at the house. The finished patio surface must sit at least 150mm below the damp proof course, and it must fall away from the building at a minimum of 1:80 — that’s 25mm over 2 metres.
Work backwards from that to establish your excavation depth. For a 40mm slab on a 40mm bed over 100mm of sub-base, the total build-up is 180mm, so you dig 180mm below your intended finished level.
Set up a datum line before touching a spade. A string line at finished patio level, pegged at the house end and falling away, gives you something to measure down from at every stage.
What you need
| Item | For an 8 × 6 ft base |
|---|---|
| Concrete | 0.43 m³ (37 bags at 25 kg) |
| MOT Type 1 | 0.91 t (one bulk bag) |
| Formwork timber | 9m of 25 × 150mm |
| Pegs | 16 × 450mm |
| DPM | 6 m² |
| Plate compactor | 1 day hire |
| Mixer | 1 day hire |
Excavation
Dig the full area to the depth you calculated, extending 150mm beyond the finished patio edge on all sides so the sub-base supports the perimeter slabs properly. A sub-base that stops at the slab edge leaves the outer row unsupported, and that’s where patios start to fail.
Remove all topsoil and vegetation. Organic material rots and settles. Dig down to firm subsoil even where that means going deeper than planned, and if you hit a soft area, dig it out and backfill with compacted sub-base material.
Sub-base
Lay 100mm of MOT Type 1 for a foot-traffic patio, more if vehicles will ever cross it. Spread it in two layers of 50mm and compact each with a plate compactor — four to six passes in overlapping runs.
A plate compactor only densifies the top 75 to 100mm, so a single 100mm layer compacted once leaves the bottom half loose. Two thinner layers is the difference between a base that holds and one that settles.
Test by walking on it. No give, no footprints, no movement when you rock your weight. Dampen the material lightly if it’s bone dry — the fines need a little moisture to migrate into the voids.
The mortar mix
Use 1 part cement to 4 parts sharp sand. Sharp sand, not building sand — the angular grains lock together and resist settlement, where soft building sand stays plastic and lets slabs sink.
Mix it to a firm, damp consistency: it should hold a shape when squeezed but not be sloppy. Wet mortar squeezes out under the slab and gives you no bedding depth. A useful test is that a trowel should stand up in it briefly before toppling.
For 20m² at 40mm bed thickness you need roughly 0.8m³ of mortar — about 1.3 tonnes of sharp sand and 12 bags of 25 kg cement. The sand and cement calculator works this out for your area and bed depth.
| Patio area | Bed at 40mm | Sharp sand | Cement, 25 kg bags |
|---|---|---|---|
| 10 m² | 0.40 m³ | 0.65 t | 6 |
| 15 m² | 0.60 m³ | 0.96 t | 9 |
| 20 m² | 0.80 m³ | 1.28 t | 12 |
| 30 m² | 1.20 m³ | 1.92 t | 18 |
| 40 m² | 1.60 m³ | 2.56 t | 24 |
Build-up depths at a glance
| Layer | Depth | Notes |
|---|---|---|
| Slab | 20–50mm | Depends on material |
| Mortar bed | 30–40mm | Full bed, compressed |
| Sub-base | 100mm | 150mm if vehicles cross |
| Total excavation | 150–190mm | Below finished level |
Check that figure against the damp proof course before you dig. The finished patio must sit at least 150mm below the DPC, and if your build-up puts it higher you need to lower the whole patio rather than thin the sub-base.
Priming the slabs
Porous slabs — most riven sandstone and limestone — should be primed on the underside with a cement slurry before bedding. A thin coat of neat cement mixed with water and a bonding additive, painted on with a brush.
Without it, the dry slab draws water out of the mortar too quickly, the bond never develops properly, and you get picture framing: dark staining round the slab edges where moisture tracks up.
Dense concrete slabs don’t strictly need priming, though it does no harm.
Laying
Work from the house outwards, and from a straight datum edge. Lay a bed of mortar slightly larger than the slab and about 45mm deep — you’ll compress it to 40mm.
Place the slab, then tap it down with a rubber mallet, working from the centre outwards, checking level and fall constantly with a spirit level and a straight edge across to neighbouring slabs.
The mortar should compress fully under the whole slab with no voids. If you lift a slab to check and see gaps, add mortar and try again.
Keep joints consistent — 8 to 12mm is standard. Use spacers if you want them uniform, though on riven slabs with irregular edges a slight variation looks more natural.
Don’t walk on newly laid slabs for at least 24 hours. If you must cross the area, lay a scaffold board across several slabs to spread the load.
Cutting
Measure and mark on the face, then cut with an angle grinder and diamond blade, or a bench saw for a straighter cut. Wet cutting suppresses the dust, which matters — stone dust contains crystalline silica and a mask alone is not adequate protection for dry cutting.
Score both faces before cutting through on thick slabs, and support the offcut so it doesn’t snap and chip the edge as you finish.
Pointing and curing
Leave the bedding to set for at least 24 hours before pointing, so you’re not disturbing slabs that haven’t gone off.
Point with a 1:3 or 1:4 cement to sharp sand mix, or use a brush-in jointing compound. Work it well into the joints with a pointing bar so there are no voids, and finish slightly recessed so water runs off rather than sitting on the joint.
Clean mortar off the slab faces as you go. Cement staining on stone is very difficult to remove once it has set, particularly on light-coloured sandstone.
Keep foot traffic off for 48 hours, and don’t pressure-wash for at least a month.
Common questions
Can I lay patio slabs on sand?
Only for small, informal areas with no vehicle access. Sand bedding lets slabs move and sink over time, and it washes out at the edges. For anything you want to last, use a full mortar bed.
How thick should the mortar bed be?
30 to 40mm compressed. Thinner and there’s no room to adjust levels; much thicker and the bed becomes unstable and expensive.
What is the correct fall for a patio?
A minimum of 1:80 away from the house, with 1:60 preferred. That’s 25 to 33mm over 2 metres. Never fall towards the building.
Why do I need to prime the slabs?
Porous slabs suck water out of the mortar before it can bond, leading to weak adhesion and picture-frame staining. A cement slurry primer on the underside prevents both.
How long before I can walk on a new patio?
24 hours before light traffic, 48 hours after pointing before normal use. Avoid heavy loads or furniture for about a week while the mortar gains strength.