Layer 1: the formation
The formation is the natural ground the patio sits on. We strip turf and topsoil, which is soft and full of organic matter, and keep digging until we reach firm, undisturbed ground. Anything that will rot or compress, such as old roots, builder's rubble, ash or soft fill, comes out.
How deep that is varies a lot. In an old, well-tended garden in Blackwell or Hummersknott, topsoil can be deep. Behind a 1930s semi in Harrowgate Hill there may be an old concrete patio and its rubble base to lift first. On a newer estate there can be compacted fill left from building.
What Darlington's ground is like
The British Geological Survey maps a real mix of ground across the town:
- Glacial till under Cockerton, Harrowgate Hill and Haughton-le-Skerne: a stiff, stony clay that drains slowly, softens in wet weather and can shrink in a dry summer.
- Glaciofluvial sand and gravel under Blackwell, washed out by meltwater from the ice. It drains well and moves far less with the seasons.
- Hummocky glacial deposits under Mowden: mounds of mixed gravel, sand and silt, so conditions can change across a single garden.
- River terrace sand and gravel under much of Hurworth, left by the Tees when it flowed at a higher level.
Beneath these lie mudstones, sandstone and the dolostone of the Magnesian Limestone, but at patio depth it is the surface deposit that matters.
Layer 2: a membrane where the ground is soft
On clay or silt we lay a geotextile membrane across the formation. Water can pass through it, but clay cannot work its way up into the stone. Without it, a sub-base on clay slowly fills with fine soil, loses strength and lets slabs dip. The BGS explains that clay soils swell when wet and shrink as they dry; keeping the stone separate from the clay is part of coping with that movement. On clean sand and gravel, as in parts of Blackwell, a membrane is less critical, but it still helps where pockets of silt turn up.
Layer 3: the sub-base
The sub-base carries the load and spreads it into the ground. We use a well-graded crushed stone such as MOT Type 1, which locks together when compacted. It goes down in layers, each compacted with a plate compactor, rather than in one thick layer that only compacts on top.
How thick? For a garden patio carrying people and furniture, a typical compacted sub-base is around 100mm to 150mm. On soft clay we go thicker, and where the dig has gone deep to remove fill, we make up the level with more compacted stone rather than soil. On firm, free-draining gravel the standard depth is usually enough.
What we avoid:
- Sand on its own as a base. It washes out and moves.
- Loose, uncompacted hardcore, which settles.
- Rounded gravel, which does not lock together.
- Laying straight onto soil, which almost guarantees sinking.
Layer 4: the bed and the slabs
Full mortar bed
We lay each slab on a full bed of mortar, so it is supported across its whole underside. "Dot and dab", five blobs of mortar under each corner and the middle, leaves voids where water collects, and slabs can crack under point loads or rock as the dots break up.
Primer on natural stone and porcelain
Porcelain has a smooth, non-absorbent back that will not bond to mortar on its own, so it needs a slurry primer. Indian sandstone also benefits from a primer, which improves the bond and helps prevent mortar staining showing through the stone.
Falls
The surface falls away from the house, typically about 1 in 60 to 1 in 80, and finishes at least 150mm below the damp-proof course, as Approved Document C expects. Our guide to patio levels and the damp-proof course explains that in detail.
Jointing
Finally, the joints are filled with mortar, a brush-in compound or a grout to suit the slab.
Cement-based mortar is hazardous to skin: the HSE warns that it can cause dermatitis and burns, which is one reason we keep wet mortar off the surface and clean down as we go.
How the ground changes the build
| Ground | Typical areas | What changes |
|---|---|---|
| Glacial till (clay) | Cockerton, Harrowgate Hill, Haughton | Deeper dig to firm clay, membrane, thicker sub-base, a planned outlet for water |
| Sand and gravel | Blackwell, much of Hurworth | Standard sub-base, run-off can often soak into borders |
| Mixed glacial deposits | Mowden | Check across the garden; treat soft pockets as clay |
Questions people ask
What is the best sub-base for a patio?
A well-graded crushed stone such as MOT Type 1, compacted in layers over firm ground, with a membrane on clay.
Is a 50mm sub-base enough for a patio?
Rarely. A garden patio normally needs around 100mm to 150mm of compacted sub-base, and more on soft clay.
Can I lay slabs straight onto soil?
No. Soil compresses and moves with moisture, so slabs sink and rock. Remove the topsoil and build a compacted stone base.
Is it better to lay slabs on sand or cement?
For a patio that lasts, a full bed of mortar over a compacted stone sub-base. Sand alone moves and washes out.
Sources: BGS Geology Viewer; BGS: swelling and shrinking soils; GOV.UK: Approved Document C, resistance to moisture; HSE: cement, health risks in construction.