What is under a Middlesbrough garden?
The British Geological Survey's maps show two main kinds of clay across the town, both left behind by the last ice age.
- Glacial till. Across Marton, Coulby Newham, Hemlington and Nunthorpe the BGS maps Devensian till, a dense mix of clay, sand and stones dumped by glaciers. It is firm when dry, sticky when wet and slow to drain.
- Lake clays. Under Linthorpe and parts of Acklam the BGS records glaciolacustrine deposits, fine clay and silt that settled in a lake as the ice melted. Pockets are sandier around the Acklam Hall grounds, but clay is never far away.
Below both lies mudstone, Mercia Mudstone under most of the town and Redcar Mudstone towards Nunthorpe and Great Ayton, but at patio depth it is the clay that matters. On estates built from the 1960s to the 1980s there is often a layer of builder's fill, broken brick and old hardcore between the turf and the clay as well.
Why clay causes patio problems
Clay holds water. After a wet spell, a lawn on till can stay soft for days, and if a patio is built so that its stone base sits directly on that clay, two things happen:
- The base pumps into the clay. Each time the ground is saturated and someone walks across, fine clay works up into the stone and the stone works down into the clay. Over a few winters the base loses its strength and slabs start to dip and rock.
- The clay moves. The BGS explains that clay soils swell when wet and shrink as they dry, the "shrink–swell" behaviour that is most noticeable in the top two metres. A patio laid on a thin base can rise slightly in winter and drop in a dry summer, which cracks mortar joints. Trees and large shrubs make it worse by drying the ground around their roots.
Water is the other issue. Rain that falls on a patio has to go somewhere. On sandy ground it soaks away beside the patio; on clay it sits, and if the patio falls towards the house it ends up against the wall.
How we build a patio on clay
Dig out to firm ground
We strip the turf and topsoil and dig down to undisturbed, firm clay. Builder's fill, organic soil and soft spots are removed rather than compacted over. On many sites that means digging deeper than the textbook depth for a patio, and it is the main reason quotes on clay vary.
Separate the clay from the stone
A geotextile membrane goes onto the formation before any stone. It lets water pass but stops clay mixing into the sub-base, which is what keeps the base strong for decades rather than a few years.
A proper crushed-stone sub-base
We lay a well-graded crushed stone such as MOT Type 1 in layers and compact each one with a plate compactor. On clay, a thicker base spreads the load and gives a buffer against seasonal movement. Rounded gravel or loose hardcore is not a substitute.
Full mortar bed
Slabs are laid on a full bed of mortar, not on five spots. For porcelain and sandstone we use a slurry primer on the back of each slab so it bonds to the bed.
Falls and levels
The finished surface falls away from the house at roughly 1 in 60 to 1 in 80, so water runs off without the slope being obvious when you walk on it. Approved Document C says a damp-proof course should be at least 150mm above the finished level of adjoining ground or paving, so the patio finishes at least two brick courses below the damp-proof course.
Where the water goes
On sandy ground, letting a patio drain onto the lawn is often enough. On Middlesbrough clay we plan the water route as part of the design:
- A border or gravel strip at the low edge, wide enough to take the run-off from a modest patio.
- A channel drain across the low edge, connected to an existing surface water drain or a soakaway.
- A soakaway, but only where a percolation test shows the clay will take the water. Building regulations guidance in Approved Document H puts soakaways as the first choice for rainwater, but till often fails the test, and soakaways should not normally be within five metres of a building.
- A permeable patio, possible on clay, but it needs a deeper open-graded base that acts as a storage tank, so it costs more.
The Environment Agency's long-term flood risk service maps scattered surface water ponding around Linthorpe, Acklam and Marton after heavy rain. If your garden is one of those low spots, ask us to look at drainage first and slabs second.
Timing on clay
Clay is easier to work in late spring, summer and early autumn, when it is drier. In a wet winter, a dug formation can turn to slurry overnight. We can work through the winter, but on clay we protect the dig with the membrane and first layer of stone as soon as it is open, and we will not lay mortar onto frozen ground.
Questions people ask
Can I lay slabs straight onto clay?
No. Slabs bedded directly on clay, or on sand over clay, move as the clay swells and shrinks, and they sink where water collects. A compacted stone sub-base over a membrane is needed.
How deep should a patio base be on clay?
It depends on what is found when we dig. We go down to firm, undisturbed ground, then build up a compacted stone sub-base and a mortar bed. Soft spots and fill can make the dig deeper in places.
Will a patio on clay crack?
A well-built one should not. Cracking usually comes from a thin base, slabs laid on spots of mortar, trees close by or water collecting under the patio.
Do I need a drain for a patio on clay?
Not always. A small patio can drain to a border. Larger patios, or ones next to the house with nowhere for the water to go, usually need a channel drain or soakaway.
Sources: BGS Geology Viewer; BGS: swelling and shrinking soils; GOV.UK: Approved Document H, drainage and waste disposal; GOV.UK: Approved Document C, resistance to moisture; Environment Agency: check long-term flood risk.