Four kinds of ground, one borough
The British Geological Survey's Geology Viewer maps the "superficial deposits" left by the last Ice Age and the sea on top of the bedrock. Around Redcar they fall into four broad groups:
| Area | What the BGS maps | What it means for a patio |
|---|---|---|
| Parts of Coatham | Blown sand over Redcar Mudstone | Drains freely but is loose and moves if not held in |
| Dormanstown, Marske, Saltburn, Eston, New Marske, Brotton, Loftus | Glacial till, a stony clay | Holds water and softens when wet |
| Much of Guisborough | Glaciofluvial sand and gravel over Redcar Mudstone | Generally drains well and makes a firm base |
| Centre of Skelton | No superficial deposit; sandstones, siltstones and mudstones close to the surface | Shallow rock in places, clay patches elsewhere |
Every garden is different, and made ground, old foundations or imported topsoil can sit on top of any of these. We dig a trial area at the start of every job to see what is really there.
Building on blown sand in Coatham
Blown sand was carried inland off the beach by the wind. It is clean and free-draining, so puddles are rarely the problem. The issue is that loose sand has little strength of its own and flows sideways when it is loaded or disturbed. A patio laid on a thin bed with no proper edging can creep outwards, open its joints and dip at the corners.
On sandy ground we:
- dig out topsoil and any loose, organic material
- lay a geotextile membrane so the sand does not migrate up into the stone
- build a crushed-stone sub-base in layers, each compacted with a plate compactor
- set a firm edge, such as a concreted edging course or a haunched kerb, so the base is confined on all sides
- bed every slab on a full mortar bed rather than dabs
Coatham's walled yards help here, because the walls already contain much of the ground, but the base still needs a solid edge where it meets a lawn or border.
Building on clay across Dormanstown, Marske and Eston
Glacial till, or boulder clay, is the ground under most of the borough. The BGS explains that clay soils swell as they get wet and shrink as they dry out. Till also softens in winter and drains slowly. The two common failures are slabs sinking where they were laid on soft clay, and water ponding on and under the patio.
On till we:
- dig deeper, down past topsoil and any soft clay, to a firm formation
- use a geotextile to stop clay pumping up into the stone
- build a thicker compacted sub-base
- give the surface a clear fall away from the house
- plan where the water goes, whether a border, a channel drain or a soakaway
The Approved Document H guidance on rainwater puts soaking into the ground first, then a watercourse, then a sewer. On clay, a soakaway needs testing, because water may not drain away fast enough; our Eston page explains why gardens below the Eston Hills often stay wet.
Building on sand and gravel in Guisborough
Guisborough is the easiest ground of the four. Glaciofluvial sand and gravel, laid down by meltwater, drains well and compacts firmly, so a standard compacted sub-base usually performs well and soakaways are more likely to work. The things to watch are the becks: the Environment Agency maps Flood Zone 3 along Chapel Beck and the streams running off the hills, so gardens near them need a closer look.
Shallow rock in Skelton
Where the BGS records bedrock close to the surface, as at the centre of Skelton, we may hit sandstone or shale before reaching full depth. Sound rock is an excellent base, but it can mean breaking out high spots, and water can run along the rock surface towards the house. On sloping plots we often add a drain at the back of a patio cut into the hillside.
Levels and the damp-proof course
Whatever the ground, the finished patio must sit at least 150mm below the house's damp-proof course. That is the guidance in Approved Document C, and it stops rain splashing up and bridging into the walls. In Coatham's terraces, yards are often already close to floor level, so we sometimes have to lower the ground rather than build on top of old paving.
Checking flood risk
The Environment Agency's long-term flood risk service shows the risk from rivers, the sea and surface water for any address. Our location pages note that it maps Flood Zone 3 within 750 metres of central Coatham, Marske, Saltburn, Loftus and Guisborough, mostly following the becks and low ground. A patio will not change your flood risk on its own, but it is worth knowing before deciding where water from a large new hard surface should go.
Questions people ask
Can you lay a patio straight onto sand?
Not safely. Loose sand needs to be confined with edging and a compacted crushed-stone sub-base, with slabs on a full bed.
How deep should a patio base be on clay?
Deeper than on sand or gravel, because soft clay has to come out first. We set the depth on site once we have seen the ground.
How do I find out what soil my garden is on?
The free BGS Geology Viewer shows the mapped deposits for any location, and a small trial hole confirms it.
Will a soakaway work in Redcar?
On sand and gravel, often yes. On clay till, only after testing, and sometimes a connection to a drain is needed instead.
Sources: BGS Geology Viewer; BGS: swelling and shrinking soils; Environment Agency: check long-term flood risk; GOV.UK: Approved Document C, resistance to moisture; GOV.UK: Approved Document H, drainage and waste disposal.