Most patio failures we get called out to in and around St Austell have nothing to do with the slabs and everything to do with what is underneath them. Get the excavation, sub base and drainage right and a patio will sit flat for decades. This guide covers how we prepare ground for paving, so you can do it properly yourself or know what to expect from a contractor.
Start by marking the patio area with string lines and pegs, then check what the ground is doing. You need the finished surface to sit at least 150mm below the damp proof course of the house, which is usually two brick courses down from it. If your garden slopes towards the house, as plenty do on the hillier plots around St Austell and the clay country villages, you will need to plan where that water goes before you dig.
Build in a fall of about 1 in 60 to 1 in 80 away from the building, which works out at roughly 12 to 17mm of drop per metre. Set your pegs to those levels now, because it is far easier to dig to a plan than to correct levels after the sub base is in.
For a standard garden patio you are digging out roughly 150 to 200mm below your finished level. That allows for 100 to 150mm of compacted sub base, a 30 to 40mm mortar bed and the thickness of the slab itself, typically 20 to 40mm depending on the product. If the patio will take vehicle weight or sits on soft ground, increase the sub base to 150mm minimum.
Strip off all topsoil and anything organic, including roots. Topsoil compresses and rots down, and any left under a patio will settle unevenly within a couple of years. In this part of Cornwall you may also hit heavy clay or, on former mining ground, made up fill of unknown quality. Clay holds water and moves with the seasons, so dig slightly deeper and add more stone rather than less. A typical 20 square metre patio produces around 4 to 6 tonnes of spoil, so plan for a grab lorry or skip before you start.
The sub base is what actually carries the patio. Use MOT Type 1, a crushed graded stone available from any builders merchant in mid Cornwall, and lay it in layers of no more than 50 to 75mm at a time, compacting each layer with a vibrating plate before adding the next. Tipping in 150mm of stone and running the plate over it once will leave soft spots that show up later as rocking slabs.
Rake the stone to follow the falls you set with your pegs, then check with a straight edge and spirit level. A properly compacted sub base should feel solid underfoot with no movement, and you should be able to walk across it without leaving impressions.
Cornwall gets a lot of rain, and a patio sheds nearly all of it. If your falls direct water onto a lawn that already sits wet through winter, or towards a neighbouring boundary, deal with it now. Options include a channel drain along the low edge connected to a soakaway, or a simple French drain where the patio meets the garden. A soakaway needs to sit at least 5 metres from the house and will not work well in solid clay, so dig a test hole, fill it with water and see how fast it drains before committing.
This is also the stage to think about anything that needs to run under the patio, such as drainage pipes, an outside tap feed or ducting for garden lighting. Laying a spare duct costs a few pounds now and saves lifting slabs later.
A flat, well drained garden is a realistic DIY job with a hired plate compactor and a free weekend or two. It gets harder when there are significant level changes needing retaining walls, when you find soft fill or old foundations, or when the site has poor access for removing spoil. Around St Austell we regularly work on sloping sites where the dig for a level patio runs to a metre or more at one end, and at that point machine excavation and proper retaining design stop being optional.
If you bring in a groundworks contractor for the preparation, expect them to talk about dig depth, sub base spec and where the water goes before they talk about slabs. Cost depends heavily on access, spoil volume and ground conditions, which is why any sensible firm will want to see the site before pricing it.
Around 150 to 200mm below your finished patio level for normal foot traffic, which allows for a compacted sub base, mortar bed and the slab. Dig deeper on soft ground or clay, and always remove all topsoil regardless of depth.
No. Slabs laid on soil or a thin sand bed will rock, sink and spread within a season or two, especially with Cornish rainfall. A compacted Type 1 sub base of at least 100mm is the minimum for a patio that stays flat.
Usually not, as patios at ground level are normally permitted development. The main legal point is drainage: surface water should soak away within your own boundary rather than run onto a highway or neighbouring land, and front gardens over 5 square metres need permeable construction or drainage to a soakaway.
Got a job in mind across Cornwall or Devon? A quick call or message with Robert is usually all it takes to get a quote moving.