How to prepare a patio base

Video by Wickes. This walk-through is based on the video "How to Lay a Patio | Wickes" from the Wickes YouTube channel, which covers the full patio process from ground preparation through to laying and pointing slabs. The groundworks section is especially worth watching before you put a spade in the ground.

1. Mark out the area and check for underground services

Start with string lines and pegs to define the exact footprint of the patio. Take your time here — it is much easier to adjust a string line than to dig out extra soil later because you misjudged the edge. Double-check the measurements from the house and from any boundaries before you dig anything.

Before breaking ground, use a cable avoidance tool (CAT scanner) to check for buried pipes and cables. You can hire one from most tool hire shops in the area. In East Kent a lot of older properties have drainage runs crossing the garden at surprisingly shallow depths, so do not skip this step even if you think you know where everything is.

2. Calculate the depth you need to dig

For a standard domestic patio you need to excavate to a depth that allows for: 100 mm of compacted hardcore sub-base, a 50 mm mortar bed, and the thickness of your chosen slab. Most concrete or sandstone slabs are 35–50 mm, so you are typically digging 185–200 mm below your finished patio surface. Porcelain tends to be thinner — around 20 mm — so adjust accordingly.

If the patio sits adjacent to the house, the finished surface must be at least 150 mm below the damp-proof course. Ignore this and you will be pushing water against the wall with every shower of rain. That said, not every East Kent terrace has a clearly visible DPC line, so check carefully or probe the mortar courses to locate it.

3. Excavate and remove the spoil

Hire a mini-digger for anything bigger than about 10 square metres — shifting a couple of tonnes of soil by hand is punishing work and rarely saves money once you factor in the time. For smaller areas a square-mouth spade and a wheelbarrow will do the job. Work in sections, keeping the sides of the excavation as vertical as you can.

Dispose of the spoil responsibly. Most councils in Kent offer a green waste or soil collection service, or you can arrange a skip. Do not just pile it at the side of the garden and hope for the best — it always looks worse than you expect, and it will compact and sink unevenly if you try to use it later as a raised bed without proper treatment.

4. Lay and compact the hardcore sub-base

MOT Type 1 crushed limestone or granite is the standard material for a patio sub-base in the UK. Spread it in layers of no more than 100 mm and compact each layer with a vibrating plate compactor before adding the next. You can hire a plate compactor from most local hire shops for a day rate that is well worth paying — compacting by hand with a rammer gives a much weaker result and you will see settlement later.

The finished hardcore should be firm underfoot with no soft spots. Walk the whole area and stamp on any sections that feel spongy. If an area sinks noticeably, dig it out and check what is underneath — you may have hit a buried tree root or a patch of disturbed ground that needs addressing before you continue.

5. Set your levels and falls

A patio must drain. Water sitting on slabs for hours will eventually find a way through the joints, freeze in winter, and lift your paving. The standard fall is 1:80 — roughly 12 mm drop for every metre of patio — and it should always run away from the house, not towards it.

Set up a datum peg at the highest point of the patio, usually where it meets the house wall (remembering the DPC clearance from step two). Use a long spirit level or a laser level to transfer the fall to pegs across the whole area. Check in both directions, not just the one running away from the house, or you can end up with a patio that channels water sideways into a wall or fence post. It depends on the garden layout, but a corner fall is often the safest option.

6. Add a sharp sand blinding layer

Once the hardcore is compacted and levelled to your falls, add a thin blinding layer of sharp sand — around 25 mm — to fill the voids in the surface of the hardcore and give a consistent bed for the mortar. Tamp it lightly and check the level again with your pegs. Do not make it too thick: you still have a full mortar bed to come, and any soft layers now will show up later as slab movement.

At this point the base is ready for your mortar and slabs. To be fair, most of the hard work is already done. The actual slab-laying goes relatively quickly once you have a solid, level sub-base to work from. If you rush the groundworks, no amount of care with the slabs will fix the result.

When to call a handyman

Call Richard if the excavation reveals tree roots, buried rubble, or a drainage run that needs rerouting before you can proceed. Likewise, if you want the whole job — from ground preparation through to laid and pointed slabs — handled in one go, that is exactly the kind of garden project he takes on around Sandwich and the surrounding villages. Getting the base right is the bit most DIYers underestimate, so having someone experienced check the levels before you start laying is never a wasted conversation.