Getting the Levels Right From the Start

A patio looks effortless when it's finished, and there is nothing effortless about it. Two things decide whether it still looks that way in five years: the level it sits at, and the way water leaves it. The slabs are the easy part. Beneath them, the excavation, sub-base and falls do the real work.

The first measurement to fix in your head is the damp-proof course. Your finished patio surface should sit at least 150 mm below it — two brick courses — and must never bury an airbrick. If you're building up close to the house, that constraint decides everything else, including how much you have to dig out. Work it out before you order a skip, not after.

Then look at how the ground already falls. If it slopes towards the house, you'll need to dig deeper and rebuild the fall away from it. That's a good thing to discover at the planning stage rather than halfway through a bank holiday weekend.

The Fall: Small Numbers, Big Consequences

Aim for a fall of between 1 in 60 and 1 in 80 away from the building — roughly 12 to 17 mm per metre. Across a 4 m patio that's only 50 to 70 mm of drop in total, which is exactly why it needs setting out properly instead of being eyeballed.

Go steeper than 1 in 40 and the patio feels odd underfoot, garden furniture sits at a tilt and the slabs start to look wrong. Go shallower than 1 in 100 and surface water will pond, algae will follow, and the first hard frost will lift the edges.

Set the fall into the sub-base itself, not just the bedding layer. Use pegs and a string line, or a laser level, and check with a straight edge at several points across the width. Water runs to the lowest point, so decide deliberately where that lowest point is — an open lawn, a gravel margin, a channel drain — and make sure your levels genuinely deliver water there.

Building a Sub-base That Drains

Excavate deep enough to allow for 100 mm of compacted sub-base, a 30 to 40 mm bedding layer, your slab thickness and the drop needed for the fall. Then work through it in order:

• Line the excavation with a geotextile membrane. It stops fine soil pumping up into the sub-base and silting up your drainage over time.
• Fill with MOT Type 3 or another permeable sub-base material. Type 1 is perfectly good for a conventional patio but it holds water; Type 3 lets it pass through to the ground beneath.
• Compact in layers no deeper than 75 mm using a plate compactor, damping the surface lightly if the material is dusty.
• Finish each layer to the correct fall, checking against your string line rather than trusting your eye.

For a patio taking nothing heavier than foot traffic and a barbecue, 100 mm compacted is a sensible minimum. Go to 150 mm if planters, steps or anything with wheels will sit on it.

Choosing Frost-Resistant Slabs

Frost is what turns a decent patio into a wobbly one. Water soaks into a porous slab, freezes, expands, and quietly crumbles the surface from the inside. Choose with that in mind:

• Porcelain is the safest bet, with water absorption typically under 0.5%. Twenty-millimetre slabs are fine on a full mortar bed; 30 mm gives you more tolerance on an uneven base.
• Concrete slabs should be certified to BS EN 1339 and have been freeze-thaw tested. Cheaper pressed slabs often aren't.
• Natural stone varies enormously. Good sandstone and granite perform well, but absorption rates differ between quarries, so ask for the figure before you buy rather than after the first winter.
• Avoid thin, unsealed or seconds-quality paving. The saving rarely survives two years of British weather.

Whatever you choose, lay it on a full bed of 4:1 sharp sand and cement. Dot-and-dab bedding leaves voids where water collects and slabs crack — a shortcut that costs far more than it saves.

Jointing, Edges and Finishing

Joints are not decoration; they're part of the drainage system. On porcelain, leave 3 to 5 mm gaps and fill with a resin jointing compound or kiln-dried sand. On concrete block paving, 10 to 15 mm joints brushed full of kiln-dried sand let water pass through to the permeable layers below.

Then restrain the edges. A 100 mm wide concrete haunch along the perimeter stops the whole field of slabs spreading outwards, which is the usual cause of a patio that looks fine for a year and then develops gaps at the edges. Mix it at 6:1 and let it cure properly before pointing the joints.

Where the Water Actually Goes

Water has to end up somewhere you're happy with. An open lawn or border works if the levels carry it there. A gravel-filled trench along the low edge acts as a simple French drain. A channel drain across a doorway stops water running back towards the threshold, and a crate soakaway wrapped in geotextile and surrounded by clean gravel handles larger volumes.

Surface water must not discharge onto a neighbour's land or into the foul sewer. If your patio is impermeable and drains to the mains, it's worth checking whether you need permission first; permeable paving or a soakaway usually avoids the question entirely.

Get the fall, the sub-base and the slab choice right and everything else becomes easy. The patio stays level, frost leaves it alone, the wall stays dry and you spend your summers sitting on it rather than repairing it.