What fall should a patio have?

About one in eighty, or 12mm of drop per metre, running away from the house. Enough to shed water quickly, not enough for a table to feel wonky.

How far below the damp-proof course should a patio be?

At least 150mm, roughly two brick courses. Where the levels make that impossible, a drainage channel along the wall is the recognised alternative.

Where should patio water drain to on clay ground?

Usually to a channel drain connected to a surface water drain, or spread onto a lawn or border. Soakaways struggle in boulder clay, and foul drains must not be used for surface water.

Can a patio cause damp inside the house?

Yes, if it bridges or sits above the damp-proof course, or if it drains towards the wall. It is one of the most common causes of a new damp patch on a ground floor wall after garden work.

Falls, drainage and damp-proof courses

A patio should fall away from the house at roughly one in eighty, about 12mm over every metre, and its finished surface should sit at least 150mm below the damp-proof course. On the clay ground common across Teesside, where the water goes after it leaves the patio matters as much as the fall itself.

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Why levels decide whether a patio works

Almost every complaint about a patio traces back to water. Puddles that never clear, green slippery patches by the back door, a damp stripe appearing inside on the kitchen wall, joints washing out at one end: none of those are surface problems, they are level problems. Getting the levels right costs almost nothing at the setting out stage and is close to impossible to correct afterwards without lifting the lot, which is why it is worth understanding before the work starts rather than after.

The fall

A patio is never laid flat. The working standard is a fall of about one in eighty, which is 12 or 13mm of drop for every metre travelled, running away from the building. Steeper than one in sixty and furniture starts to feel like it is on a slope; flatter than one in a hundred and surface tension keeps water sitting on the slabs. The fall should be consistent and it should have somewhere to end: a lawn edge, a gravel margin, a drainage channel or a gully. A perfect fall into a boundary wall just moves the puddle.

The 150mm rule

Every house has a damp-proof course, a barrier in the brickwork usually visible as a thin line two courses above ground level. Its job is to stop moisture rising into the wall, and it only works if the outside ground stays below it. Building Regulations guidance puts the finished external level at least 150mm, about two brick courses, beneath it. Raise a patio to the height of the door threshold, as plenty of well meaning DIY jobs have, and rain now sits against the wall above the barrier. The result is a damp patch inside that no amount of internal decorating will fix.

Where the garden simply will not allow 150mm, for example on a low threshold or a converted bungalow, the accepted answer is a drainage channel: a linear channel or a gravel-filled trench running along the wall, taking the water away before it can stand against the brickwork. Any quote proposing to bring a patio right up to the door should explain exactly which of those two solutions it is using.

Teesside clay, and where the water actually goes

Much of the ground across the patch is boulder clay: it drains slowly, holds water through a wet winter and moves as it wets and dries. Practically, that means a soakaway near the house is often disappointing, because clay will not swallow the water fast enough. Better answers are usually a channel drain connected to an existing surface water drain, a discharge onto a lawn or border where the run-off can spread, or a permeable gravel margin along the low edge of the patio. Sending patio water into a foul drain is not allowed, and connecting to a public surface water sewer needs the water company's agreement, so a good installer will say up front which route they are proposing.

The details that keep it working

Keep gutters and downpipes off the surface: a downpipe discharging onto slabs will undermine them. It needs a gully or a channel connection.
Do not bury the airbricks: underfloor ventilation blocked by a raised patio causes damp and rot in a suspended floor.
Give manholes proper treatment: a recessed cover tray, filled with matching slab, is the tidy answer and it keeps the chamber accessible.
Leave joints doing their job: resin jointing sheds water and is not a drainage system in itself, so the fall still has to be there.
Watch the edges: the low side needs an escape route, whether that is a channel, a gravel strip or a lawn set slightly below slab level.

If your existing patio has the problem

Standing water in one area, with the rest sitting flat and sound, is usually a local relay job rather than a full replacement, and it falls under patio repairs. Water running towards the house, or a finished level within a couple of brick courses of the damp-proof course, is more serious and worth getting looked at properly before the next wet winter. Describe what happens after heavy rain when you ask for a quote, because it changes the answer more than the material choice does.