Patio Drainage, Levels and Foundations Explained Simply

A patio that sinks, ponds or floods the house wall usually has a problem you cannot see: the foundations, the levels or the drainage. The paving on top is only the finish. What lies beneath decides whether the surface stays flat for years.

This guide explains patio levelling, patio foundations and paving foundations in plain terms, along with surface water drainage, raised patio construction and the practical details that stop things going wrong.

Levels, falls and the damp-proof course

Every patio needs a gentle fall so water runs off instead of sitting on the surface. The fall normally runs away from the house, towards a lawn, border or drain. It should be slight enough that furniture sits level, but enough that puddles do not form after rain.

The finished surface must sit below the damp-proof course in the house wall, with a visible gap, otherwise rain splashing or lying against the wall can bridge it and cause damp indoors. Setting levels carefully at the start is more important than any other single decision. If your existing ground is already high against the house, the soil or old paving may need to be lowered.

Falls also need to consider where the water ends up. Sending it onto a neighbour's land or towards a doorway simply moves the problem, so the direction is worked out before the first slab is laid. Where a patio sits against a door threshold, level access and drainage must be balanced, and a channel drain at the door is a common solution. Falls are usually set across the whole patio in one direction, which is easier to build and easier to live with than several small falls.

Excavation and sub-base preparation

Excavation and site preparation means removing turf, topsoil and soft material until you reach firm ground. Roots, rubble and soft spots are dealt with at this stage, because anything compressible left in the ground will settle later. Where there is an old patio, it may need breaking out and removing, which affects waste removal and time.

The sub-base is normally compacted hardcore or crushed stone, often MOT type 1, laid and compacted in layers instead of tipped in one go. Each layer is compacted with a plate compactor so it locks together. The depth depends on the ground and the use: a seating area needs less than somewhere a heavy vehicle or bin lorry might cross. A sand-only bed is not suitable for heavy loads, since sand alone shifts and washes out.

• Remove topsoil, roots and soft material.
• Lay hardcore or MOT type 1 in compacted layers.
• Use a geotextile membrane over poor or clay ground where advised.
• Add a bedding layer only once the base is firm and set to level.

Why patios sink or rock

Most failures trace back to the same few causes. The sub-base was too thin or not compacted, the ground was soft, water collected underneath and washed fines out, or edges were not restrained and slabs spread outwards. Spot bedding, where mortar is placed only in blobs under each slab, leaves voids that crack under load.

Trees and large shrubs can also cause movement as roots take water from the soil, and downpipes discharging onto paving can undermine a base over time.

Heavy use is another factor. A patio built for chairs and a table is not the same as one that a wheelie bin, skip or delivery lorry crosses. If you expect vehicles, tell your contractor beforehand so the base can be built stronger. Repairs are rarely quick or simple once a base has failed, which is why getting the foundations right first time is worth more than a finer slab.

• Thin or uncompacted sub-base.
• Soft ground or buried rubbish left in place.
• Water draining into the base without an outlet.
• No edge restraint, so paving creeps.
• Spot bedding instead of full support.

Drainage, soakaways and permeable paving

Surface water drainage can be as simple as falling the patio onto a lawn or border. Where that is not possible, a linear drainage channel across the paving or at a threshold takes water to a soakaway or drain. A soakaway needs suitable ground and distance from buildings, and it is not suitable everywhere, so it should be assessed on site. Our guide to garden drainage solutions, soakaways and channels goes into more detail.

Permeable paving lets water pass through the joints into an open-graded base beneath. It is useful where runoff is a concern, and in front gardens it can help avoid planning permission for larger paved areas, but you should check with your local council. It needs the right sub-base and jointing material, and the joints need occasional cleaning. It is not a cure for poor ground: if the soil underneath does not drain, water has nowhere to go, and a drainage outlet may still be needed.

Edges, walls and paving against buildings

Edge restraints hold the paving in place. These can be concrete haunching behind kerbs or edging stones, or a mortared border course. Without them, the outer slabs can slowly move and open up.

Where paving meets a wall, the joint often needs a small gap or flexible filler so movement does not crack the wall or slab. The base should never block airbricks or ventilation on the house.

Existing drain covers and inspection chambers also need to stay accessible. Paving around them is normally cut neatly or fitted with a recessed cover so it can be lifted without breaking the slabs, and any pipes found during excavation should be identified before they are covered.

Raised patios, retaining walls and steps

On sloping plots, such as gardens nearer the Malvern Hills, a level patio may need raised patio construction or multi-level patios. A raised area needs a retaining wall built on proper foundations with drainage behind it, since water pressure builds up against solid walls. Our article on retaining walls, raised beds and sleepers covers that in depth.

Patio steps and edging should have even risers and treads, because uneven steps are a common trip hazard. Treads need a slight fall for water, and the sub-base must be firm under each step. We carry out groundwork, drainage, laying and finishing ourselves, and offer a free site visit to check levels before quoting.