Laying Patio Slabs on a Grit Bed: How to Build a Level Patio
From excavation to the compacted base course to the last slab – the proven unbound construction method
Disclaimer
This guide has been prepared with great care. Nevertheless, we accept no warranty for the accuracy, completeness or currency of its contents. You follow this guide at your own risk – any liability for personal injury, property damage or financial loss arising in connection with its use is excluded. The contents do not replace professional advice for your individual situation. Always observe the manufacturer instructions of your tools and materials as well as applicable local regulations (e.g. building codes, neighbour law, utility line enquiries before digging). Work on electrical, gas or water lines and on load-bearing structures must only be carried out by qualified professionals.
From excavation to the compacted base course to the last slab – the proven unbound construction method
The unbound method on a grit bed is the standard for patios made of concrete and natural stone slabs: it needs no mortar, lets rainwater drain away, and individual slabs can be lifted and re-set at any time later. The real work is not in laying the slabs but in the sub-base – excavation, base course and the precisely screeded grit bed decide whether the patio stays level or develops waves after two winters. Plan a fall of 2 % away from the house from the very start, so rainwater reliably runs away from the facade.
Tools
- Spade and shovel
- Plate compactor (rental)
- Screed board and two screed rails or pipes
- Rubber mallet (light-coloured head, non-marking)
- Spirit level and mason’s string
- Wheelbarrow
Materials
- Patio slabs of concrete or natural stone (min. 4 cm thick)
- Crushed stone or mineral mix 0/32 for the base course
- Grit 2/5 mm for the slab bed
- Edging stones or deep kerbs plus lean concrete for the edging
- Joint spacers 3–5 mm
- Jointing grit or joint sand
Step by Step
Plan the area and set the levels
Stake out the patio area with string and pegs and define the finished height – at the house wall at least 15 cm below the door threshold or below the damp-proofing level. From there, plan a 2 % fall away from the house, i.e. 2 cm per metre. The string, stretched at finished slab height, is your reference for all following layers.
Excavate
Dig out the area 25–30 cm deep, measured from the finished slab surface, and completely remove roots and topsoil. The base should already follow the later fall so that all layers end up equally thick. On cohesive, clay-rich soil, it pays to go a few centimetres deeper and install more base course.
Set the edging
Set edging stones or deep kerbs in a bed of earth-moist lean concrete and support them with a concrete haunch at the back. The edging holds the grit bed and slabs permanently in position – without it, the edge slabs migrate outwards over time. Align to the string and let the concrete set for a day.
Install and compact the base course
Fill in 15–20 cm of crushed stone or mineral mix 0/32 in layers and compact each layer of no more than 10 cm with the plate compactor in several passes. The base course already follows the 2 % fall. A properly compacted base course is your insurance against settlement – this is where the lifespan of the patio is decided.
Screed the grit bed
On top of the base course go 3–5 cm of grit 2/5 mm. Lay out two pipes or rails following the fall, level them in and screed the grit flat over them with the screed board. Then remove the pipes and fill the grooves with grit. Important: do not step on the finished bed – work your way backwards out of the area.
Lay the slabs
Start at the house wall or at a right-angled reference edge and work forward from the already laid surface – never walk across the screeded bed. Set the slabs down carefully, bring them to an even spacing of 3–5 mm with joint spacers and tap them lightly with the rubber mallet. Continuously check height and alignment with spirit level and string; mix concrete slabs from several packs to distribute colour variations.
Fill the joints
Finally, sweep jointing grit or coarse joint sand diagonally into the joints until they are full. The material settles after the first rains – then top up once more. It is better not to compact the finished surface with the plate: unlike thick paving blocks, large-format slabs can crack in the process.
At a Glance
Safety First
- Before excavating, check whether utility lines (power, water, sewage, irrigation) run through the working area – ask the utility company if in doubt.
- Plate compactors are heavy and vibrate strongly: wear safety boots and get an introduction from the rental company.
- Large-format slabs quickly weigh 25–50 kg: carry them in pairs or use slab-lifting tongs, and lift from the legs.
- When cutting slabs with an angle grinder, wear safety glasses, hearing protection and a dust mask, and cut wet if possible.
Mistakes You Should Avoid
Laying without or with too little fall
A patio without the 2 % fall away from the house backs water up against the facade and forms puddles on the surface. This can only be corrected later by completely re-laying – so plan the fall in from the very first layer.
Skimping on the sub-base
If you lay slabs on a few centimetres of grit directly over the soil, you will have waves and rocking slabs after the first winter. The 15–20 cm of compacted base course are not an optional step but the foundation of the patio.
Laying slabs tightly butted without joints
Concrete and natural stone slabs need a 3–5 mm joint so they can absorb stresses from temperature and the smallest movements. Tightly butted slabs spall at the edges. Joint spacers cost little and give you an even joint pattern into the bargain.
Common Questions
What goes under patio slabs?
The proven build-up from bottom to top: compacted subsoil, 15–20 cm of compacted crushed-stone base course 0/32 and on top a screeded grit bed of 3–5 cm in 2/5 mm. Slabs laid directly on soil or sand subside and shift.
How much fall does a patio need?
The standard is 2 %, i.e. 2 cm per metre, always away from the house. For very rough or large-format slabs, 2.5–3 % is also fine. Less than 1.5 % experience shows leads to puddles and lime marks on the surface.
What does it cost to have patio slabs laid?
DIY, material costs are around €40–80/m² depending on the slab. Done by a professional company, labour of roughly €60–120/m² is added depending on ground conditions, slab format and cutting work – including excavation, sub-base and disposal.
Can I lay patio slabs on top of my old patio?
Only if the old surface is level, load-bearing and has sufficient fall, and if the build-up height fits below doors and damp-proofing – then pedestals or a thin grit bed are possible. Removal is usually the cleaner solution, because old surfaces are rarely level enough.
Why do individual patio slabs rock?
It is almost always the bed: the slab has no full contact with the grit, for instance because the bed was screeded unevenly or walked on after screeding. Lift the slab, re-screed the bed and set the slab down fresh – that is the great advantage of the unbound method.
When is a professional worth it?
For large areas, large-format slabs from 60×60 cm, ceramic pavers, sloping sites, or when steps, drainage channels and connections to the building’s damp-proofing come into play, professional execution pays off. We build patios from natural stone, concrete slabs, ceramic and wood – including mechanical excavation, a professional layer build-up and clean connections to house and garden. It is precisely the combination of correct fall, drainage and a load-bearing sub-base that makes the difference between a patio that looks good for two years and one that lasts for decades.
