What is the base under block paving?
The base is the load-bearing structure beneath a paved surface. It spreads the weight of feet and vehicles over the natural ground, provides the levels and falls for drainage and gives the pavers a stable support. The blocks themselves are only the visible top of the system.
In a typical build-up, the natural ground, called the subgrade, is excavated, shaped and compacted. On top comes the sub-base, usually crushed granular material known in Spain as zahorra, laid and compacted in layers. Where needed, a bound layer such as soil-cement or a concrete slab, called solera, is added. Then come the laying bed of grit, the pavers and the jointing material. Our guide to layers and base describes each layer with a cross-section.
Is a concrete base better than a granular sub-base?
Not as a general rule. A compacted granular sub-base is the standard, flexible solution for most domestic paving and performs very well when it is deep enough and properly compacted. A concrete base adds stiffness for heavier loads or weak ground, but it changes how the paving behaves and how water must be removed.
| Granular sub-base (flexible) | Concrete or soil-cement base (rigid or semi-rigid) | |
|---|---|---|
| How it carries load | Spreads load through interlocking stone particles | Spreads load through a stiff bound slab |
| Water | Can let some water pass and drain away | Water on top must be led off by falls and drains |
| Repairs | Blocks and base can be lifted and corrected locally | Local repairs are harder; cracks in the slab affect the paving |
| Typical use | Paths, patios, most private drives | Heavier traffic, weak ground, some steep or special areas |
The common mistake is to treat a concrete slab as a cure-all. A rigid slab under a sand bed can trap water that seeps through the joints, so the bed stays wet, washes out and the blocks start to move. If a slab is used, it needs falls and outlets so that water draining through the joints can escape. When the real problem is poor compaction or a thin sub-base, building a correct granular base is usually the better answer than pouring concrete.
When does a bound layer make sense?
A bound layer is worth considering where the natural ground is weak or variable, where heavier vehicles will use the area, or where the paving has to bridge trenches and disturbed ground. It is also used under some natural stone and mortar-bedded finishes that need rigid support.
- Yards and access routes used by vans or occasional heavier vehicles
- Areas over backfilled trenches for services, where the ground may settle unevenly
- Ground that softens noticeably when wet and cannot be improved by compaction alone
- Mortar-bedded stone slabs or crazy paving, which need a stiff, continuous support
- Some steep slopes, where a stiffer layer helps resist downhill creep of the build-up
Soil-cement, known in Spain as suelo-cemento, is a granular or natural material mixed with cement and compacted, giving a semi-rigid layer that is less stiff than a slab but stronger than an unbound sub-base. A concrete slab is fully rigid and needs formwork, joints and time to harden before paving. We decide between them on site, based on the ground and the use; we do not prepare geotechnical studies, and where an architect or engineer is involved we follow their specification.
How is the base built on site?
The base is built in layers, each compacted and checked for level before the next goes on. Edging is set before the final layers so that the base and the paving are contained from the start. Photos of each stage are sent to owners who are not on site, because none of it is visible later.
- Remove the old surface, vegetation and soft material down to firm ground.
- Shape and compact the subgrade to follow the planned falls.
- Spread the granular sub-base in layers and compact each layer with a plate compactor or roller.
- Where specified, lay soil-cement or cast a concrete slab within formwork, with joints and outlets for water.
- Set kerbs or edging on a concrete bed and haunch.
- Check levels and falls across the base before the laying bed is screeded.
Compaction is where many cheap jobs fall short. Thick layers compacted only at the surface leave loose material underneath that settles later. Compacting in thinner layers takes longer but produces a base that stays put. Our guide to compaction and falls covers this step in detail, and the article on cheap versus well-built paving shows what the difference looks like a few years on.
Do you pour concrete slabs as a separate service?
Partly. We do not build structural slabs or foundations, but we do pour new stamped concrete, known in Spain as hormigón impreso, as a finished outdoor surface, and we also lay asphalt. On this page, though, concrete appears only as the support for the paving that we lay on top of it.
Keeping the base and the paving in one pair of hands has a practical benefit: the levels, falls and edge details of the base are set with the finished surface in mind. If the paving above a concrete slab later shows problems, there is no argument between two trades about whose layer is at fault. For renovating an existing concrete surface without replacing it, the sister site's page on renovating outdoor concrete explains that service.
How can you tell whether an existing base has failed?
Signs of a failing base include sunken areas, ruts in wheel tracks, blocks that rock underfoot, puddles that did not exist before and cracks in mortar joints. These point to a base that is too thin, poorly compacted, washed out or disturbed by trenches or roots.
A local problem can often be repaired by lifting the blocks, correcting the base and relaying them; the sister site's page on diagnosing why paving sinks explains how the cause is established first. Where the whole base is inadequate, replacing the paving together with the base is usually the lasting answer, as described on our page on replacing old paving. For the paving itself, see concrete block paving.
The depth and type of base have a direct influence on cost. See factors that move the price for how this is reflected in quotes. A rigid layer adds material, formwork and hardening time compared with a granular sub-base. Send us a photo and the approximate m² – free, no-obligation quote.
Frequently asked questions
What is zahorra?
Zahorra is the Spanish term for crushed granular material used as sub-base under paving and roads. It is spread in layers and compacted to form a dense, load-bearing base. Its grading and compaction largely determine how well the paving above performs.
What is soil-cement?
Soil-cement, or suelo-cemento in Spanish, is soil or granular material mixed with cement and compacted. It forms a semi-rigid layer, stronger than an unbound sub-base but less stiff than a concrete slab. It is used where extra support is needed without a full slab.
Is a concrete slab under paving always a good idea?
No. A slab adds stiffness but can trap water that seeps through the joints, keeping the bed wet and causing movement. If a slab is used, it needs falls and outlets. For most domestic paving, a well-compacted granular sub-base is the more forgiving choice.
Can I lay pavers directly on my old concrete yard?
Sometimes, if the old concrete is sound, at a suitable level and drains properly. Often it is cracked, too high against doors or has no falls, in which case it is better removed. The existing slab is assessed before deciding which approach to take.
How deep should the sub-base be?
The depth depends on the ground and the traffic. A garden path needs less than a drive used by cars, and a yard used by vans needs more still. The exact depth is decided on site for each job rather than taken from a fixed rule.
Why is compaction in layers so important?
Compactors only reach a limited depth. If a thick layer is compacted from the top, the lower part stays loose and settles later, causing the paving to sink. Spreading and compacting the sub-base in thinner layers gives a dense base throughout.
Do you prepare geotechnical reports?
No. We are a paving contractor and execute the work. We do not prepare geotechnical studies or structural designs. If your project has an architect or engineer, we build the base to their specification; otherwise we choose a standard build-up based on what we see on site.
Can a base be built over a filled-in trench?
Yes, but backfilled trenches are a common cause of later settlement. The trench filling must be compacted thoroughly, and in some cases a bound layer is used to bridge the area. Trenches crossing the paved area are identified before the base is designed.
Why does water appear between my pavers after rain?
Water rising between pavers often means the bed is saturated, for example because a rigid slab underneath has no outlet or the falls are inadequate. The cause is checked from the base upwards, because simply refilling the joints will not solve it.
Can you build only the base so someone else lays the pavers?
We prefer to build the base and lay the paving together, so that levels and edges match and responsibility stays with one contractor. If another party is laying the finish, we discuss what is practical, but standalone base or slab work is not our usual service.