Modular surfacing is only as good as the base it sits on. We repeat that on every project - and every time it proves true. Bulging, tile rocking, puddles after rain, connections wearing out too quickly - in 9 out of 10 cases the cause is an unsuitable or poorly executed base, not the surface. This article answers one question: what to lay under the modules so the court lasts for years.

The iron rule - the base must be hard and compacted

Before we compare options, one rule with no exceptions: modular surfacing requires a hard, stable, compacted base. Soil, grass, gravel, sand, crushed stone without a top layer - none of these is suitable under a permanent modular court.

Why? Modular surfacing transfers loads vertically to the base. If the base is soft or unstable, modules work on every step, connections loosen, tiles start to shift. After a few seasons the court looks like a jigsaw after a storm. On top of that: soil settles unevenly, depressions form, and puddles remain even with good fall.

Temporary laying of modules on grass or sand during an outdoor event - yes, that works for a few days. For permanent use: no.

Concrete slab - the best choice

A concrete slab is the gold standard under a modular court. It gives the best stability, longest service life, and the most even surface of all available options. If you are building a court from scratch and can install a slab - that is the right choice.

Parameters of a proper slab under modular surfacing:

• Concrete class - minimum C16/20, reinforced with steel mesh or distributed reinforcement (polypropylene or steel fibres). Reinforcement prevents cracking during temperature changes.
• Slab thickness - minimum 10 cm, ideally 12-15 cm. A thinner slab is prone to cracking when the ground freezes.
• Expansion joints - every 3-4 m in both directions. Concrete moves thermally - without joints it cracks in random places. Joints are planned together with the court layout so they do not run through the middle of the playing area.
• Drainage fall - 0.5-0.6% in one direction. On a 10 m court that is 5-6 cm level difference. Players do not feel it; water drains immediately.
• Curing time - minimum 28 days before laying the surface. Fresh concrete under modular surfacing loses moisture more slowly - modules can bulge.
• Surface flatness - maximum deviation 2 mm over 2 m with a straightedge. Modular surfacing transfers unevenness 1:1 - every bump will be felt underfoot.

The slab requires prior preparation: excavation, geotextile, crushed-stone sub-base (15-20 cm mechanically compacted crushed stone, 5-10 cm levelling sand) - and only then concrete. This stage is done by a construction crew before the surfacing installation team arrives.

Important: if you plan a fixed hoop, the pole foundation (around 60 × 60 × 100 cm of concrete) is done together with the slab - not afterward. More on planning the whole process: How to prepare the base for a basketball court step by step.

Asphalt - a good alternative, especially for upgrades

Asphalt is the second-best base under a modular court. It gives stability comparable to concrete, is frost-resistant, and handles dynamic loads well. In Polish conditions asphalt stays stable for many seasons - provided it is well executed and has no deep cracks.

When asphalt makes particular sense:

• Upgrading an existing court - you have an old asphalt court or polyurethane on asphalt. Instead of stripping - lay modules directly on top. One day of installation, zero demolition cost. This is one of the most common upgrade paths for school and housing estate courts.
• Existing car park or yard - an asphalt yard at a facility can quickly become a court with no earthworks.
• Faster and cheaper delivery than concrete - asphalt is laid faster than a slab and does not require 28 days of curing. Modules can be laid once it has cooled.

What to watch for: asphalt must be in good technical condition - no deep cracks, no sinking sections, no areas where the top layer is separating from the sub-base. Minor surface cracks are not a problem. Large cracks with material loss - require repair before installation.

Paving blocks - yes, but with conditions

Paving blocks as a base under modules are acceptable - but not a first-choice solution. Why? Blocks have joints that can move under dynamic load. A module resting on a joint instead of solid surface is more exposed to micro-movement and connection failure.

When blocks work well:

• blocks are stable and do not sink anywhere,
• the surface is level - no blocks standing above the plane,
• joints are filled and do not crumble,
• the surface manufacturer allows this installation method (Enlio does - with stable paving).

When blocks are not enough: if the paving is old, uneven, sinking in sections, or has wide, raised joints - better to level with bituminous mass or consider a slab. There is no point building a court on a base that will need repair in a year.

One important note: if you are starting from scratch and considering new paving solely to lay modules on top - it makes no economic sense. A concrete slab is in that case a better and often similarly priced solution.

Existing polyurethane court - yes, it works

You have an old polyurethane court - faded, cracked, with a peeling top layer. The classic approach says: strip and start again. Modular says: lay modules on top and play tomorrow. And it really works - provided the polyurethane lies stable on the base and has no active blisters or peeling sections. More on this: Old polyurethane court? Lay modules and play tomorrow.

What absolutely not to lay under modules

To remove any doubt - bases that are not suitable under a permanent modular court:

• Soil and native ground - settles unevenly, becomes soft after rain, plant roots destroy connections from below.
• Grass and lawn - absolutely not. Grass rots under modules, the ground is unstable, and after a week the court looks like a wave.
• Gravel and crushed stone without a top layer - aggregate is sub-base under concrete or asphalt, not a base for modules. Modules on gravel shift with every step.
• Sand and sand fill - like gravel. Soft, unstable, uneven.
• Concrete paving slabs without joint filling - unstable, they move under dynamic load, joints too wide.

Summary - which option for whom

• Building a court from scratch - concrete slab. Most expensive upfront, cheapest over the years. Best stability, longest service life.
• You have existing asphalt in good condition - lay modules directly. You save base preparation cost and time.
• You have stable paving blocks - acceptable; check level and joint condition before deciding.
• You have an old polyurethane court - lay modules on top if the polyurethane is stable. No stripping, no downtime.
• You have soil, grass, or aggregate - hard base first, then modules. No shortcuts.

Not sure about the condition of your base? Hoop And Court assesses the base during consultation and advises whether it is suitable for direct module installation or needs prior work. Contact us before you start - it saves problems afterward. Also read: Most common mistakes when building a backyard court.