The vast majority of cracks, leaks and deformed pool walls do not originate in the shell, but underneath it. The base slab has to spread the load evenly — and for an ordinary family pool holding 30 m³ that means 30 tonnes of water alone, plus the weight of the structure, the backfill and the surrounding paving. If the base settles unevenly by even a few millimetres, the shell is the first thing to suffer — and the repair costs an order of magnitude more than a properly built slab. This article explains what happens under a pool and what belongs in the design; the specific slab design, however, always belongs to a structural engineer or designer who knows your subsoil.
Why the slab decides the pool’s service life
Structurally, a pool is an unusual building. Empty it weighs a few hundred kilograms, filled it weighs dozens of tonnes — and that load keeps changing: you fill it, drain it, winterise it. So the subsoil does not receive one constant load it can gradually “settle” under, but a repeated cycle.
A simple calculation: 1 m³ of water = 1,000 kg. A 6 × 3 × 1.5 m pool holds roughly 27 m³, i.e. 27 tonnes of water. Spread over an area of 18 m², that works out at approximately 1.5 tonnes per square metre. That in itself is not extreme; the problem is unevenness — if one part of the subsoil compresses more than another, the slab bends and the shell bends with it.
The slab’s job is therefore not just to be a “solid foundation”. It performs three functions at once:
- Spreading the load over a larger area so that the stress in the subsoil stays within acceptable limits.
- Creating a precise horizontal plane on which the shell or the panel structure rests across its whole area, with no local point support.
- Providing stiffness — the slab acts as a single unit, so it does not transfer small differences in the bearing capacity of the subsoil into the structure.
Failure of any of these three functions shows up differently: insufficient load spreading tilts the pool, poor levelness deforms the walls and causes leaks at the penetrations, and low stiffness produces a crack in the slab and subsequently in the structure above it.
Build-up of the base from the bottom up
Treat the following parameters as indicative ranges that recur in manufacturers’ installation manuals and in both Czech and German practice. The actual values for your plot are set by the design on the basis of knowledge of the subsoil — a slab on clay looks different from one on sandy gravel.
1. Excavation with an overhang
The excavation is always made larger than the pool itself, typically by 50–100 cm on each side, to leave room for formwork, backfill and any drainage. The slab itself is usually 25–40 cm larger than the pool footprint on each side. All topsoil and humus-rich layers are removed — organic soil under the slab gradually decomposes and settles.
2. Compacted gravel bed
On the levelled and compacted bottom of the excavation comes a gravel sub-base, usually 16–32 mm grading, around 15–25 cm thick. The key phrase is compaction in layers: a plate compactor can compact roughly 10–15 cm at a time, not half a metre of loose material in one go. An uncompacted sub-base is one of the most common hidden mistakes — nobody sees it, and it only shows up two years later.
3. Geotextile and separation where needed
A geotextile between the soil and the gravel prevents fine soil particles from gradually mixing into the gravel and the bed losing its bearing capacity. On clay and clayey-loam subsoils it is practically a necessity. Whether to use one, and at what weight, is again determined by the design.
4. Blinding concrete and waterproofing
A blinding layer (often 5–10 cm of plain concrete) creates a clean, firm surface for laying the reinforcement and any waterproofing. Without it, the reinforcing mesh sinks into the gravel and you fail to maintain the concrete cover to the reinforcement, which is usually required to be around 3 cm.
5. Structural slab with reinforcement
For in-ground pools you will commonly encounter structural slab thicknesses from 15 to 25 cm, and more for larger projects or poorer subsoil. It is reinforced with steel mesh (typically 100 × 100 or 150 × 150 mm grids), with one layer in thinner slabs and two — bottom and top — in thicker ones. Concrete is usually chosen in strength class C20/25 to C25/30, with an exposure class corresponding to a wet environment with freeze-thaw cycles.
To stress it again: these are figures that practice works with, not a prescription. Whether you should have 15 cm with one mesh layer or 25 cm with two is a structural engineer’s decision. The difference in the cost of the concrete is a few hundred euros; the difference resulting from the wrong choice runs into tens of thousands.
Levelness: the millimetres nobody sees, but the water does
Levelness of the slab is the parameter on which the quality of the whole build stands or falls. Water is a perfect spirit level — every millimetre of fall is visible on the surface, especially in overflow pools, where the water surface is the main visual feature.
In practice there are two levels of requirement. With skimmer pools a tolerance in the order of several millimetres over the whole length is normal; with overflow pools the requirements are considerably stricter — there we are talking about single millimetres across the entire length of the channel. The difference between the two concepts is examined in the article Skimmer or overflow.
Levelness is checked with a two-metre straightedge in two perpendicular directions and with a level or rotating laser around the perimeter. The check belongs before the shell is set in place, not after — you cannot level things out under a full shell.
With liner and fibreglass shells, unevenness has one more unpleasant consequence: the shell rests only at certain points, and where support is missing, local stress builds up. That is exactly where most of the cracks that appear after the first or second season come from.
Thickness and reinforcement by pool type
The demands on the slab differ according to how the structure transfers the load. A monolithic concrete pool uses the slab as part of the load-bearing system; a plastic shell needs it mainly as a flat, stable support; an above-ground pool essentially relies only on the levelness and bearing capacity of the surface. An overview of the individual construction types and their requirements can be found in the article Pool construction types compared.
| Pool type | Nature of the load | Typical demand on the slab (indicative) | What to watch out for |
|---|---|---|---|
| Monolithic concrete | The slab is part of the load-bearing structure and acts together with the walls | Thicker slab, usually two layers of reinforcement, continuity of reinforcement into the walls | Connection of slab and wall reinforcement, construction joints and their sealing |
| Concrete blocks / permanent formwork | The load is transferred along the perimeter of the walls | Reinforced slab with starter bars prepared for connecting the walls | Exact position of the starter bars, otherwise the wall does not sit on its axis |
| Polypropylene (PP) shell | Evenly across the whole floor area | Flat reinforced slab according to the manufacturer’s manual | Levelness and gradual filling simultaneously with the backfill |
| Fibreglass (GRP) | Across the floor area, sensitive to point support | Flat reinforced slab, often with a levelling layer | No stones or coarse unevenness under the shell |
| Steel / panel structure with liner | Combination of floor area and perimeter | Reinforced slab, precise level around the perimeter | Accuracy of the perimeter — a panel cannot be “levelled out” |
| Above-ground frame pool | Floor area, but the structure is flexible | Thinner slab or a carefully compacted and levelled base | Levelness is just as critical as with in-ground pools |
Problematic subsoils: when a design is a must
On sandy gravel subsoil the slab is a relatively straightforward matter. But there are situations where, without an expert assessment, even a properly built slab risks failing:
- Clay subsoil — it changes volume with moisture, shrinking when dry and swelling after rain. This cycle can gradually lift and tilt the slab.
- Made ground and uncompacted fill — a very common situation with new builds where the garden was levelled using excavated material. A slab laid on fill without an assessment of its depth and compaction is a lottery.
- Slope — the combination of a cut into undisturbed ground on one side and fill on the other is the riskiest variant of all in terms of differential settlement.
- Proximity of mature trees — the roots not only exert mechanical pressure but, above all, draw water out of the clay and increase its volume changes. A designer will determine the safe distance for the specific tree species.
In all these cases there is only one sensible recommendation: have the subsoil assessed and have the slab designed. The cost of a geotechnical survey and a structural calculation is a fraction of the price of the pool — and what a complete construction budget looks like is shown in the article What a pool really costs.
Groundwater and drainage
Groundwater is the second factor that often “decides” the slab. An empty plastic or fibreglass shell in a waterlogged excavation behaves like a boat — buoyancy can push it upwards, which is one of the most expensive possible scenarios. The slab therefore sometimes also acts as an anchoring element, and its dimensions are derived precisely from the groundwater level.
Preparation therefore usually includes perimeter drainage led into a soakaway or a safe outfall, or an inspection chamber from which the water can be pumped out. The topic is covered in detail in the article Groundwater and your pool. The essential point is that the drainage must be designed before concreting — adding it around a finished slab afterwards is only possible with great difficulty.
Penetrations and conduits: a decision you cannot undo
Penetrations are often routed through the slab for pipework to the equipment pit, for the main drain, for electrics to the lighting or for conduits carrying data cabling to the controls. All of them have to be installed before concreting.
Practical rules that are worth following:
- Run penetrations in a conduit one size larger than seems necessary — pulling another cable through is then a matter of minutes.
- Add at least one spare conduit. Lighting, a counter-current unit, dosing or a heating connection are often dealt with only later.
- Seal the conduits against concrete getting in and blow a draw string through them.
- Survey and photograph the position of the penetrations before the pour. Three years from now, that photograph will save you drilling blind.
Drilling into a finished reinforced slab is possible, but it means damaging the reinforcement and the waterproofing at a point where that should not happen. On a DIY build it is one of the most frequent complications — you can find out more about the division of responsibility between you and the contractor in the article Turnkey pool or DIY.
Concrete curing and when to continue building
Concrete gains strength gradually; the standard reference point is 28 days, when it reaches its declared strength. After roughly a week it typically has around 70 % of that value, which is usually enough for follow-up construction work, but not for full loading.
Curing in the first days is essential. The concrete must stay damp — by watering, or by covering it with sheeting or geotextile. Drying out of the surface in the first 48 hours leads to shrinkage cracks and a noticeably lower final strength. In a hot summer that means watering several times a day.
Concreting at temperatures below +5 °C without winter measures (heated mixing water, admixtures, thermal protection) is a risk: fresh concrete that freezes will never make up the lost strength. So plan the date of the pour so that the forecast for the first few days is reliably above freezing.
Above-ground pools: different demands, the same physics
With above-ground frame and hard-sided pools it is often assumed that the base will “somehow work out”. In fact the opposite is true: the structure is flexible and any unevenness transfers into it immediately. A 4.5 × 2.5 m pool with a depth of 1 m contains around 10 tonnes of water, and if it stands on a slope with even a slight fall, the water column loads one side of the wall incomparably more than the other.
An above-ground pool usually does not need a massive reinforced slab, but it does need a flat, load-bearing and stable surface: well-compacted gravel with a levelling layer, paving on a compacted bed, or a thinner concrete slab if you expect the pool to stay in place for years. Lawn and freshly dug beds do not fall into this category. Choosing one is covered in more detail in the article How to choose an above-ground pool.
Typical mistakes that cost the most
- A slab on uncompacted fill. The most expensive mistake of all, because it only shows up after filling and cannot be fixed other than by taking everything apart.
- Forgotten or undersized penetrations. They mean drilling into a finished structure or routing pipework the long way round.
- Too thin a slab with no reinforcement. It saves a couple of hundred euros and delivers a crack in the least convenient place.
- Levelness not achieved. In overflow pools it means a non-functioning overflow; in shells, deformation and leaks.
- Concreting in frost or without curing. It looks the same, it behaves differently.
- Topsoil left under the slab. The organic layer decomposes and creates a void.
- An empty shell in a waterlogged excavation. Groundwater buoyancy pushes the shell up or deforms it.
Frequently asked questions
How thick should the slab under a pool be?
In practice, for in-ground pools it commonly ranges between 15 and 25 cm with corresponding reinforcement; for above-ground pools it tends to be thinner. The specific thickness, however, depends on the type of pool, its size and above all on the bearing capacity of the subsoil — the design belongs to a designer or structural engineer, not to a table from the internet.
Does the slab have to extend beyond the pool footprint?
Yes. The overhang, usually a few tens of centimetres on each side, ensures the load is spread beyond the outline of the shell and gives room for formwork and assembly. Without it, the edge load is concentrated at the edge of the slab.
How long should you wait before putting the pool on the slab?
A week or so is usually enough for follow-up work; concrete reaches full strength after 28 days. The specific procedure is set by the contractor according to the concrete mix, the weather and the type of structure — with heavy monolithic pools the wait is longer than with a lightweight shell.
Is compacted gravel alone enough under an above-ground pool?
For smaller seasonal pools usually yes, provided the base really is flat, load-bearing and does not settle. For larger frame pools that stay in place for several years, a concrete slab is a considerably more reliable solution.
What should I do if I forgot a penetration?
There are solutions — core drilling with subsequent sealing, or routing the pipework around the slab. Both should be designed and carried out by a professional, because they involve intervening in the reinforcement and the waterproofing. Improvisation in this spot shows up as a permanent leak.
In conclusion
The base slab is the only part of a pool you will never see once it is finished, and at the same time the only one that cannot be repaired later without dismantling the entire structure. It is therefore worth investing in an assessment of the subsoil, in a design and in honest compaction before you decide on the colour of the liner.
Once you know what structure you are dealing with, it is easier to specify the requirements for the slab as well. An overview of pools, their dimensions and technical documentation can be found in the pools category; the installation manual for the specific model is always the document a designer should use as the basis for designing the slab. Further articles on construction and operation can be found in the Guide section.
