Preparing the Base

If an above-ground pool is going to fail, this is where it starts. Not the liner, not the pump — the ground underneath. A pool set up on a slope puts uneven load on the frame, pushes all the water to one side, and either distorts the wall permanently or lets go entirely. Getting the base right is a full day of genuine physical work — topsoil off, ground compacted, levelled to within a couple of centimetres across the entire footprint — and it is the difference between a pool that lasts six summers and one that does not survive the first.

Why Level Matters So Much

Water finds its own level, and it does not care about your garden. On a base that falls even 5 cm across a 4.5 m pool, the deep side carries substantially more pressure than the shallow side, and that load is concentrated on a handful of frame legs. The wall bulges, the legs splay, and the pool either deforms permanently or fails suddenly. A collapse releases more than fourteen tonnes of water in seconds — which is a genuine hazard to anyone nearby, not just an inconvenience.

Preparing the Ground, Step by Step

Allow a full day, and do not start filling until step 11 checks out.

1. 1
   Choose the position: full sun for most of the day, at least three metres from large trees, well clear of septic tanks and soakaways, and within reach of a weatherproof socket.
2. 2
   Keep it away from the house wall. Splash-out and the inevitable drips will sit against the render, and you want room to walk right around the pool anyway.
3. 3
   Mark out a circle or rectangle at least 60 cm wider than the pool on every side, so you have working space and room for the ground cloth to sit flat.
4. 4
   Strip the grass and topsoil from the whole marked area. Do not simply flatten the grass — it rots, settles unevenly, and roots will come back through the liner.
5. 5
   Remove every stone, root and sharp fragment. Then walk it barefoot; you will find things your eyes missed.
6. 6
   Cut the high side down rather than building the low side up. Filled soil compacts unevenly and will settle under load within weeks.
7. 7
   Check level properly: a long spirit level on a straight timber board, rotated across several axes. A short level on its own will lie to you across four metres.
8. 8
   Compact the surface — a hired plate compactor is ideal, a hand tamper is acceptable, treading it down is not. Portuguese summer soil is often dry and dusty, so damp it lightly first.
9. 9
   Spread 2–3 cm of washed sand or fine crushed stone and screed it flat with the timber board. This is the layer that protects the liner and takes out the last small imperfections.
10. 10
    Lay the geotextile ground cloth over the whole prepared area, smoothing out every wrinkle. A fold under the liner becomes a permanent crease and a wear point.
11. 11
    Re-check level one final time with the ground cloth down, then part-fill the pool to about 3 cm and check level again before adding any more water. This is your last chance to correct anything.

Ground Conditions in Portugal

Much of Greater Lisbon and the Silver Coast sits on clay or clay-limestone soils that behave very differently between February and August. Ground that seems rock-hard in July can be soft and mobile after the first autumn rain, so a base prepared in high summer wants slightly more compaction than it appears to need. Sandy coastal soils around Cascais, Estoril and Oeiras drain beautifully but shift under concentrated load, which makes the compaction step and the sand screed more important, not less. Never set an above-ground pool directly onto paving slabs or a patio: the joints transfer point loads straight into the liner, and the pool cannot drain if it does overflow. If you would rather not judge the ground yourself, this is exactly what our team assesses on the site visit — it is the part of the job we would not let a client skip.

If you would rather not handle this yourself, these are the services that cover what you just read.

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