Underlay, sand or old carpet under a pond liner: which protection to choose?
To save a few euros, it is tempting to lay sand, an old carpet or cardboard under the liner. Each option has its limits, and a punctured liner costs far more than the protection. Here is how they compare.
— The comparison at a glance
- Geotextile underlay: rot-proof, tough, the benchmark.
- Sand: fine on the bottom, but slides off the sides.
- Old carpet: rots over time, may contain chemicals.
- Cardboard, newspapers: break down within a few months, best avoided.
Geotextile underlay: the benchmark solution
Underlay is rot-proof and resists roots and stones. It protects the vertical sides just as well as the bottom. It cuts with a utility knife and is laid in strips that overlap by 10 to 20 cm.
Geotextile protective underlay for ponds
Rot-proof underlay to lay beneath EPDM or PVC liner. Live price and instant add to cart:
Sand: useful, but not on its own
A 3 to 5 cm layer of sand on the bottom smooths the ground and fills the hollows. But it slides off the sides and planting shelves, which are left exposed. The best approach: sand on the bottom to level it, underlay over everything.
Old carpet: not as good an idea as it seems
Natural-fibre carpets rot. Synthetic ones last longer, but may contain stain-repellent treatments or adhesives. Their uneven thickness also creates folds in the liner.
Cardboard and newspapers: best avoided
They break down within a few months. The liner then sits directly on soil and stones, often once the pond is already full of water and fish.
Which underlay weight?
- Loose soil with no stones: a standard underlay is enough.
- Stony ground or ground with roots: a thicker underlay, or two layers.
- Large pond, natural pool: a thick underlay everywhere, sides included.
OaseFol SeamTape - EPDM seam tape
For joining two sheets of EPDM liner watertight. Live price and instant add to cart:
For your project, discover our EPDM liners and installation accessories.
Calculating the underlay: a worked example
Work out the liner first: pond length + 2 times the depth + a margin for the edges, then the same calculation for the width. The underlay covers the same area, but you need to allow for the roll width and the overlaps. Our 400 g/m² underlay, for example, comes in a 2 m width.
Take a 4 x 3 m pond, 80 cm deep, with a 50 cm margin on each side:
- Length: 4 + 1.6 + 1 = 6.6 m.
- Width: 3 + 1.6 + 1 = 5.6 m.
- Liner: 6.6 x 5.6 m, i.e. about 37 m².
- Underlay: three 6.6 m strips in a 2 m width. With two 15 cm overlaps they cover 5.7 m, so the 5.6 m needed. That means ordering about 40 m².
Always redo this calculation with the actual dimensions of the hole, measured with a flexible tape following the planting shelves. Sizing is explained in detail in our article EPDM liner: calculating and ordering the right size.
Laying the underlay without creating weak spots
Underlay will not make up for badly prepared ground. First remove stones, roots and shards, then compact the bottom. On flat areas, spread the sand and smooth it with a straightedge.
Then unroll the strips from the bottom towards the edges, following the planting shelves without stretching the underlay. Let it follow the corners: underlay stretched across thin air tears or sags as soon as the weight of the water arrives. In windy weather, weigh the edges down with a few rounded stones, and remove them before laying the liner. Never fix the underlay with nails or staples: each one is a hard point under the liner.
As for roots, the product sheet for our Oase 200 g/m² underlay notes a useful limit: the protection works against ordinary roots, but not bamboo. If bamboo grows nearby, site the pond further away or plan for a rhizome barrier.
Walk over the underlay before laying the liner, barefoot or in socks. You will feel any forgotten stone or root straight away. Removing it now is far easier than hunting for a leak later.
What about difficult ground?
On very stony soil, double the underlay on the bottom and the planting shelves, where the weight of the water is greatest. On a plot visited by moles or water voles, look into it before laying: fine wire mesh buried under the protection is a common precaution. In wet ground, also check that groundwater cannot lift the liner, because no underlay solves that problem. To see the whole project, read Building a pond with a liner.
Oase geotextile underlay 200 g/m²Flexible 2 x 5 m roll, to protect the liner from stones and roots.View productGeotextile underlay 400 g/m²Rot-proof, reinforced with a polypropylene mesh, 2 m wide, sold by the m².View productGravel-coated liner 1 mTo cover the liner edges and protect them from UV.View product
Frequently asked questions
— Do you need underlay on top of the liner?
Yes, wherever you will place stones, pebbles or low walls, so they cannot puncture the liner from above.
— Is underlay needed under a preformed pond?
It is not essential: the pond sits on a well-levelled bed of sand.
— How much underlay should you allow?
The same area as the liner, plus the overlaps.
In summary
- Geotextile underlay everywhere: the safe solution.
- Sand to level the bottom, never on its own.
- No cardboard or natural-fibre carpet.
- Thicker underlay on stony ground.
Sources and references
- Recommendations from EPDM liner manufacturers
- Hands-on experience from Aquiflor, a water garden centre in Nodebais (Belgium) for over 30 years
- Questions received by Foudebassin.com customer service
Last updated: 28 September 2026.
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