Written by the Pondliner.org team · reviewed
Yes. Always. Pond liner underlay costs a fraction of relining a pond, and the vast majority of liner failures start from underneath — a stone, a root or a shard you didn't see during excavation. Skipping underlay to save £40 on a £400 liner is the most expensive false economy in pond building.
Which pond liner underlay weight do I need?
- 300 g/m² — most standard garden ponds on prepared soil.
- 500 g/m² — stony, rooty or reclaimed ground; consider adding a 50 mm sand bed.
- 700 g/m² — large or deep ponds, swim ponds, reservoirs. Double-layer on shelves.
The weight number refers to the geotextile's mass per square metre and correlates strongly with puncture resistance. 300 g/m² is genuinely fine for most builds; the temptation to over-spec is real but rarely justified on prepared loam. The exception is shelves, which we'll come back to below.
Where people skimp (and shouldn't)
The shelves and the deep zone. Both take the most concentrated load — shelves from rocks and plant pots, the deep zone from water pressure. Both deserve a layer of underlay, even if you're being thrifty everywhere else. On deeper ponds (over 1.2 m), doubling up the underlay across the deep zone is a five-minute job that materially extends the liner's life.
The other place people cut corners is the overlap at the pond edge. Underlay should run a full 30 cm beyond the liner edge, not finish flush. This protects the liner where coping stones, paving or rockery edging press down, and where small mammals occasionally try to take a shortcut across the lip.
Geotextile vs old carpet
Old carpet rots, holds water, and the backing breaks down into the pond. Use a needle-punched polyester geotextile — it's inert, drains, and lasts as long as the liner above it. Hessian, cardboard and builders' membrane are all worse choices for the same reasons: they break down, hold abrasive grit, or fail to drain.
What about a sand bed underneath?
On stony or reclaimed ground, a 50 mm bed of sharp sand under the geotextile is excellent insurance. It absorbs the last of the point loads from anything the stone-pick missed and gives a smooth substrate for the underlay to sit on. Don't use builders' sand — it's too fine and turns into a slurry once water arrives.
On chalk, flint or shale subsoils — common across southern England — a sand bed is effectively mandatory for a long-life build. Pair it with a 500 g/m² geotextile and a reinforced 0.7 mm liner from our reinforced pond liners range and you'll forget the pond is lined.
Underlay above the liner?
Yes, if you're laying cobbles, slate or any stone finish over the liner. A second layer of geotextile above the liner protects it from the sharp edges of decorative aggregate and from the weight of larger feature stones. The same logic applies to plant baskets in the deep zone — a small geotextile pad under each basket prevents abrasion as the basket settles.
For the maths of how much underlay to order, see our companion guide on how to measure your pond for a liner — the formula is identical, just buy the underlay to the same dimensions. You can also run your numbers straight through the pond liner calculator to get the sheet size in seconds.
Buying a pond liner with underlay
Most people search for a pond liner and underlay together for good reason — they're a system, not two independent purchases. Order the underlay to the exact same length and width as the liner (same formula: length + 2× depth + 0.6 m overlap, by width + 2× depth + 0.6 m). Any area the underlay doesn't cover is an area where a buried stone can find your liner over the first few frost cycles.
When you're getting a quote, ask whether underlay is included or priced separately. A reputable supplier will list the underlay weight (300, 500 or 700 g/m²) alongside the liner spec and quote both to the same dimensions. If a quote looks unusually cheap, it's almost always because the underlay has been left out — add it back in before you compare prices, or use the pond liner calculator to price both together.
Frequently asked questions
Can I use old carpet instead of pond underlay?
No. Carpet rots, holds water, and its backing breaks down into the pond. Use a needle-punched polyester geotextile rated 300–700 g/m² depending on ground conditions.
Do I need underlay if my soil is soft and stone-free?
Yes. Even on prepared loam, small stones work their way up over the first two seasons of frost-heave. Underlay is cheap insurance against a leak you can't easily find later.
Should I put a layer of sand under the underlay?
On flint, chalk, slate or reclaimed ground, yes — a 50 mm bed of sharp sand under a 500 g/m² geotextile is the long-life standard. On clean loam it isn't necessary.
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