A pond liner needs support from the ground beneath it. Underlayment is a layer selected to cushion and separate the liner from soil, small stones, and other uneven surfaces. It reduces some installation risks, but no fabric makes a liner puncture-proof or fixes a site with groundwater problems. Choose the material and installation method that your liner manufacturer specifies for the pond you are building.
What does underlayment do?
A suitable nonwoven geotextile can provide a consistent protective layer under a flexible liner and may be easier to fit to the pond's shape than improvised household material. The required weight, coverage, and any additional protection under heavy rocks depend on the liner and design. Ask for the liner's current instructions rather than substituting a material because it looks soft.
Why not reuse carpet or cardboard?
Old carpet can be difficult to fit cleanly and may contain backing, adhesive, or debris whose suitability beneath a pond liner is unknown. Cardboard can break down and does not provide a predictable long-term layer. Neither is a reliable substitute for the underlayment called for by a liner system. Do not assume that stacking extra improvised layers offers the same protection as compatible materials installed correctly.
Check the ground and the full design
Remove sharp debris and address obvious uneven or unstable areas before laying any protective layer, following the liner installation instructions. Plan around rock placement, seams, edges, and any pipes or fittings that cross the liner. If water is rising beneath a liner, underlayment alone is not a groundwater-control plan. For that separate issue, read What Do I Do About Water Under a Pond Liner?.
Keep material choice separate from leak repair
Underlayment goes beneath a liner as part of its installation. If an existing pond is losing water, first locate the cause rather than assuming more material underneath will fix it. See Is My Pond Losing Water to Evaporation or a Leak? for the initial water-loss checks. A qualified pond installer can help match the liner, underlayment, site, and rock protection when the manufacturer's directions do not cover a specific design.