We used scraps of pond liner to cushion under heavy rocks. You can use one piece or layer them if you feel more layers are needed.

Thanks for the tip lisak1, will give it a try :))

I agree with lisa. The liner is really tough; however i would imagine if your boulders or whatever were really sharp, there could still be an issue. If you weren't already planning to have underlayment, I believe it would help the liner move down from any sharp weight. I only have a ~700 gal. pond but I believe my experience should carry over to large pond - large boulder installs as well. I used old carpet procured for free from a carpet dealer/installer. Ask; they should be happy to save one of their dump runs. I have not seen commercial pond underlayment, but I bet it isn't as protective as the free carpet. Some of my carpet is under a utility pond. It is still exposed and none of it gets moldy. It is great stuff. Did I mention that it was free?!

Hi chas045 and thanks for your thoughts. I should maybe give a bit of background and detail: my pond was installed with commercial underlay and pvc liner back in 1993 and continues to perform magnificently. We're currently changing the style of the pond and correcting the pond edging level in one or two places. The changes include the addition of largish boulders (I can barely lift them one at a time) and pebbles on the first ledge about 9" below the water level. Also, the liner is being held in place by very heavy "brick" edgings (actually made of concrete 18" x 9" x 4") that will be level with the surrounding lawn thus providing a mowing edge. A separate section of the pond overflows into a bog garden area when it rains.

I guess I could use spare bits of black underlay for the heavy boulders as an alternative to lisa's recommendation of spare bits of liner - after all, the boulders will be in the pond and therefore wicking is an irrelevancy. But, for seating the heavy concrete edgings on the liner, I really don't want to use anything that wicks water onto the surrounding lawn area. I'm worried that carpet will wick.

Hope that helps and thanks again for your helpful suggestions and advice!

For the large bricks, since they are bound to be flat, I am certain that lisa's solution would be more than fine and that is probably true for boulders as well since they are not being put in with equipment, but just an extra hand or two.

Thanks chas045 - will definitely go ahead with lisa's suggestion of the pondliner leftovers :)