Used an old 3mm pond liner for mine inside a cedar raised bed a couple years back. I repurposed it from an old pond I filled in. Are you talking about the underlayment or the liner?

I'm curious now about the reason why the distributor said not to use it. I had fish in the pond for years so I would assume the leaching would be minimal if any against wet soil versus running water.

FWIW, I don't see how a liner would make a significant difference in the life of the frame - the only thing I know that can withstand constant contact w/ soil is concrete or stone...?

Thanks for your comments. I purchased a pond liner. Before I installed it, I had a conversation with a neighbor who indicated that over time it could leech chemicals into the soil. The liner was bought from home depot. They gave me the distributer where it came from. I told them of the use and they said don't use it. It comes from China and I don't trust them ala sheet rock toxins. I found a company on the web with non toxic plastic. After I talk to them I'll post more info.

My new and old beds are made from local cypress. The old one after 3 or 4 years is showing rot. I believe if I can reduce the amount of moisture on the inside of the lumber I'll get more mileage.

I have several raised beds for both veggies also roses, and they are ordinary concrete blocks, have been out there over 30 years now and no problems...after awhile they get green moss and other stuff growing on them and you don't realize what they are....we just laid them on the ground , one of them we mixed some quik-mix cement and put it between them to hold them in line.... relatively cheap also....sally

Re: Raised bed liners. Been there, done that. Wood rots and concrete blocks won't stay aligned. I went with old tires. The bigger the better. Soil environment is easy to manage and the large truck and tractor tires provide a place to sit while tending the plants. I cut the sidewall out on the up side and leave the down side intact to retain water. They last forever. Mine are going on 30 years. Haven't found any proof that they leach out. You have to burn them to produce toxic waste. Try local tire and/or truck shops. They are happy to get rid of them.