Allotments 4 All

I'm going to be screwing my greenhouse to some scaffold planks I got from a mate (nuff said) and I want to make them last as long as possible.

What should I use as a preservative to increase its life as its going to be next to the earth? It would have to be a non toxic substance.

Can only think of Cuprinol, used Cuprinol treated scaffold boards for gravel board for my fence in contact with the earth 16 years ago, they are rotten now. I used old bricks to build a wall to stand the GH on, they will last for ever.