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Here is a link that might be useful: FAQ HFGH

As always, thanks for the help!

Gardiner is the HFGH construction expert here, but I would like to suggest an alternative to a sheet of plastic between your aluminum base and copper treated timbers. Check your local big box home center for the rolls of 3- 1/2" wide vinyl sold for sealing sills under standard 2 x 4 framed exterior walls and unlike plastic sheeting, made to last as long as the structure.

Every time we visit our local Menard's and/or Home Depot or Lowe's, I'm always amazed at the latest in materials and just as regretful that they weren't around when our old house was constructed, so do wander the construction materials areas, and you too may find a product that may not be advertised exactly for your intended use, but one that could prove ideal anyway.

Another possibility is vinyl siding. It comes in 10 ft. lengths, is very cheap per piece, but again, made to withstand the elements and just a few pieces of the 5" width laps, would cut to fit and cover the tops of the timbers.

Of course, the ultimate and perhaps, in the end, best of all timbers for a GH base, might be the resin timbers that will never rot, are always perfectly straight, will never bow or warp, and don't require any barrier at all.