I've stacked 2x12 - it worked well. I have about a 8" slope. The maximum I've stacked is a 2x12, 2x6 and 2x4 all together. I put the 2x12 on the bottom, and put either a 2x6 or 2x4 on top. I put a couple of plates like this or this ) or slightly thinker on the back (non water/liner side). That ...