As Fundo Ligno base is pre-sloped and 3/4 .in thick at the edge, how does an extension work? Doesn't extending the slope make it more than 3/4 .in thick?

Yes, an extension would make it thicker. The extension is 12”x60” starts at 3/4” and ends at 1” thick. Please let us know if you have any other installation questions, we are here to help, and you can call us at 800-938-7925.