It always starts with the tree. Here we have two ash trees, one with serious rot on the back side, the other a double-trunk with a V-crotch. Not ideal. However, in this case the addition of a treehouse to the double-trunked tree will actually strengthen the V-crotch and help avoid a catastrophic rupture. How? Through the magic of through-bolting. Instead of cutting the through-bolt flush with the tree, we extend it several inches and use it as support for a small treehouse.