Just a quick observation that I made regarding having re-planted a birdhouse: wood was starting to disintegrate and thus integrity of structure was at risk. I am wondering whether the plastic barrier if placed only between wood and plants would help keep strength of wood but that the run off of water from the sides would prevent water from sitting on the roof. I don't imagine that anyone could/would want the planting area to be completely sealed off to prevent water runoff unless you were planting something that needed every drop of water? Semps, sedums need good drainage anyway.
