Thing is there are still roots deep in the ground, and water has not had any effect, and I've had heaps of rain over the years they have been in. I think the roots provide a run-off because big puddles never sit around long. Also the water has successfully run -off aboveground as well, when it rains heavily. Last week we had over 229 mm of rain in the space of 2 and half days.
I layered concrete tiles, and some bricks before I covered it. Thanks for your interest. If it happens I'll let you know.