Our neighbours are having a area of traditional cobbles laid. The old base
has been dug down (not to the bottom - it must be quite deep) and a new one
constructed using a couple of inches of the old material (is this good
practice?) then a membrane and then a couple more inches. All this is
compacted before concrete is laid (a few inches) and finally the cobbles.
What is the purpose of the membrane? Under all that concrete it can't be to
stop seeds? The only thing I can think of is to stop roots penetrating
through all the old base into the concrete layer. Can anyone help?