Membrane Question

[Harry Ziman]

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?

Thanks

Harry

[BigWallop]

The membrane and loose rubble is laid to give water a low resistance soak away rather than gathering and washing the soil out from under the foundations. It does do other things like stop root and weed growth, but the main thing is for good drainage as far as I know.

[Harry Ziman]

[BigWallop]

It's what is under the concrete and mortar that make the foundation. If it is set on soft soil, then good drainage is most certainly needed to stop the soil being washed away from underneath it. If it was laid on harder ground then the fitting method might have been quite different.

Pooling water is not something you want under any construction, so the best way to stop it from puddling and gathering under the foundations is give it good drainage right at the start.

The proof is in how long the build is going to last. If it was left to fend for itself through frost and weather, then it would most like heave and crack. If it is given good drainage and nothing to gather and freeze and swell, then it will happily stay put for many, many years with little maintenance.

It does really depend on what's underneath the foundations.