Thickness of sub base

[kop202]

Should all soil be excavated out down to a subsoil. Some front gardens have 450mm + thicknesses of topsoil. Or is it OK to lay 100 -150mm of MoT type 1 on a reduced level suboil that is firm but still contains "organic" content.

[Tony McC]

All topsoil should be removed

[kop202]

I thought so. I just wonder sometimes how people quote for work if they dont go to the trouble of finding out how much needs to come off and more importantly go back in as Mot.
Credit crunch cowboys on the march

[Tony McC]

Most *reputable* contractors work on a minimum 200mm construction, often 250mm. In general, topsoil tends to be the uppermost 150-300mm, but then the clever contractor will apply a bit of local knowledge.

F'rinstance, if it's a relative new build, say 20 years or less, its most likely that the site was soil-stripped prior to construction, so the existing drive will be built on sub-soil and imported fill, with a bit of topsoil brought in at the end of the build to make a front garden. As is the way with most housebuilders, the imported soil will be minimal depth (and minimal quality), so assuming a 250mm dig will usually guarantee you're out of the topsoil.

[Dave_L]

If you have a spec from the subcontractor, the excavated depth should be indicated. We do.