How level does sub grade need to be?

[onionsa]

Hi all, I've been digging out a large part of our garden to get it ready for a split level patio with retaining wall.

My question for this thread relates to how accurate the sub grade needs to be? I know we aim for 150mm depth from the desired patio level, but just how close to 150 does it need to be? Is 120 enough if the ground is particularly hard? Is 200 too much as I dig out bricks and roots?

I'd really appreciate some real world answers to this as general information I've found only really states the "ideal" 150mm.

Thanks in advance!

[seanandruby]

You can lay a patio straight onto a firm subgrade so 120 ml is ok on a good subgrade. Take out the vegetation and any soft spots though.

[onionsa]

Thanks for confirming, I'll get it down to a more even level on the more obvious high spots and dig out all the roots before getting the MOT1 on there.