The Brew Cabin

Does a french drain edging the house wall remove the need for patio surface to be 150mm or more below DPC?

Can mortar bed for a patio be laid directly on earth that is said to be sufficiently compacted by previous patio?

Backgrd to questions:

Just had Indian Sandstone patio laid with french drain (channel of shingle) between patio edge and house wall. The company doing the work said with this arrangement there is no requirement for patio surface to be 150mm or more off DPC. The surface is in fact 80mm. Too, the company laid the sandstone paving on a mortar bed straight on to the earth. This earth had however sat under the orginal house contractor paving for the last 15 yrs and it was explained to me that this was firm enough base.

It's ok for a patio to be directly bedded over the sub-grade, as long as the sub-grade has been cleared of all vegetation and organic/deleterious matter, so no problem there, but I'm not convinced about this tale regarding a splash strip and dpc levels.

Firstly, is it a genuine 'french drain' or is it just a splash strip? The difference being that a so-called French Drain is usually at least 450mm deep, while a splash strip is often just 50mm or so of chippings.

Does the ground drain well or is it clayey and heavy? To where does this so-called French Drain drain?

There's some discussion in the trade at the moment about the relevance of the 150mm below dpc rule of thumb that has served us well for the last 100 years, and I'm not convinced that ignoring it is a good move. I know the question becomes more complex when the property involved is a new one, but for any properties built before, say 2000, I would be loathe to ignore the 150mm rule.