I've seen various different recommendations for laying porcelain paving slabs. The main difference seems to be whether a concrete layer is added on top of the hardcore.
Most guides (like this one) suggest digging about 200mm (20mm slab, 50mm mortar, 100mm+ hardcore).
Other guides suggest up to 100mm of concrete in addition, which seriously increases my excavation work (+ several cubic metres of soil removal).
Is the concrete layer really necessary? How deep should I dig?
If it depends on the quality of the soil substrate - in my case it's pretty firm (London clay)
If it depends on the size of slabs - they will be large. Possibly 600x900.
UPDATE:
After some comments below I consulted some manufacturer documentation and also contacted their technical support:
The docs make no mention of concrete at all, but support staff said adding this would be a judgement call based on ground conditions. Adding that most installations would not require it.
I'd be surprised if my ground conditions fell outside the category of "most" so I'll dig an initial 200mm and take a view and/or invite contractors to finish the job. Hopefully another 100mm of excavation won't be required, but I'm certainly not going to dig the full 300mm for the hell of it.
