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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.

Tim
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    What do the makers of the porcelain paving slabs say? They should have the proper instructions for using their product for longest life. Internet instructions have to be taken with a grain of salt and checked with the makers of anything. – crip659 Apr 12 '22 at 20:11
  • I haven't chosen them yet, but looking at one on my shortlist it says "sub-base of 100mm should prove to be sufficient" .. but also that it "be based upon the prevalent ground conditions". Notably, it says nothing about a layer of concrete. – Tim Apr 12 '22 at 20:20
  • Contact the manufacturer, share your concerns with them, and see what they say. They'll probably repeat "100mm and no concrete is necessary" (since that's what their literature also says). Not sure we can do much more to assuage your fears than that. If you're still really concerned, then dig deeper and put in a concrete layer. – FreeMan Apr 13 '22 at 17:20
  • This is a Question & Answer board, not a discussion forum. Please make your "update" an answer. – FreeMan Apr 21 '22 at 10:46
  • It's not an answer to my question. It's a likely course of action in the absence of an answer. If someone wants to provide some reasoning about when you should or shouldn't add concrete layer then I'll accept it. – Tim Apr 21 '22 at 11:14

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