Answer
What thickness should patio slabs be?
Porcelain around 20mm, natural stone 22mm or more; the base matters more than the slab.
In short
It depends on the material and the use. Porcelain paving is typically 20mm, natural stone often 22mm or more, and both are laid on a full bed so the thickness works with the base. For heavy use the build-up underneath matters more than the slab. We spec it for the job.
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