Patio Grid installation

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Hello,

Thank you in advance for all who will take time to support!

I am planning to install patio grid in my garden to give some firmness to my heavy clay soil.

Now I understood I need to dig around 130mm (as it will be light traffic and only foot traffic) and put around 60mm hard subbase 30mm of Sharp sand and the 40mm of the grid filled with probably 10/20mm chipping.

However as I am on clay soil and need permeability (yes raining a lot in Wales and I have a conservatory adjacent to where I am going to do the patio), I am not sure what to use as hard subbase.

I read MOT 1 is the most common but my understanding is that it will not be good for the permeability so can I use 20mm Limestone instead? or 20mm gravel?

It is for an area of 10sqm approximately.

Thank you!