What sub-base for Porcelain tiled patio?

[windsor-tg]

My current patio is block paved (laid about 20 years ago) but I am looking to have a porcelain tiled patio. My house is on clay soil and I have two large oak trees in my neighbouring garden. I find that during the summer, a gap opens up between the paving blocks and also some of the paving blocks sink due to effectively ground movement beneath the paving blocks caused by clay shrinkage.

Can anyone advise how the sub-base should be best constructed so that I get minimal movement once the porcelain tiles are laid. I am concerned that if the sub-base is not properly constructed, the porcelain tiles will move/crack over time. The overall size of the patio will be 70 sq metres (I have attached a photo to show the layout of the new patio). 
 

I did speak to one structural engineer recently who suggested the following sub-base:

1. remove existing paving blocks/bricks and excavate soil to required depth
2. lay lay approx 3" of compacted sharp sand on top a decent mot type 1 base
3. Include 2 movement joints 
4. lay rebar concrete slab (mesh of steel wires/bars), minimum 100mm deep
5. lay DITRA Drain 8mm matting on flexible adhesive (standard S1 or even unmodified will do)
6. Lay porcelain tiles on Ditra Drain using flexible adhesive but adhesive must contain polymers in the mix for proper adhesion of the tiles to the Ditra Drain (standard S1 polymer modified adhesive at normal bed depth - solid bed approx 4mm deep)
7. On back of tiles, should be 'back butter' (thin spreading of adhesive on the back of a tile to ensure it correctly bonds to your tiling substrate. It's required when fixing large format porcelain tiles with a standard non-pourable adhesive and takes mere seconds but ensures a lasting bond! to aid in achieving solid bed)
8. Use epoxy grout between tiles - better weathering and is water impermeable

 

Does what this structural egineer suggest an over-kill? Is there an alternative option?

[markc]

That’s a detailed high spec base and should do the trick.

always a difficult decision when building on ground that will heave and fall. If it was a structure you would take foundations down below the affected ground and support the slab above it(beam and block style).

as your application is a patio, some slab movement isn’t as detrimental so strip and sub base etc is probably the cheapest and easiest
 

[tonyshouse]

Movement joints are needed, is the retaining wall to retain ground above patio level or under it? 
 

if it were mine I would dig out, geotextile, compacted hardcore or MOT type 1, no sand, reinforced concrete with A252 mesh, 125mm concrete. 
 

if patio slabs then the three bays as plan would be ok laid on leanmix or sand and cement 

 

more nervous with the fancy membrane but too expensive for me, I would divide the two larger areas  into 4 bays each, continuous mesh but movement joints using wood ‘3x1” above and below mesh. 

 

slabs all laid to same falls, tiles not to cross movement joins, slabs all laid to same fall and perfectly flat, suggest tile as normal for outdoors, polysulphide the movement joints 

 

mitigate cracking by design 

[markc]

Oh yes, only just noticed the 3” sand ? no idea what that does … apart from wash away if the area is quite wet.