Required sub-base for Porcelain tile patio

Tgrewal

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 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 sand
3. lay concrete slab, minimum 75mm deep
4. lay DITRA Mat above concrete slab
5. lay poreclain tiles with flexible adhesive

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

casper_gutman

Based on the guidance here... https://www.milesstone.co.uk/blogs/news/a-guide-to-laying-porcelain-paving
...it looks like a concrete slab may be unnecessary and a good depth of well-conpacyed granular material might do the job.

On the other hand, a slab is one of the approaches they suggest but they recommend using a granular sub base below the slab for a total depth of 180 mm.

So, on one hand it might be overkill and on the other not enough!

DB1904

casper_gutman said:

Based on the guidance here... https://www.milesstone.co.uk/blogs/news/a-guide-to-laying-porcelain-paving
...it looks like a concrete slab may be unnecessary and a good depth of well-conpacyed granular material might do the job.

On the other hand, a slab is one of the approaches they suggest but they recommend using a granular sub base below the slab for a total depth of 180 mm.

So, on one hand it might be overkill and on the other not enough!

If I were faced with the OP's ground conditions and was only laying a concrete slab I would be going for a suitable sub base. So asking someone to just lay some sand is asking for problems. 

FaceHead

You've spoken to a structural engineer, so are unlikely to get better advice on a forum. Go with it.

A few observations:
-The recommended approach is very heavy duty, but that seems to make sense, as the ground conditions are quite challenging 
-Why sand and not MOT Type 1, which would be more usual? I suppose it makes little difference. 
-As casper says, it may not be enough - If the ground is moving a serious amount, there is really not much you can do about it. So find and alternative design or risk it on this one.  

stuart45

A flexible gravel bed can be used.