Indian sandstone..mixing subases? - Indian sandstone can i mix subase

[majorvonhapen]

Hi Tony,

Congratulations on such a great, informative site.

Please forgive me if I am repeating a question already asked, I don't want to be a time waster. I combed through the archives and hopefully I'm the first with this problem/query.

Cutting to the chase.

Indian sandtone, 18 by 12 inch, one inch thick flags, going down on a full mortar bed.

Covering our existing 3 foot wide concrete path (well under DPC),
............then in a semi circle adjoining the path, and going down on a 100mm subase type A.

'Somebody' has just told me that a cracks could form as I putting down the stone on two different subases, due to movement, etc.

Is he right?

Thank you very much for taking the time to read this.

Kind regards,

Majorvonhapen

[Tony McC]

In essence; yes - you could experience what is known as 'differential settlement', because one section of the paving is laid over a rigid concrete base while the other is laid over a flexible sub-base of Type 1 material. Any 'crack' is most likely to occur in the jointing between the flags nearest to the interface of the sub-base and concrete base, so you can pre-empt this by having a joint run directly above, or damn close to, this interface.

As I said, it would be the mortar that cracks, not the flags, so any differential settlement that does occur can be pretty easily remedied by re-pointing the affected joints. After a couple of years or so, if you find there is a trip developing between the two, then it's a simple matter to re-lay a few flags on the 'low' side and bring them up to the level of those on the higher side, thereby eliminating the trip.

In short - nowt to worry about.