On 19/02/2021 13:38, Scion wrote:
A friend had a low brick garden wall built with slabs on top for seating.
A couple of slabs came loose after very little use and it is evident that
none of them are fixed properly - a sharp tug will make them come off.

Builder is blaming friend, for having the temerity to sit on the seating
area. (Yes, he was told it was for seating.)

It doesn't help that the slabs are wider than the wall; the overhang
allows leverage forces to pull the slab up.

I'm happy to have a bash at fixing, my question is, what mix of cement/
sand/whatever should I use to obtain maximum "tensile strength"? Any prep
on the underside of the slabs? They are smooth-ish - think slate texture -
but fairly thick.


The problem is not just the mortar mix (which was probably too weak)
but the differential thermal expansion and contraction of wall and
slabs.
Also 'slate texture' slabs won't have the suction required to
create a proper cement bond, which could be the major issue which
you won't cure even with a stronger mortar mix.
I presume the mortar is still well adhered to the wall but the slabs
have parted company with the mortar ?. You might have to consider a
chemical adhesive which allows some flexibility as the slabs expand
and contract.