So I have a wee a conundrum on a bit of patio I'm hoping to finish tomorrow. I've laid an area of patio using 600*600 ~20-25mm riven slate slabs on a ~50-70mm full wet mortar bed above 75-100mm compacted type 1. Most of it has been done and is now well set. I just have to lay the last section which runs up to a small but thick blockwork/stone faced retaining wall I've built and must match the level of an existing bottom step (slabs already in place have been laid to this level).
About half the width of the last row of slabs will run over the top of the retaining wall (or the concrete bottom step at one end) which is ~40 mm higher than the level of the sub base. The other half of the slab will lay over the sub base. This means, to get the correct level, the thickness of the mortar bed under the part of the slab which rests on top of the retaining wall will be about 10mm, possibly thinner while the half over the type 1 will be about 50mm deep. The step and top of the wall have both been primed with SBR.
On reflection I should have built the wall and laid the step a little shorter, but that ship has sailed. I'm considering two options:
1. Mix a stronger mortar, say 1:3 or 4 using a mix of sharp sand and soft sand to get something that's a bit closer to brick laying mortar but with a little extra grit for using on this row and hope it'll be ok with half the slab on a much thinner mortar bed (I guess block walls only have a 10mm or sometimes thinner mortar layer so maybe a thin bed on top of a totally solid immovable base will be ok). The rest of the slab would still rest on a thicker mortar bed down to the sub base level. Both slab and wall will have an sbr/cement slurry applied as a bond bridge.
2. Use a flexible exterior tile mortar to bed the half of the slab which sits over the wall then a normal slab laying mortar mix for the deeper section under the half which sits over the type 1 sub base. Again a bond bridge will be applied to both surfaces first. My thinking is that the tile adhesive is designed for thinner layers.
Would appreciate any thoughts on this (good or bad, even just to say that sounds fine) or alternative suggestions you could offer.
Have added some pics for context. Last one shows the slabs rested in their intended layout (the pattern will fill the gap between this last row at the step and the main section)
About half the width of the last row of slabs will run over the top of the retaining wall (or the concrete bottom step at one end) which is ~40 mm higher than the level of the sub base. The other half of the slab will lay over the sub base. This means, to get the correct level, the thickness of the mortar bed under the part of the slab which rests on top of the retaining wall will be about 10mm, possibly thinner while the half over the type 1 will be about 50mm deep. The step and top of the wall have both been primed with SBR.
On reflection I should have built the wall and laid the step a little shorter, but that ship has sailed. I'm considering two options:
1. Mix a stronger mortar, say 1:3 or 4 using a mix of sharp sand and soft sand to get something that's a bit closer to brick laying mortar but with a little extra grit for using on this row and hope it'll be ok with half the slab on a much thinner mortar bed (I guess block walls only have a 10mm or sometimes thinner mortar layer so maybe a thin bed on top of a totally solid immovable base will be ok). The rest of the slab would still rest on a thicker mortar bed down to the sub base level. Both slab and wall will have an sbr/cement slurry applied as a bond bridge.
2. Use a flexible exterior tile mortar to bed the half of the slab which sits over the wall then a normal slab laying mortar mix for the deeper section under the half which sits over the type 1 sub base. Again a bond bridge will be applied to both surfaces first. My thinking is that the tile adhesive is designed for thinner layers.
Would appreciate any thoughts on this (good or bad, even just to say that sounds fine) or alternative suggestions you could offer.
Have added some pics for context. Last one shows the slabs rested in their intended layout (the pattern will fill the gap between this last row at the step and the main section)
