2

I've been contemplating a DIY patio re-do vs having a hardscaping pro do it.

I've used a demolition hammer to remove mortared brick from a concrete slab, leaving a rough surface in which the aggregate is clearly visible on the surface. The loose mortar has been power-washed off. There is a long 3/16" crack in the concrete slab, which was poured back in the 1960s or 1970s.

I'd like to install cleft flagstone.

Two hardscaping pros have told me you can't put a dry sand bed on top of concrete. They want to set the stone in mortar and grout the joints.

What would prevent using a dry screeded sand bed atop concrete? Does it have anything to do with water?

mr blint
  • 1,840
  • 2
  • 24
  • 49
  • @crip659: What do you think of using a ground-grid or a geo-cell type of product to contain an aggregate + sand mix in place in order to mitigate the mush/migration issues? An extra two or three inches in height on the patio would be no problem. – mr blint Aug 01 '22 at 16:49
  • The benefit IMO to that sort of experiment is that the flagstones could always be taken up if they started to move, and placed in a mortar bed, but if you start with a mortar bed, you're stuck with that solution. – mr blint Aug 01 '22 at 16:51
  • It might work, but if it does not, you have to start at zero again, remove everything. The mortar bed solution is known to last. – crip659 Aug 01 '22 at 17:06
  • It's a non-destructive experiment. The flagstone will be salvageable, which won't be the case if we start with mortar, and the crack propagates up through the mortar to the flagstone. – mr blint Aug 01 '22 at 17:27

1 Answers1

1

When flags are set on top of sand which is on top of gravel, then water soaks through the sand and runs through the gravel which does not move given the spaces between bits of gravel.

If you put the sand on a solid surface then the water movement will eventually create channels and the flags will move and/or break.

I suggest you take the advice of the professionals - they are professionals for a reason. Not just trying to "rush" you for money but trying to do you a good value job for your money.

FreeMan
  • 47,262
  • 25
  • 88
  • 193
Solar Mike
  • 27,371
  • 2
  • 32
  • 61
  • The two pros had very different attitudes regarding the crack. One wanted to demolish the entire slab and start fresh (extra $3,000 to demolish and an extra $3,000 to pour a new slab) and the other said the crack wasn't going anywhere because the slab was probably 50 years old or older, and not to worry about the crack ruining the flagstone the way substrate movement cracks floor tile. I didn't know which one to trust. – mr blint Aug 01 '22 at 17:37
  • 1
    @mrblint that is totally different to the info you put in your original \question. If you don't give the true story then how do you expect us to help? Neither conflict with what I told you though. – Solar Mike Aug 01 '22 at 22:16