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My concrete sidewalks are an inch or so below grade (presumably sunken since installation by previous owners?) and are beginning to crack. I'd like to install flagstone pathways in their place. It looks like, in general, laying stone over existing concrete is acceptable. It also looks like I can use 1/2" of sand and 1" thick stone to get the walkaway about 1/2" above grade which seems suitable.

In addition to aesthetics, I'm interested in replacing the walkway with stone for long term durability. Will the pathway be as durable laid over existing concrete as it would be if I demolished the concrete and installed it more conventionally?

  • The big question is—did the concrete "sink" or did the grade rise? I have an example: My neighbors own an 80 year old house. When they moved in, the front walk was concrete and about an inch below grade. They decided to put in pavers and hired a contractor to remove the concrete. The contractor found that the concrete was only an inch or so thick, laid on top of... pavers. They did some research and found that over the previous 80 years the lawn on either side of the walkway had "risen" about two inches. This meant that the original pavers were stable, so they laid the new ones over the old. – Jurp Feb 3 at 16:26
  • In your case, if the grade rose over time, then you can lay the flagstones as you describe since the base is not only compacted already, but stable. If, on the hand, the concrete sank, then you need to determine a) is it still sinking? and b) why is it/did it sink? If you don't remedy this problem, then anything you do with stone in that area will also sink. – Jurp Feb 3 at 16:28
  • I believe it sunk because someone at some point watered the grass (there's evidence of a very old sprinkler system), but it probably hasn't been watered regularly in 10 years in a dry climate. We intend to xeriscape the whole area, so the water quantities are unlikely to change from the recent past. How can I tell if its restabilized without waiting five more years? Also, if I'm going to dry install the flagstone, does it even matter since jacking up sagging will be as easy add adding more gravel/sand underneath? – ericksonla Feb 3 at 21:31
  • Do you have public sidewalks in front of your house? If so, then you can get a sense if the grade rose by examining the grass that's right next to the concrete there - is it higher than the sidewalk? If so, then the grass built soil over time, which means the grade rose. Since you intend to xeriscape, then you can more easily move slope the soil down to the stone should it sink. Jacking the existing concrete after you lay the stone could be a pain. – Jurp Feb 3 at 21:35
  • Upon rexamining to compare to public sidewalks, I take back everything I said before. I think the sidewalks are settled but the grade rose due to tree root growth. So in that case it seems likely to be stable? – ericksonla Feb 3 at 22:26
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About durability: it is difficult to tell. Concrete is a artificial rock/stone, not so far from flagstone.

There are a lot different type of stones, some cracks more often then other (also between flagstone). Also the size matter. I seen thin layer stone used for pavement (because they are cheap), but so they crack quickly. Also too large stones tend to crack more easily if the base in uneven (underground water!).

As far i remember, flagstone are not the more hard and resistant stones.

But the main problem is the "sunken". This is independent of material. Or it was not placed as one should ("foundations"), or the soil is not even (maybe because of changing water in soil (differences between dry and wet weather)). I would check this, and maybe I would do a oversize pavement, so that I would not park/go on the same spots every time. If actual concrete is still sinking, I think there is no advantage to keep it as foundation.

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