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I'm going to be putting pavers over an old slab patio. The current patio has settled and spalled, so I'm looking to pitch the paver patio in such a way that it drains as the original slabs did. In some places that will mean 2" of substrate while 1/2" in others.

It is surrounded on two sides by brick wall, and a third side by pressure treated wood (the side of a house built on concrete tubes). The fourth side is the down hill side. Since there is something constraining on all sides, do I really have to cement the perimeter pavers down, or can I just install again existing surfaces?

I plan on using polymeric sand for the cracks, but what is the best type of substrate to use under the pavers if they are on top of solid concrete? Should I compact the substrate or not?

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    @isherwood These questions were separate but sharing in detail and general composition, so I combined them into this and flagged the first question. – Ben Welborn Jun 8 '16 at 19:20
  • so weird though, on other stack exchanges people have told me to separate questions if it contains two questions ... but here they are put together ... – user379468 Jun 8 '16 at 21:18
  • Normally I would say that too, but these are kind of the same question... it's one problem... combined, your question is like, "How do I put new pavers on top of old ones? Or is that okay? I need guidance on the substrate." – Ben Welborn Jun 9 '16 at 13:24
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Since nobody else it going for this, then I'll give my two cents (based on limited experience, so please, anyone else feel free to chime in).

When it comes to concrete roads, for example, after patching it a few times the road eventually becomes too worn out, and the concrete is rubblized (into crush and run). Then fresh pavement is layed on top. Here's a look at some overlaying options for roads: Concrete Pavement Overlay Design. This isn't a manual and it's not extremely helpful but some of the pictures might help you visualize your own possible solutions.

With that being said, I realize this is just a patio... but the similarities to highways is not so remote that you couldn't glean something from studying how they're done. I think the biggest issue you have to deal with is levelling the area. I'm afraid that I don't really know what's going to work for you. Your idea of whitetopping may be okay... but you also said something about 2 inches... so, I think that the official answer must be that you have to remove the old stuff... it's the only way to be sure it's done correctly... and yes, soil compaction is important. After leveling and compacting the soil you need a full 2" to 3" layer of crush and run before laying down 1" to 2" of sand and new pavers.

Edit- After giving this some thought, my worry would be that the underlying pavers would cause the sand and underlayment to wash out due to non-drainage... so a soil base (which drains) is probably better. Plus, you need room for the crush and run (or gravel), sand, and pavers (but perhaps you already have that depth). The grade should be about 1/4" per 12 ft.

Otherwise if you pour a concrete pad over the old structure, and want to add pavers, then you probably shouldn't use anything beneath the pavers, so that there is nothing to wash away.

  • what about crushed stone ? it will be anchored on 3 sides ... the fourth side is down hill, so I suppose that it could wash out, was thinking of using hydroduct on the one surface that won't be anchored ... it has a fine mesh over plastic duct system, it would allow drainage but potentially hold back anything that might wash out – user379468 Jun 16 '16 at 19:32
  • @user379468 hydroduct sounds interesting. I suppose it stops working when the mesh becomes clogged so it might be better to sue something like pea gravel (rather than crushed stone which has lots of dust). Or if you could break up the old pavers so that they could drain it would probably work out. – Ben Welborn Jun 17 '16 at 14:09

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