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An existing patio has flag stones set over a concrete slab. If you were to remove the flag stones and thinset, what would be the best way to raise the level of the existing slab to make up for the lost height once the flagstones are removed? Height difference is about 1".

Is there a recommended mix for this? Are there any additives, bonding agents, mesh or anything along those lines that would be recommended?

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    There are several ways this can be done, however you try to add concrete over it know that a 1 inch pad won't last as long outside.
    – DMoore
    Commented May 12, 2014 at 18:38
  • I would agree with @DMoore, one inch would be lucky to make it a year depending on climate/weather/traffic etc. Long term you would want to tear it all down and build it back up to the desired height.
    – Chris W.
    Commented May 12, 2014 at 18:51
  • @ChrisW. - there was a reason why someone put flagstones there. They choose a "temporary" solution and if you don't mind doing this every 7-10 years just put in pavers.
    – DMoore
    Commented May 12, 2014 at 19:12
  • @DMoore the flagstones are slate and they were installed from the beginning (maybe 30+ years ago) to create a more decorative surface instead of plain concrete. It wasn't a temporary solution.
    – OrganicLawnDIY
    Commented May 13, 2014 at 0:48
  • @OrganicLawnDIY - I was just guessing that they threw flagstone over an old patio - as that is what I see all the time in my area. Actually prepping the area with concrete is hardly temporary. I still think your new one inch pad can't be expected to last as long. I guess it depends on your location (how bad your winters are). Not saying it will be bad in a couple years but I have seen issues with doing an inch of concrete in a temperature controlled basement.
    – DMoore
    Commented May 13, 2014 at 15:13

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Something got lost in translation and the question I asked here was totally different than what the person asking had in mind. Not a patio and they didn't even want to raise the height of the slab! That said... I still found the answer to the question I asked by contacting Sakrete, a manufacturer of bagged concrete mixes found in many home improvement and hardware stores in the US.

They recommended:

  1. Surface be thoroughly cleaned and all thinset or tile adhesive completely removed. Any cracks should be repaired.
  2. Existing slab should be painted with their Concrete Bonder and Fortifier and allowed to dry.
  3. They recommended their Sand Mix which as far as I can tell has either no, less or smaller aggregate. It's used for pouring 1/2" to 2" thick. For the liquid, use the bag recommendation but replace half of the water with Bonder and Fortifier. Combine the admix with water before adding to cement.
  4. Any expansion joints in the original slab need to be duplicated on the top coat.

No mesh or fibers necessary.

I called Sakrete because that's what's easiest for me to get locally. Quikrete has a similar Sand/Topping Mix and an Acrylic Fortifier. You can mix your own sand mix I guess if you're into that sort of thing.

One of the comments above about being lucky to last a year was a bit extreme. I have seen a heavily trafficked sidewalk leveled with some straight out of the bag mix that held up for over 10 years in the NYC area. DIY job with no bonding agents or other additives.

I have personally used Quikrete's acrylic fortifier and can attest that it creates a better bond.

I agree it's always better to rip out the old slab and pour a new one but sometimes the added labor and budget isn't always practical.

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    Adding 1 inch over existing can be successful with the advise you received from Sakrete. The key step you didn't mention is the etching of any exposed old concrete. A good bond is dependent on a properly prepped base. I would always recommend etching the old base with muratic acid, neutralize and be sure to flush completely. Then do the bonding steps and re-pour. Ever good bonding agents will not adhere well to dirty or efflorent concrete.
    – shirlock homes
    Commented May 13, 2014 at 10:35
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And the WINNER is................................"remove all " I have the problem of three uneven steps that need the height adjusted, the slope of the first step outside the threshold, and extend the three of them. The absolute simplest, after extensively pursuing this research is: The more cures the higher the risk. Tear everything out. Worth the work for the simplicity of the replacement that follows.

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