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Old 07-08-2013, 11:26 AM   #1
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flagstone over bad concrete


We have an old concrete slab that's starting to disintegrate -
can we fill with sand and lay a flagstone patio over the old slab.

If so - how?

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Old 07-13-2013, 07:43 PM   #2
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flagstone over bad concrete


I don't think sand alone will provide sufficient strength to fill the cracks, and support the heavy flagstone. If you did that, I think the flagstone would crack, as well. I would put another layer of concrete over the existing bad slab to provide a good foundation for the flagstone.

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Old 07-14-2013, 05:51 PM   #3
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flagstone over bad concrete


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Originally Posted by marrilee View Post
We have an old concrete slab that's starting to disintegrate -
can we fill with sand and lay a flagstone patio over the old slab.

If so - how?


Sure you can do that,BUT be prepared to replace over and over and over,just tear it out and do it right,and you will be done with it.
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Old 07-14-2013, 08:22 PM   #4
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flagstone over bad concrete


And where is located? Update your profile as that helps with location-related advice.

Why did the old stuff fail? Freezing? Undermined? Or what?

A picture of the affected area would be helpful.
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Old 07-14-2013, 09:18 PM   #5
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flagstone over bad concrete


I think you can put the flagstone right on top. I would suggest a self-leveling cement on top of the slab with the flagstone put on it wet. Flagstone will stick to cement, and the self-leveling cement would fill the cracks. It is not cheap, but a lot cheaper than trying to remove that slab. A 40lb box of the quickrete stuff is about $25 at the big box.

I used the self-leveling cement on a front porch that was not level with a lot of pock marks and cracks. It looked like brand new cement when I finished. The self-leveling cement went from 1/16th inch thick to 3/4 inch thick. I also used it to attach flagstone to a little "patio" (of sorts). The flagstone stuck well. I think it might help with your broken slab.
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Old 07-15-2013, 07:42 AM   #6
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flagstone over bad concrete


Self leveling on a patio would be a pretty bad idea. When it rains you want there to be a slope to the surface so the rain will move away from your house. Using a self-leveler would lose that slope. You'd end up with water left standing on it. That and I'm not sure how many of those kinds of cements are intended for exterior use.

But without knowing WHERE you're located and WHY that existing concrete failed it's rather difficult, if not impossible, to offer useful advice. What works for one situation (and it's questionable how well that would work over time) may be disastrous for another.

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