Question
You recently answered a question about cementing flagstone over an existing patio.  I have a stamped conc. patio that has flaked and spalled.  The surface looks OK but the colorant and sealing is chipping off.  I only have 3 1/2" to the bottom of my door thresholds.  Can I lay a bed of sand, and just butt flagstone or some other paving stone over the top, without mortaring them?  I don't think mortar would bond to the sealed and stamped conc. anyway.  I'm trying to salvage the poor job the contractor did on the patio.  Thanks for your insight.

Answer
Yes you could do that... but I wouldn't. The sand will track into your house and be a mess or even damage wood and other floor types susceptible to scratching. Sand also will splash out of joints onto the house when it rains.

I only use sand set stone out in the garden away from the active ling areas or high use areas near your home and entrances.

So in order to get the stone to bond to your old patio you need to scarify the surface of the bad stamped concrete. You can start by mechanically scraping, chipping, etc. You can rent a belt sander with a very coarse grit, you can rent a cement grinder, or even a small jack hammer will work. Just drag these over the surface and be sure you make lots of chips and scrapes. Also you could do it manually if it is not to big an area and old hammer and a few hours of scraping and chipping will do. Then you can acid etch the surface. Spray it with Muriatic Acid/water mixture (be careful and follow safety instructions). After this has etched the surface you can pressure wash with a high power washer. Rent the big one at 3000psi. After it is completely dry you are ready to lay the new stone.

Sorry this is not easy, nothing worth doing ever is.
Good Luck.


Sean J Murphy, LA,ISA, LEED AP
Please check out my blogs and articles on landscape topics like this on my websites.
http://www.seanjmurphy.com
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