Question
I have a concrete patio about 12 Ft X 32 ft I want to know if I can cover the existing concrete pad with flagstone rock ? I so what materials and adhesives are required Thank you Bill

Answer
The simple answer is yes you can.

As long as your patio is in good shape and does not have too many cracks in it or a bad slope, you can cover it with stone. Stone is best laid just like tile. You will use a standard gray mortar to set the stone (make sure you get the appropriate type for your climate and conditions). Typically 1/2" to 1" of mortar is spread and then 1/2" to 2" thick stones are set in the wet mortar. Use more mortar to make up for thinner pieces of stone and less for thicker stones. The stones are tapped or tamped into the mortar by  hand or with a rubber hammer or by tapping on a block of wood on top of the rock. Use a 3' or 4' level to be sure the stone's surface is uniform and even. Occasionally you will have to remove a stone and add mortar or scrape some off to get the stones even.

Where there are cracks in the patio it is a good idea to lay a piece of tar paper or plastic across the cracks as a bond breaker so that the stone will not adhere to the concrete near the cracks. This bond breaker should be at least 18" wide. The professional belief is that this will keep the crack from transferring through to the stone above. However, some masons and designers, including me, don't believe the bond breakers really work in the long run and just set the edges of stones along the cracks so that if and when the crack transfers through to the stone it will just follow the edges of the stones and be less visible.

Once the mortar is cured for a day or more you can go back and grout the joints. I personally like to do it just like tile by applying plenty of mortar between each stone using a trowel and sponge to press it in. I scrape of the excess and then let is set up for about an hour or two and then I wipe off the excess several times with a big wet sponge and wring out the water in a 5 gallon bucket. This is a very neat and clean look and leaves no gaps. Some people like the look of leaving it un-wiped while being very careful about pressing the mortar in the joints, its a rougher look, but some people like it. The mortar or grout in the joints can and is often a colored mortar, with buff, brown and black being popular choices.

After all the mortar and grout is fully dried and cured you should seal the stone and the grout with an appropriate masonry/stone sealant for your climate. Consult your local masonry supplier for brands and types. The sealer will keep water from migrating between the stones and then freezing and popping them loose in winter. If you live where it does not freeze then sealing is not as important. Note that sealants often bring out or enhance the colors of the stones, but can also be glossy and commercial looking so be careful to choose a sealant and finish that will give you the look you want (glossy, matte, satin, etc.).

Also, when selecting the stone, be sure to do the finger scratch test. Especially on Crab Orchard stones from the south, if you can easily scrape away rock with your own finger nail it is probably too soft. Also make sure it won't brake easily in your hand. Make sure you are getting good, hard stone that is fairly consistent in thickness and finish. Some stones like 'Tennessee Blue Gray' which is often sold as "Pennsylvania Stone/Sate" is actually not nearly as dense as the real thing and will discolor in wet conditions due to high iron content. The best stones to use for this application are 3/4 to 1" thick and consistent.

Good Luck on your project.


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
http://amenityarchitects.com