DEAR TIM: I want to dress up the look of our existing concrete patio. Is it possible to install flagstone or similar material directly over the concrete? What do you use to adhere the flagstone to the concrete? Can I enlarge the patio without fear of future cracking where old meets new? Kathy D., Batavia, IL
DEAR KATHY: Wow! What a gorgeous patio you will have. Flagstone can be installed directly on concrete. In fact, it is the only way I would do it if you desire an easy to maintain surface and one that stays flat for the entire time you intend to use it. Your biggest challenge will be getting used to working with flagstone. It is one of those natural building materials that can drive you crazy. But the insanity is worth it once you are finished.
To maintain the top of the patio in the same plane, you install different thicknesses of Portland cement mortar beneath each individual piece of flagstone. It is wise to get all of your material delivered at one time and sort through it until you find the thickest piece. This sole piece dictates the height above the current patio that you must reach as you install all of the pieces.
I prefer to install flagstone in a wet bed of Portland cement mortar. This is a simple blend of medium sand, regular Portland cement and clean water. Mixing two measures of sand to one measure of cement will yield a very rich blend that will permanently bond the flagstone to the existing concrete. If you desire an insurance policy to make sure the flagstone does not delaminate, apply a thin coat of wet cement paint on the concrete just before you place the mortar onto the concrete. The cement paint is simply pure Portland cement that is mixed with water until it resembles paint. Only mix as much mortar and paint as you can use in 2 hours.
There are two schools of thought with respect to the consistency of the mortar that you work with. Flagstone and many other natural stones absorb water slowly. Masons refer to this as suction. Low or poor suction means that the mortar does not firm up and the rock can swim or float on the wet mortar for a period of time. Some masons prefer to make the mortar damp so that the flagstone is very firm the moment it is set. I have found that there is indeed a happy medium.
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Mortar that has a higher moisture content and is blended to the above specifications produces millions of tiny crystals that grow into the pores of the flagstone and concrete patio. These are the mechanical connections that lock the stone into place. Damp mortar might not yield as many crystals. The trick is to make up a small batch of mortar and practice. Trust me, you will know when it is too wet. I prefer my mortar to have the consistency of cake icing that just barely pours out of a mixing bowl.
Enlarging your patio is not a problem. You simply need to bond your existing patio with the new concrete base. I do this by drilling one half inch diameter holes in the middle of the edge of the existing patio. Drill the holes approximately four inches deep and drive twelve inch long one half inch diameter steel reinforcing rods into the holes. A four pound hammer does this very quickly.
Install long pieces of the same type of steel in the new slab. Place the steel on two foot centers both directions and support it with small pieces of stone so that the steel ends up in the middle of the concrete once it is poured. Be sure the long rods are tied to the dowel rod pins that have been driven into the existing patio. This steel will hold the concrete together in the event it cracks in the future.
Keep in mind that the shape of your new flagstone patio can vary a little bit. You do not have to extend the flagstone to the edges of your existing patio. Furthermore, the new patio section can be flared or curved. You can also add landscape features and fountains that overlap on to the existing patio. Curves always seem to add interest in the event your existing patio is a traditional square or rectangle.