A beautiful flagstone patio should have uniform base
Q. I plan to install flagstone over an small concrete patio which is crumbling and to enlarge it. Do I use the same method to lay the flagstone over the concrete as I do for the new enlarged section?
A. Flagstone makes a beautiful patio, certainly much better looking than concrete that is crumbling in areas. Its appearance is totally different from a uniform concrete patio because each piece of flagstone varies in shape and color.
The flagstone can be laid over the old concrete patio. Since you are planning to enlarge the patio though, it would be wise to remove the old concrete so the entire flagstone area will be on the same type of foundation. Using the old patio as a partial base would require the rest of the area to be built up higher. This may result in future settling and unevenness.
Flagstone is a completely natural product from the earth. Its name comes from the way it is processed into individual stones, not from the type of stone used to make it. Flagstone is cut into thin layers (flagged) to create the pieces you purchase. Depending upon your region of the country, it can be made from limestone, sandstone, bluestone, slate, etc.
Most flagstone is priced by the square foot and it is not inexpensive. It actually is somewhat difficult to precisely determine how many square feet you are getting because of its irregular size. To be safe, purchase about 10 percent more than you calculate you need. Shaping the stones for a good fit results in some waste and you surely will break a few pieces no matter how careful you try to be.
If you are building a large patio, plan on taking the good part of an afternoon to select the flagstone. You will have to go through many pieces to match ones that fit together reasonably well. It is best to select ones that are about two inches thick for adequate strength. If you do put some over concrete, one-inch-thick flagstone is adequate because the concrete provides support.
To place the patio over new ground, remove any grass and level the area. Dig down to a depth of the average stone thickness plus two inches for the sand bed. If you prefer to raise the height, first lay a bed of medium crushed gravel. The pieces of gravel lock together to form a stable base.
Using pressure-treated 2-by-4s, make some forms around the perimeter of the new patio. Pour the sand into the area and tamp it down. The end of a 4-by-4 post works well if you do not have a vibrating compactor. Dampen the sand slightly so it compacts.
While the sand is still damp, size and place the pieces of flagstone on the sand. Tamp each one down so it settles firmly into the sand bed. Place a level across each two adjacent pieces to be sure you are getting them level. Stretching several pieces of string across the stones is another leveling indicator.
The final step is to pour additional sand over the flagstones. Using a brush, push the sand into the gaps between the stones. Brush off the excess sand. Spray it with water and let it dry. Sweep more sand over the top to fill any areas that have settled.
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